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Ultra‐low frequency (ULF) waves transfer energy and momentum into the ionosphere‐thermosphere system. To quantify this energy, this paper first presents a new method to quantitatively detect ULF waves in Incoherent Scatter Radar (ISR) data based on 2D fast‐Fourier transforms and subsequent reconstruction of the wave. In parallel with other data sets, including optical, magnetometer, satellite, and models, we present the first full ionospheric energy dissipation rates for a ULF wave, split into electromagnetic (EM) and kinetic fluxes. The EM energy deposition is calculated from the use of the Poynting theorem, looking at Joule and frictional heating rates, where both rates show the same order of magnitude (1.24 × 1013 and 7.3 × 1012 J) respectively when integrated over the wave lifetime of 2 hr 15 min and an area of 4° magnetic latitude × 74° magnetic longitude. However, contrary to the common assumption that the EM flux is dominant, we determined the kinetic flux, to be almost equal in magnitude (8.7 × 1012 J). This indicates that previous papers might have underestimated the total energy dissipation by ULF waves. Compared to the substorm energy budget, we find that locally, the ULF wave event studied here makes up approximately 10% of a typical substorm cycle budget. Plain Language Summary: The Earth's magnetic field lines can move back and forth regularly, like waves on a guitar string. When this happens on the time scale of minutes, we call these ultra‐low frequency (ULF) waves. ULF waves play an important role in the transfer of energy from the Earth's magnetic field to the Earth's upper atmosphere. In this study, we developed a new method to detect ULF waves in radar data to obtain information about the wave. By combining several data sets, such as radar, optical, satellite, and magnetic field measurements, we were then able to determine how much energy is being transferred in one ULF wave event. Both electromagnetic and kinetic energy are important in this transfer of energy. Key Points: We present a new method to detect and reconstruct ULF waves in ISR data with a 2D fast‐Fourier transformWe estimate ionospheric energy dissipation rates due to EM and kinetic energy sources using the new methodWe find that kinetic and EM energy rates are of similar magnitude and should both be included in ionospheric energy deposition by ULF waves [ABSTRACT FROM AUTHOR]

The Mid‐latitude All‐sky‐imaging Network for Geophysical Observations (MANGO) employs a combination of two powerful optical techniques used to observe the dynamics of Earth's upper atmosphere: wide‐field imaging and high‐resolution spectral interferometry. Both techniques observe the naturally occurring airglow emissions produced in the upper atmosphere at 630.0‐ and 557.7‐nm wavelengths. Instruments are deployed to sites across the continental United States, providing the capability to make measurements spanning mid to sub‐auroral latitudes. The current instrument suite in MANGO has six all‐sky imagers (ASIs) observing the 630.0‐nm emission (integrated between ∼200 and 400 km altitude), six ASIs observing the 557.7‐nm emission (integrated between ∼90 and 100 km altitude), and four Fabry‐Perot interferometers measuring neutral winds and temperature at these wavelengths. The deployment of additional imagers is planned. The network makes unprecedented observations of the nighttime thermosphere‐ionosphere dynamics with the expanded field‐of‐view provided by the distributed network of instruments. This paper describes the network, the instruments, the data products, and first results from this effort. Plain Language Summary: A network of cameras and optical sensing instruments is deployed across the continental United States and is used to study the processes in the earth's upper atmosphere. These processes range from response to explosive events on the sun, as well as, response to strong events in the troposphere like thunderstorms or hurricanes. This paper describes this network, how it works, how the data are being produced, and initial scientific results. Key Points: A network of red and green line cameras and Fabry‐Perot interferometers is being developed in mid‐latitude USAThe network observes dynamics in two altitude regions of the upper atmosphereThe data are used to understand the upper atmospheric response to energetic events generated in the lower atmosphere or the sun [ABSTRACT FROM AUTHOR]

Two solar eclipse events in 2023 appeared to produce considerable enhancements in the thermospheric column density ratio of monatomic oxygen to molecular nitrogen (ΣO/N2) as measured by TIMED GUVI. We quantify potential sources for eclipse‐induced ΣO/N2 changes and find that the observed enhancements arise from the ionospheric O+ radiative recombination contribution to the OI 135.6 nm emission from which ΣO/N2 is derived. Variations in the solar Extreme Ultra Violet (EUV) and X‐ray spectrum, due to the difference between the disk spectrum and the coronal spectrum, are also considered but shown to have negligible contributions to the ΣO/N2 enhancements. After accounting for the radiative recombination contribution, we constrain the real thermospheric compositional change to the uncertainty level of the measurements of 5%–10%. These results are valuable for the interpretation of eclipse‐induced ΣO/N2 changes that will further first‐principle model comparisons and lead to a better understanding of the response of the thermosphere to localized variations in solar EUV and X‐ray forcing. Plain Language Summary: The Thermosphere Ionosphere Mesosphere Energetics and Dynamics' Global Ultraviolet Imager provides an estimate of the column ratio of monatomic oxygen (O) to molecular nitrogen (N2) in the upper atmosphere (ΣO/N2) as a data product under normal, sunlit conditions. Eclipses are special conditions. In this paper, we model the measured ultraviolet emissions from O and N2 that go into the retrieval algorithm to understand why this increase in the radiance ratio occurs within the totality of the eclipse. There are two components to the O emission, the glow created by electrons (photoelectrons) that result from the ionization of the upper atmosphere and the glow created by recombination of ionospheric O+ ions. We find that the photoelectron source decreases significantly during the eclipse, but near the magnetic equator, the ionospheric recombination emission does not, which causes the retrieval algorithm to overestimate the ΣO/N2 column density ratio during eclipses. The observations and models are consistent with this explanation. Key Points: The apparent thermospheric ΣO/N2 enhancements (10%–30%) for two eclipse events are found to be predominantly of ionospheric originVariations in the solar spectrum due to wavelength‐dependent attenuation are found to have a negligible impact on ΣO/N2 during the eclipseThe true eclipsed‐induced thermosphere compositional changes are determined to be within 5%–10% of the pre‐eclipse values [ABSTRACT FROM AUTHOR]

There are growing concerns about the effect of solar flares on the ionosphere, mainly due to possible deterioration or damage to our communication and navigation satellite systems. On 3 July 2021, and 28 October 2021, there were solar flares (SFs) classified as X1.59 and X1.0, respectively. These two SFs were the only ones of X‐class that occurred during the last low solar activity (LSA:2018–2021). Data from magnetometers and Global Positioning System (GPS)—Total Electron Content (TEC) are used to investigate the spatial‐temporal electrodynamics of the ionosphere from pole‐to‐pole in the American sector. Employing ∆H and vertical TEC, along with the ROT (rate of change of VTEC) parameter. Rapidly ∆H disturbances closely follow the X‐ray variation and the ∆H valleys and peaks are well‐synchronized during the SFs, indicating that they are linked. Major disturbances in the ∆H are noticed in the mid‐low‐equatorial latitudes. However, minor disturbances were seen at high latitudes. Also, |ROT| is a good indicator of the electron density changes during the SFs, especially when the X‐ray intensity rises to the peak. Plain Language Summary: There are growing concerns about the effect of solar flares on the ionosphere, mainly due to possible deterioration or damage to our communication and navigation satellite systems. This kind of space weather event known as solar flare, releases energy in the form of radiation in the entire electromagnetic spectrum. However, the UV, EUV, and X‐ray radiation burst penetrates deeper into the Earth's atmosphere and is absorbed in the D and E regions (lower ionosphere) and F‐region. In this investigation the ionospheric disturbances are investigated, from pole‐to‐pole, using a magnetometer and GPS‐TEC networks. Key Points: The primary objective of this paper is to investigate the ionospheric response to solar flares across the entire latitudinal rangeThe ∆H exhibited synchronized peaks and valleys, during SF. The ROT is the most effective parameter to study electron density disturbancesThe methodology employed in this study involves the calculation and analysis of ∆H, VTEC, and ROT (Rate of TEC change) [ABSTRACT FROM AUTHOR]

The paper presents the effects of the storm‐time prompt penetration electric fields (PPEF) and traveling atmospheric disturbances (TADs) on the total electron content (TEC), foF2 and hmF2 in the American sector (north and south) during the geomagnetic storm on 23–24 April 2023. The data show a poleward shift of the Equatorial Ionization Anomaly (EIA) crests to 18°N and 20°S in the evening of 23 April (attributed to eastward PPEF) and the EIA crests remaining almost in the same latitudes after the PPEF reversed westward. The thermospheric neutral wind velocity, foF2, hmF2, and TEC variations show that TADs from the northern and southern high latitudes propagating equatorward and crossing the equator after midnight on 23 April. The meridional keograms of ΔTEC show the TAD structures in the north/south propagated with phase velocity 470/485 m/s, wave length 4,095/4,016 km and period 2.42/2.30 hr, respectively. The interactions of the TADs also appear to modify the wind velocities in low latitudes. The eastward PPEF and equatorward TADs also favored the development of a clear/not so clear F3 layer in northern/southern regions of the equator. Plain Language Summary: The thermosphere‐ionosphere‐magnetosphere system is largely affected during events of geomagnetic storms. Its dynamics, mainly in the ionized environment, may impact modern lives by degradations in the satellite signals affecting, for example, all applications involving Global Navigation Satellite System. The thermosphere and ionosphere responses to the large geomagnetic storm of 23–24 April 2023 are analyzed here and the main discoveries are the unexpected spatial plasma density distribution over Boa Vista (MLat ≈ 8°N) due to resulting effects of a disturbed eastward electric field and traveling atmospheric disturbance (TAD). The interactions/interferences of TADs were able to explain the unexpected moment of the peak in thermospheric neutral wind speed measurements over the equatorial station, as well as all variations in ionospheric parameters (foF2, hmF2, and total electron content) recorded in pairs by seven Digisondes and GNSS receivers spread across Brazil. Key Points: Interactions of traveling atmospheric disturbances (TADs) cause unexpected thermospheric neutral wind speeds in the American equatorial and low‐latitude sectorsThe effects resulting from a disturbed electric field and TADs were able to produce anomalous electron density distribution at low latitudesThe variations in the ionospheric observational measurements recorded around 4 UT in Brazil on 24 April 2023 were explained by the passages of TADs [ABSTRACT FROM AUTHOR]

In this study, we used observations from multiple instruments (all‐sky imager [ASI], digisonde, and Beidou geostationary satellite global positioning system receivers) to show the occurrence and evolution of two plasma density enhancement structures (PDESs) during the night of 8 February and 4 November 2018. The PDESs were elongated from northwest to southeast. Their zonal scales were about 200–400 km and longitudinal ranges were about 800–1,400 km. Within the PDES, the plasma density can be enhanced by about two times over the background density. The PDESs appeared before midnight and gradually dissipated after midnight. Plasma density depletion structures appeared within the PDES at the later stage of PDES. Each PDES event lasted for over 4 hours. The morphology and evolution of the PDESs presented in this paper are different from those reported in previous studies. In addition, based on the thermosphere‐ionosphere‐electrodynamics general circulation model simulations and digisonde observations, we proposed a possible cause for the PDESs in the events, in which the occurrence and evolution of PDES are related to the variations of thermospheric meridional winds in the equatorial region. Plain Language Summary: Ionospheric plasma irregularities or disturbances are frequently observed in the equatorial region and have been the focus of ionospheric research. These irregularities include plasma bubbles, traveling ionospheric disturbances, and plasma density enhancements. They have a great impact on the trans‐ionospheric radio wave signals. Most of the previous studies show that the plasma density enhancements occur over large scales and the occurrence of plasma density enhancements is usually accompanied by plasma density depletions in the adjacent areas, and hence, the formation of the density enhancement is suggested to be related to the plasma density depletion. In this study, two unique plasma density enhancement structures (PDESs) were observed using multiple instruments. The observed plasma density enhancements were elongated from northwest to southeast, which is different from previous studies. This paper presents the full evolution of the plasma density enhancement from its appearance to disappearance. In addition, we proposed a possible formation process for the equatorial plasma density enhancement in which the occurrence and evolution of PDES are related to the variations of thermospheric meridional winds in the equatorial region. Our results provide new insight into the dynamics and variability of the coupled thermosphere and ionosphere system in the equatorial region. Key Points: Evolution of equatorial ionospheric plasma density enhancements from occurrence to disappearance was illustrated with multiple data setsThe plasma density enhancement structures (PDESs) were elongated from northwest to southeast, with their sizes changing with time, and they produced Spread F in ionogramsVariations in neutral winds may contribute to the occurrence of the observed PDES [ABSTRACT FROM AUTHOR]

We investigated statistically the orientation of about 10,000 auroral arcs observed during 2016–2021 in Abisko (68.36°N, 18.81°E, Sweden) in the equatorward part of the nightside auroral oval. The observations were made between 19 and 06 magnetic local time (MLT). On average, the orientation of the arcs, that is, the angle between an arc and the West‐East direction, linearly changed with MLT at the rate 2.2°/hr. In most cases the mean orientation of the auroral arcs near midnight was parallel to the geomagnetic latitude of Abisko, except a few late winter or spring months. These anomalies cannot be explained by geomagnetic disturbances or interplanetary conditions. They do, however, coincide with spring transitions in the atmospheric circulations from winter‐to summer‐type. The spring transitions were manifested in the data of a meteor radar in the region of observations as reversals of the zonal wind at altitudes 90–100 km from eastward to westward. We propose that these spring transition changes in the global atmospheric circulation couple also the overlying thermosphere at 100–150 km where neutral winds may affect the ionosphere and subsequently the whole ionosphere‐magnetosphere system. Then, changes in the coupled ionosphere‐magnetosphere system are manifested in changes of the orientation of auroral arcs. Plain Language Summary: Aurora is a manifestation of the precipitation of charged particle of the magnetospheric origin into the Earth's ionosphere. The most prominent auroral forms populating in the auroral oval are relatively narrow multiple auroral arcs extended for many hundred kilometers within the oval. In the past a number of studies were made to understand how auroral arcs are oriented with respect to the East‐West direction in different local time and what may affect this orientation. It is commonly believed that the orientation of the arcs primary depends on the solar wind conditions and geomagnetic activity. However, precise analysis of about 10,000 auroral arcs observed by the optical camera located in the Northern Europe during a 6‐year period of 2016–2021 has revealed anomalous orientations of the arcs which cannot be explained by interplanetary or solar conditions. Unexpectedly we have found that the global atmospheric circulation apparently affects orientation of auroral arcs. This effect, which has been never reported before, is the main subject of the present paper. Key Points: Orientation of nightside auroral arcs may be affected by the neutral windThe effect is noticeable during the spring transition in the global atmospheric circulationThe neutral wind can influence the ionosphere‐magnetosphere system through the ionosphere dynamo processes [ABSTRACT FROM AUTHOR]

The thermospheric density and its variations are crucial to aerospace activities as well as space weather research and operation. However, due to the difficulties in observing the thermosphere, there has been a lack of effective descriptions for the general characteristics of thermospheric density. In this paper, the Two‐Line‐Element data sets (TLEs) from multi‐target low Earth orbit satellites are used to derive a proxy of the daily average atmospheric density in the thermospheric shell located in the vicinity of LEOs' orbital altitude. It captures the overall characteristics of the thermosphere and exhibits good correlations (∼0.9) with modeled and observed thermospheric density. By applying the spectral whitening method to this proxy, a new index JsT ${J}_{s}^{T}$ is derived to describe non‐periodic perturbation of the density where the specific satellite passed by. The fact that the JsT ${J}_{s}^{T}$ obtained from different satellites within the same thermospheric shell presents significant consistency to each other means that the new index is a good indicator for the overall feature of the variations of thermospheric density, and it is possible to define a unified regional index JrT ${J}_{r}^{T}$ to describe density disturbances for the thermospheric shell where these satellites fly through. Moreover, the JrT ${J}_{r}^{T}$ at different altitudes also present good consistency suggesting the possibility of defining a global index JpT ${J}_{p}^{T}$, capable of describing the density variation of the entire thermosphere. Plain Language Summary: Due to limited in situ observation, the thermospheric density is not easy to obtain. Herein, we have extracted the density information of the thermosphere based on the Two‐Line‐Element data sets and developed a new index using the spectral whitening method, which describes aperiodic disturbances of the density. The construction of this index series can address the limitations of existing quantitative or graded descriptions of space weather disturbances and the problem of mismatched or inconsistent parameters/indices. Ultimately, this will contribute to the monitoring and warning of space weather causal chains. Key Points: A proxy of daily average thermospheric mass density was extracted from Two‐Line‐Element data setsBy applying the spectral whitening method to this proxy, a new index is derived to describe non‐periodic perturbations in the thermosphereA unified regional index was defined to describe density disturbances for the thermospheric shell where these satellites fly through [ABSTRACT FROM AUTHOR]

In this paper, the equatorial thermosphere anomaly (ETA) is investigated using accelerometer measurements to determine whether the feature is density‐dominated, wind‐dominated, or some combination of the two. An ascending‐descending accelerometry (ADA) technique is introduced to address the density‐wind ambiguity that appears when interpreting the ETA in atmospheric drag acceleration analyses. This technique separates ascending and descending acceleration measurements to determine if a wind's directionality influences the interpretation of the observed ETA feature. The ADA technique is applied to accelerometer measurements taken from the Challenging Minisatellite Payload mission and has revealed that the ETA is primarily density‐dominated from 9:00 to 16:00 local time (LT) near 400 km altitude, with the acceleration perturbations behaving similarly between 2003 and 2004 across all seasons. This finding suggests that the perturbations in the acceleration due to in‐track wind perturbations are small compared to the perturbations due to mass density, while indicating that the formation mechanisms across these local times are similar and persistent. The results also revealed that in the terminator region at 18:00 LT the acceleration perturbations deviate appreciably between ascending and descending passes, indicating different or multiple processes occurring at this local time compared to the 9:00–16:00 LT ascribed to the ETA. These results help constrain ETA formation theories to specific local times and thermospheric property responses without the use of supplemental wind measurements, while also indicating regions where in‐track winds cannot always be neglected. Plain Language Summary: Earth's thermosphere, a layer in the upper atmosphere, is a highly variable region that contributes to satellite drag in low Earth orbit. The equatorial thermosphere anomaly (ETA) is a perturbation feature that produces increases and decreases in drag accelerations across the lower latitudes of the thermosphere. Direct measurements of the thermosphere are often limited, so satellite instruments, such as accelerometers, have been used to analyze the acceleration that a satellite experiences due to atmospheric drag to indirectly measure thermospheric properties. However, the measured drag acceleration is dependent on both mass density and wind, making it difficult to determine whether a change in drag across the ETA is due to either property. This has implications on what mechanisms are responsible for its formation. This work introduces an ascending‐descending accelerometry technique to exploit the fact that while acceleration perturbations produced by mass density are independent of satellite direction, those arising from winds can either be positive or negative depending on the relative spacecraft motion. By separating satellite orbits into ascending (south‐to‐north) and descending (north‐to‐south) passes, it can be determined whether drag acceleration perturbations are due to mass density, wind, or a mixture of the two, helping constrain possible formation mechanisms. Key Points: Accelerometer observations of the Equatorial Thermosphere Anomaly (ETA) cannot distinguish between in‐track density and wind perturbationsA new technique is applied to observations determining that the ETA, occurring between 9 and 16 LT, is a density‐dominated featureThe ETA demonstrates repeatable annual behavior for each season, suggesting a common and persistent formation mechanism [ABSTRACT FROM AUTHOR]

The impact of geomagnetic storms on the mesosphere temperature has been controversial and lacks direct observational evidence. The intricate chemical and physical processes in the mesosphere, combined with the scarcity of observations, pose challenges to achieving a thorough comprehension of storm‐induced turbulences in this region. Currently, some investigations have characterized temperature responses during geomagnetic storms and the focus has largely been on changes above mesopause (∼90 km). In this work, the responses of temperature in the mesosphere (75–95 km) to the storms on September 8 and 15, 2003 and its causes are studied using observations from the Sounding of the Atmosphere using Broadband Emission Radiometry (SABER) instrument onboard the Thermosphere Ionosphere Mesosphere Energetics and Dynamics (TIMED) satellite. It is found that (a) temperature responses for the moderate storm on September 15 manifest as increases within the latitude range of 80°S to 65°S, with peak values decreasing from approximately 15 K to around 7 K as latitude decreases, while for the minor strom of September 8 temperature changes only occur at ∼80°S with peaks of 7 K and −10 K, (b) the temperature responses can be transmitted down to 76–81 km, depending on the latitude and storm magnitude, and (c) there are significant fluctuations in both ozone (exceed 45%) and atomic oxygen (exceed 90%) after storm onset, highly correlated with temperature temporal and spatial variations. We suggest that the increases in ozone caused by the increases in atomic oxygen concentrations are the major contributor to rising temperature. Plain Language Summary: Geomagnetic storms may have a non‐negligible effect on the atmospheric circulation, neutral temperature, winds, and composition of the mesosphere and lower thermosphere. However, there is a lack of understanding of the perturbations and its chemical and physical processes in the mesosphere during geomagnetic storms. In this paper, we analyze temperature, ozone, and atomic oxygen observations acquired from the SABER instrument aboard the TIMED satellite. Our work demonstrate that variations in mesosphere temperature occur at high latitudes and are correlated with the magnitude of the storm. Additionally, significant disturbances in ozone and atomic oxygen are observed from 80°S to 65°S latitude. We propose that the variations in mesosphere temperature at high latitudes are dominated to changes in ozone caused by the heightened concentrations of atomic oxygen. Key Points: Temperatures of the mesosphere observed by SABER have remarkable responses to the geomagnetic storms of 8 and 15 September 2003The mesosphere temperature variations occur at high latitudes and can be conveyed to 76–81 km, depending on the latitude and storm magnitudeOzone variations are responsible for the mesosphere temperature variations at high latitudes during storms [ABSTRACT FROM AUTHOR]

Much of the longitude/local time dependence of the thermosphere is controlled by non-migrating tides. Observations of semidiurnal (12-hr) tides between 120 and 200 km altitude, that is, the middle thermosphere, are rare owing to the lack of systematic measurements in this region. Since late 2018, the Global-scale Observations of the Limb and Disk (GOLD) Mission has provided unique measurements of thermospheric disk temperature and the column density ratio of atomic oxygen to molecular nitrogen ratio (ΣO/N2) from geostationary orbit. In this paper, we present an approach to deduce the strongest semidiurnal non-migrating tides in the middle thermosphere by adapting the method of Krier et al. (2021, https://doi.org/10.1029/2021ja029563) that deduces diurnal non-migrating tides in simultaneous observations of temperature and ΣO/N2 made by GOLD. Testing of this approach suggests that the principal sources of uncertainties in the derived semidiurnal non-migrating tides are the limitation on the longitudes sampled, such that uncertainties are higher for tides with longer horizontal wavelength, and contamination of the local time sums by stationary planetary waves, which causes amplitudes to be overestimated. Our approach is applied to GOLD data during solstice conditions in 2019-2021. Comparison to models yield disagreements which are likely due to uncertainties intrinsic to the method and/or misrepresentation of tidal dynamics in the models. These results are the first observations of semidiurnal non-migrating tides in the middle thermosphere from a geostationary observational platform. [ABSTRACT FROM AUTHOR]

The global‐scale observations of the limb and disk (GOLD) Mission images middle thermosphere temperature and the vertical column density ratio of oxygen to molecular nitrogen (ΣO/N2) using its far ultraviolet imaging spectrographs in geostationary orbit. Since GOLD only measures these quantities during daylight, and only over the ∼140° of longitude visible from geostationary orbit, previously developed tidal analysis techniques cannot be applied to the GOLD data set. This paper presents a novel approach that deduces two specified non‐migrating diurnal tides using simultaneous measurements of temperature and ΣO/N2. DE3 (diurnal eastward propagating wave 3) and DE2 (diurnal eastward propagating wave 2) during October 2018 and January 2020 are the focus of this paper. Sensitivity analyses using TIE‐GCM simulations reveal that our approach reliably retrieves the true phases, whereas a combination of residual contributions from secondary tides, the restriction in longitude, and random uncertainty can lead to ∼50% error in the retrieved amplitudes. Application of our approach to GOLD data during these time periods provides the first observations of non‐migrating diurnal tides in measurements taken from geostationary orbit. We identify discrepancies between GOLD observations and TIE‐GCM modeling. Retrieved tidal amplitudes from GOLD observations exceed their respective TIE‐GCM amplitudes by a factor of two in some cases. Plain Language Summary: The uppermost region of the Earth's atmosphere, known as the thermosphere (∼80–600 km altitude), is connected to the lowermost region by planetary‐scale atmospheric waves, called non‐migrating tides, which are thermally driven and do not follow the apparent motion of the Sun across the sky. Understanding non‐migrating tides is essential to describing the global dynamics of the Earth's upper atmosphere. There is a gap in observations of these waves in the middle thermosphere temperature, around 150 km altitude. The NASA/GOLD instrument, in geostationary orbit above the mouth of the Amazon River, images the temperature and composition of the middle thermosphere. Conventional tidal analysis techniques cannot be applied to the GOLD data set, so we have designed a novel technique that infers important tides using simultaneous measurements of temperature and composition. For two separate time periods, we apply our technique to simulated observations and actual GOLD data. We find that our technique generally infers the most important tides in simulated data with high accuracy. The GOLD data reveal valuable observations of tides in the middle thermosphere as well as discrepancies with the simulated data. Key Points: First estimates of non‐migrating diurnal tides from an observational platform in geostationary orbit using GOLDDeduction of non‐migrating tides via known phase relationships between temperature and compositionRetrieved tidal amplitudes from GOLD observations are generally stronger than their predicted model counterparts [ABSTRACT FROM AUTHOR]

With Doppler Interferometer instruments on thermosphere‐ionosphere‐mesosphere energetics and dynamics satellite (TIMED/TIDI) observations, the 1‐day resolution, non‐migrating DE3 tide in the zonal wind component can be extracted using the method from Li et al. (2015, https://doi.org/10.1002/2015JA021577). This paper presents the spatiotemporal variability of the DE3 tide in the zonal wind component between 85 and 105 km altitude and within ±30° latitude. The global structure and long‐term variation (intra‐annual and >1 year variability) are similar to the previous results. It is found that (a) it is dominant in the Southern Hemisphere between 95 and 105 km altitude and in the Northern Hemisphere between 90 and 100 km altitude; (b) the maximum amplitudes are in August and September around the equator, and the phases are stable and hardly vary with their latitudes; and (c) the influence of the quasi‐biennial oscillation (QBO) phenomenon can be noticeably detected. Additionally, the short‐term variability (after filtering out long‐term variability) can also be presented: (a) it increases with altitudes gradually between 85 and 105 km altitude, and the peak value is above 100 km; and (b) the short‐term variability does not demonstrate significant long‐term variation. Furthermore, through comparative analysis, we found that (a) the long‐term variations in DE3 in temperature and zonal wind are consistent with each other; however, the short‐term variabilities in DE3 are not similar. This indicates that the dominant dynamic processes of DE3 in the temperature and zonal wind components are different. (b) The symmetric DE3 tide in the zonal wind component is dominant at 0°–15° latitudes, and the asymmetric tide is dominant at 15°–30° latitudes. This indicates that the DE3 tide at different latitudes is dominated by wave modes with different symmetries. Key Points: Spatial distribution and temporal variation of the diurnal non‐migrating tide with zonal wavenumber 3 (DE3) in the zonal wind componentThe Quasi‐Biennial Oscillation (QBO) signals are observed in the long‐term variability of the DE3 tideShort‐term variability (after filtering out long‐term) of the DE3 tide in the zonal wind component is different from that in temperature [ABSTRACT FROM AUTHOR]

"Ultra‐fast" Kelvin waves (UFKWs) serve as a mechanism for coupling the tropical troposphere with the mesosphere, thermosphere and ionosphere. Herein, solutions to the linearized wave equations in a dissipative thermosphere in the form of "Hough Mode Extensions (HMEs)" are employed to better understand the vertical propagation of the subset of these waves that most effectively penetrate into the thermosphere above about 100 km altitude; namely, UFKWs with periods ≲4 days, vertical wavelengths (λz) ≳30 km, and zonal wavenumber s = −1. Molecular dissipation is found to broaden latitude structures of UFKWs with increasing height while their vertical wavelengths (λz) increase with latitude. Collisions with ions fixed to Earth's magnetic field ("ion drag") are found to dampen UFKW amplitudes, increasingly so as the densities of those ions increase with increased solar flux. The direct effect of ion drag is to decelerate the zonal wind. This leads to suppression of vertical velocity and the velocity divergence, and related terms in the continuity and thermal energy equations, respectively, that lead to diminished perturbation temperature and density responses. Access is provided to the UFKW HMEs analyzed here in tabular and graphical form, and potential uses for future scientific studies are noted. Plain Language Summary: In earth's atmosphere, Kelvin waves (KWs) are eastward‐propagating oscillations with periods of days to weeks that are centered on the equator and confined to low latitudes. They are forced by the spatial‐temporal variability of the heat of condensation ("latent heating") that is released when rising moist air forms rain droplets, mainly in the tropics. As with many atmospheric waves, they propagate vertically and grow exponentially with height in concert with the decrease in atmospheric density and pressure. The KWs that survive the trip from near the surface to about 100 km altitude are called "ultra‐fast" Kelvin waves, or "ultra‐fast" Kelvin waves (UFKWs). Just above 100 km, they reach maximum amplitudes where their exponential growth is curtailed by the viscosity of this part of the atmosphere. Here they interact with ionized particles (the ionosphere) and generate electric fields that ultimately drive ionospheric variability at higher altitudes (>200 km), thus presenting an element of "space weather" to navigation and communications systems. In this paper we model a set of UFKWs to better understand how their amplitudes, vertical and latitudinal structures change as they propagate above 100 km, and in so doing advance our knowledge of the physical processes underpinning near‐earth space weather. Key Points: "Ultra‐fast" Kelvin wave (UFKW) with periods less than about 4 days can effectively penetrate above 100 km altitudeDissipation broadens UFKW latitude structures with increasing height and lengthens vertical wavelengths with increasing latitudeIon drag dampens UFKW amplitudes with increasing efficiency at higher solar activity levels [ABSTRACT FROM AUTHOR]

As activity in Earth orbit continues to grow, it is important to characterize the environment of near‐Earth space. One means of remotely sensing lower thermospheric neutrals is by measurement of O and N2 density through the observation of far‐ultraviolet (FUV) airglow of atomic oxygen at 135.6 nm and the N2 Lyman‐Birge‐Hopfield (LBH) bands (~130–180 nm), as has been done on the Ionospheric Connection Explorer (ICON), Global‐scale Observations of the Limb and Disk (GOLD), and Thermosphere Ionosphere Mesosphere Energetics and Dynamics (TIMED) missions. This technique is not without limitations, however, as the FUV measurements suffer from contamination by ionospheric emissions at low latitudes and auroral emissions excited by precipitating energetic electrons and protons at high latitudes. Previous work has shown the potential for making measurements of O and N2 density in the lower‐middle thermosphere using observations of extreme‐ultraviolet (EUV) airglow. This measurement approach has a potential advantage in that it does not have an inherent ionospheric emission that must be accounted for. Additionally, these emissions are primarily excited directly by solar UV rather than electron impact and thus have the potential to enable expansion of neutral density observations into the auroral zone and polar cap where the FUV measurement cannot be applied. This article demonstrates a new approach and algorithm designed to retrieve thermospheric O and N2 density from 150 to 400 km using measurements from the ICON EUV instrument. The retrieval results throughout 2020 are summarized and compared to measurements from ICON FUV, GOLD, and SWARM. Plain Language Summary: The region of space where most satellites orbit is almost empty ‐ but not completely empty. The small amount of nitrogen and oxygen gas present can cause satellites to gradually fall out of orbit through drag. A failure to understand and evaluate changes in the density of the upper atmosphere led to the loss of 38 Starlink satellites in February 2022, for example, This is one reason that it is important to understand how much gas is in near‐Earth space and how the amount changes over time, but orbit decay due to drag makes it difficult to make direct measurements. One common solution is to measure the amount of gas from afar, taking advantage of the fact that these gases emit small amounts of light in the far‐UV spectrum. There are some limitations to this approach, one of which being that charged particles in the same region can create an extra and unrelated signal, which requires correction. This paper presents a novel approach which uses more energetic extreme‐UV light as an alternative to the far‐UV. We show the results of this new algorithm and demonstrate that the measurements line up well with those from other instruments and spacecraft. Key Points: We present an algorithm concept for remote sensing thermospheric O and N2, novel in its use of EUV airglow measurementsComparison to related measurements during conjunctions with ICON FUV, GOLD, and Swarm are favorableRetrieval results from 2020 indicate cooler atmospheres than those predicted by MSIS [ABSTRACT FROM AUTHOR]

STEVE, Strong Thermal Emission Velocity Enhancement, was often observed by ground‐based imagers in visible wavelengths and rarely detected by global FUV imagers. We present a new event, and revisit two reported STEVE events, that were observed by the DMSP/SSUSI FUV imager at O 135.6 nm, N2 LBHS, and LBHL emissions. Coincident particle and plasma drift observations showed that the events were associated with high ion drift speed, low ion density and no energetic particle precipitation. This is consistent with earlier findings (e.g., MacDonald et al., 2018, https://doi.org/10.1126/sciadv.aaq0030; Gallardo‐Lacourt et al., 2018a, https://doi.org/10.1029/2018ja025368, 2018b, https://doi.org/10.1029/2018gl078509; Archer et al., 2019). In this paper, several new features are identified: (a) Detection rates of FUV STEVE are much lower than that of visible STEVE; (b) The STEVE on 27 March 2008 covers 3‐hr local time with a length up to 2,700 km around 60° magnetic latitudes; (c) Different widths of STEVE observed in O 135.6 nm and N2 LBH images indicate a large altitude range in the STEVE FUV emissions; (d) The true ion (assuming O+) drift speeds during the 27 March 2008 STEVE event could be up to 20 km/s, well above the DMSP SSIES sensor limit. The kinetic energy of the high speed ions is larger than the excitation potential of the observed FUV emissions; (e) The requirement of such extreme high ion drift speed explains why FUV STEVE have been rarely observed, compared to the visible STEVE; (f) The three FUV STEVE events occurred during moderate geomagnetic activity; (g) Theses events were conjugate in both hemispheres. Plain Language Summary: STEVE, Strong Thermal Emission Velocity Enhancement, was discovered by ground‐based imagers in visible wavelengths. We present examples of STEVE detected by FUV imager at O 135.6 nm, N2 LBHS (140–150 nm), and LBHL (165–180 nm) emissions. These FUV STEVE events were associated with high plasma drift speed and low plasma density in the ionosphere, and no energetic particle precipitation. These FUV STEVE events show several new features: (a) Detection rates of FUV STEVE are much lower than that of visible STEVE; (b) The STEVE length is up to 2,700 km; (c) There is a large altitude range of the FUV emission; (d) The true ion (assuming O+) drift speeds could be up to 20 km/s. Ions with such a high drift speed would carry sufficient kinetic energy to excite FUV emissions; (e) The FUV STEVE events occurred during moderate geomagnetic activity; and (f) The FUV STEVE events were conjugate in both hemispheres. Key Points: In addition to the visible emissions, STEVE is also observed in far ultraviolet emissionsThe STEVE on 27 March 2008 has a horizontal length up to 2,700 km and a large altitude extensionEstimated O+ ion drift speed for some STEVE events is ∼20 km/s and the ions' kinetic energy is greater than the FUV excitation potential [ABSTRACT FROM AUTHOR]

This paper quantifies and interprets how thermosphere dissipation in the form of molecular viscosity, molecular thermal conductivity and collisions with ions ("ion drag") determines the height versus latitude structures of a variety of migrating and non‐migrating solar thermal tides, pole‐to‐pole up to 400 km altitude. This is done through computation of thermosphere Hough Mode Extensions (HMEs); that is, solutions to the linearized momentum, thermal energy, continuity, state and hydrostatic balance equations wherein the horizontal structure of troposphere forcing for each HME corresponds to the eigenfunction (Hough function) of Laplace's tidal equation for a particular tidal mode, and the background state and thermosphere dissipation are specified for nominal solar minimum, average and maximum activity conditions. The broad features revealed, different for each HME, include changes in vertical(horizontal) structure with latitude(height), degree of vertical penetration to the middle and upper thermosphere, changes in vertical wavelength (λz) with height due to the transition in background thermal gradient around the mesopause, and insights into role of ion drag in determining tidal amplitudes and their solar cycle variability. The HMEs are particularly useful for fitting measurements of tides, and for estimating tidal fields beyond those dependent variables actually fit and including those outside the fitting domain. The HMEs reported here were created as part of the Ionospheric CONnection (ICON) mission to serve as observation‐based lower boundary conditions for Thermosphere Ionosphere Mesosphere General Circulation Model‐ICON. Access to the full set of HMEs in tabular and graphical form is provided. Key Points: Dissipation broadens amplitude(phase) latitudinal(vertical) structures with increasing height(latitude)Vertical wavelength shortening occurs with the shift to positive background thermal gradient above the mesopauseMolecular dissipation and ion drag result in decreased amplitudes with increasing solar activity at 400 km [ABSTRACT FROM AUTHOR]

This paper reports the climatology of medium‐scale traveling ionospheric disturbances (MSTIDs) observed between September 2018 and August 2019 in the nighttime redline airglow intensity measurements from an all‐sky imager located in Sutherland, South Africa (32.4°S, 20.8°E; magnetic latitude: 40.7°S). The nighttime MSTIDs appeared as either single or multiple dark bands, commenced mostly before midnight, and predominantly occurred during the southern winter solstice. They primarily propagated westward with speeds of 31–127 m/s. The periods and wavelengths of the multi‐band MSTIDs were in the ranges of 26–155 min and 134–578 km, respectively, while the temporal and spatial scales of the single band MSTIDs were 11–30 min and 43–143 km, respectively. Analysis of neutral wind measurements from a co‐located Fabry‐Perot interferometer showed that the thermospheric wind was mostly northeastward in the evenings of MSTID occurrence and that the favored westward propagation of the MSTIDs was the least restricted propagation direction by the background wind. Based on data from nearby ionosondes, the MSTIDs were mostly accompanied by sporadic E and spread‐F structures. From the seasonal occurrences of MSTIDs and sporadic E as well as the MSTID propagation and prevalent wind directions, it is concluded that Perkins instability is the most likely source of the majority of the observed nighttime MSTIDs, while the remainder may be seeded by gravity waves. Key Points: Properties of dark band structures representing nighttime medium‐scale traveling ionospheric disturbances (MSTIDs) observed over South Africa presented for the first timeMSTIDs prevalent in southern hemisphere winter and primarily propagated westwardOccurrence of MSTIDs is mostly accompanied by sporadic and spread‐F, while propagation is supported by favorable neutral wind circulation [ABSTRACT FROM AUTHOR]

Here we compare the global‐scale morphology of Earth's the Far‐Ultraviolet (FUV) emissions observed by NASA's Global‐scale Observations of Limb and Disk (GOLD) mission to those modeled using the Global Airglow (GLOW) code with atmospheric parameters provided by Thermosphere‐Ionosphere‐Electrodynamics General Circulation Model (TIEGCM). The O 5S oxygen (135.6 nm) and N2 Lyman‐Birge‐Hopfield (LBH) emissions are observed over the Western hemisphere every 30 min by the GOLD instrument. The FUV brightness of the thermosphere‐ionosphere is expected to vary in systemic ways with respect to geophysical parameters, solar energy input from above, and terrestrial weather input from below. In this paper we examine the O 5S oxygen emission and the N2 LBH emission brightnesses with local time, latitude, season, tides, geomagnetic activity, and solar activity based on GOLD observations and GLOW modeling. Early GOLD observations indicate that the model effectively reproduces the brightness variations with local time and latitude but is biased low in magnitude. However, the TIEGCM is unable to accurately represent the extraordinary nighttime equatorial ionization anomaly observed by GOLD. It is also expected from these results that the signal from geomagnetic storms may obscure tidal signals. Plain Language Summary: In January 2018, a FUV instrument, the Global‐scale Observations of Limb and Disk (GOLD), was launched as a NASA mission of opportunity aboard a geostationary satellite and scientific operations began in October of 2018. GOLD views the entire disk of the Earth every 30 min, allowing the separation of geophysical variability in both space and time. This allows an unprecedented test of global models. Here, we model the Earth's upper atmosphere using the Thermosphere‐Ionosphere‐Electrodynamics General Circulation Model (TIEGCM), which produces temperature and the densities of the major chemical components of the neutral atmosphere, ionized plasma densities, and neutral air temperatures. These atmospheric temperatures and densities are used to calculate the Earth's upper atmosphere emissions in the Far‐Ultraviolet (FUV) with the Global Airglow (GLOW) model. The calculated emission rates are then used to compute the expected FUV signals as observed from geostationary orbit. In this paper, we show how the FUV brightness varies with local time, season, atmospheric activity, and solar activity. Of particular interest is that the GLOW model excels at producing FUV brightnesses that match the local time and latitude trends of the daytime observations; however, the model fails to adequately portray the extraordinary nighttime phenomena observed by GOLD. Key Points: The Global‐scale Observations of Limb and Disk (GOLD) instrument observes the Earth in the Far‐Ultraviolet (FUV) from a geostationary orbitGLOW adequately calculates the FUV brightness for various local times, latitudes, solar forcing, and lower atmospheric forcingGLOW calculations indicate the oxygen brightness varies by ±45 R and the effective temperature varies by more than 80 K due to tides [ABSTRACT FROM AUTHOR]

Ionospheric day‐to‐day variability is ubiquitous, even under undisturbed geomagnetic and solar conditions. In this paper, quiet‐time day‐to‐day variability of equatorial vertical E × B drift is investigated using observations from ROCSAT‐1 satellite and the Whole Atmosphere Community Climate Model with thermosphere and ionosphere eXtension (WACCM‐X) v2.1 simulations. Both observations and model simulations illustrate that the day‐to‐day variability reaches the maximum at dawn, and the variability of dawn drift is largest around June solstice at ~90–180°W. However, there are significant challenges to reproduce the observed magnitude of the variability and the longitude distributions at other seasons. Using a standalone electro‐dynamo model, we find that the day‐to‐day variability of neutral winds in the E‐region (≤~130 km) is the primary driver of the day‐to‐day variability of dawn drift. Ionospheric conductivity modulates the drift variability responses to the E‐region wind variability, thereby determining its strength as well as its seasonal and longitudinal variations. Further, the day‐to‐day variability of dawn drift induced by individual tidal components of winds in June are examined: DW1, SW2, D0, and SW1 are the most important contributors. Plain Language Summary: The ionosphere is different from one day to the next, even under geomagnetic and solar quiet condition. The vertical E × B drift at the geomagnetic equator is a key parameter that influences the state of the ionosphere and atmosphere. In this paper, we study the quiet‐time day‐to‐day variability of the equatorial vertical E × B drift by ROCSAT‐1 observations and the Whole Atmosphere Community Climate Model with thermosphere and ionosphere eXtension (WACCM‐X) v2.1 simulations. Both observations and WACCM‐X show that day‐to‐day variability is large at dawn and dusk and it also changes with season and longitude. To better study the variability, we carried out numerical experiments with a new standalone electro‐dynamo model. We found that wind variability below ~130 km is the main contributor, and winds above ~130 km plays a secondary role. The ionospheric conductivity mediates the drift variability response to the wind variability and thus affects its strength and seasonal and longitudinal variations. Further, we examine the variability of dawn drifts generated by different tidal components in June and find that DW1, SW2, D0, and SW1 are the most important ones. This work emphasizes the importance of lower atmospheric variability in studying and predicting the day‐to‐day variations of ionosphere and space environment. Key Points: We quantify the quiet‐time day‐to‐day variability of equatorial vertical drift as a function of local time, longitude, and seasonBoth observations and models show the largest variability occurring at ~ LT 06, which is mainly driven by the variability of E‐region windsThe variability of dawn drift induced by tidal components are examined, with DW1, SW2, D0, and SW1 being the most important contributors [ABSTRACT FROM AUTHOR]

Numerical simulations using the National Center for Atmospheric Research (NCAR) thermosphere‐ionosphere‐electrodynamics general circulation model (TIE‐GCM) are performed to elucidate the effects of the interplanetary magnetic field (IMF) on the middle thermosphere composition during a "geomagnetically quiet" period from the day of year (DOY) 110–111 in 2019 (when the Auroral electrojet (AE) index never exceeded 300 nT and the Kp never exceeded 2). In particular, this paper aims to examine how the Global‐scale Observations of the Limb and Disk (GOLD) mission observed daytime thermospheric O and N2 column density ratio (∑O/N2) depletion at mid‐latitudes originated under such a "geomagnetically quiet" condition. A comparison of electric potential, Joule heating rate per unit mass, ion velocity, neutral temperature and winds in the middle thermosphere (∼160 km) between real IMF and without the IMF east‐west component (By) indicates that a By dominant condition can enhance their strengths under this "geomagnetically quiet" condition. Consequently, ∑O/N2 depletion with a stronger magnitude (30% compared with ∼8% without By) and larger disturbed area was introduced in the post‐midnight sector at high‐latitudes due to strong and localized upwelling associated with the enhanced Joule heating rate per unit mass. The ∑O/N2 depletion was transported equatorward and corotated from local post‐midnight to early morning, and was observed by GOLD at middle latitudes during daytime. Plain Language Summary: This study utilizes numerical simulations with a climatological tide in the lower boundary to investigate how thermospheric composition disturbance is produced and transported to mid‐latitudes during a geomagnetically quiet period (DOY110‐112, 2019, maximum Kp = 1.7, AE < 300 nT). Through comparing parameters from simulations under real IMF and By = 0, it is found that the IMF By, which was dominant over this period, can significantly enhance the ionospheric potential, Joule heating rate per unit mass, ion drift and neutral winds compared with Bz only condition. As a result, the geomagnetic activity with AE < 300 nT can trigger stronger depletion (30%) of the column density ratio of thermospheric O and N2 (∑O/N2) in the polar region. Then the accelerated meridional wind transports the ∑O/N2 depletion equatorward to mid‐latitudes. The depletion is also transported from local post‐midnight to local early morning. This study shows the crucial role of dominant By during "quiet time". Additionally, it suggests that the response of ∑O/N2 to geomagnetic activity during the "quiet period" is similar to the one during geomagnetic storms. Key Points: IMF with dominant By enhances ion convection pattern, Joule heating rate and neutral windsStrong thermospheric composition disturbance can be generated and propagate into mid‐latitudes with dominant By during "quiet" timeResponse of thermospheric composition to geomagnetic activity with AE < 300 nT is similar to its response to geomagnetic storms [ABSTRACT FROM AUTHOR]

We have measured auroral zone thermospheric neutral winds in the midnight local time sector, using ground‐based optical Doppler spectroscopy of the 630.0 nm emission from atomic oxygen, originating at around 240 km altitude over Alaska. One of the most prominent features seen in winds at these latitudes is the cross‐polar jet emerging from the polar cap at local times around magnetic midnight. The standard view is that wind flows anti‐sunward in the midnight sector and spills equatorward over magnetic latitudes extending well below those of the auroral zone. The purpose of this paper is to show that this view is too simplistic. From our observatory at Poker Flat, Alaska (∼65.12oN), the anti‐sunward flow is frequently seen to stall over surprisingly short horizontal distances (100–200 km), without spilling further equatorward. This behavior is most prevalent during a low solar activity at mid‐winter when the combination of pressure gradient established by solar heating and the ion drag is not enough to allow the jet to push through the background atmosphere on the nightside. At higher latitudes, by contrast, the flow is relatively uniformly anti‐sunward around magnetic midnight even during quiet conditions. During periods of high solar and magnetic activity, the expected spilling of the midnight sector cross‐polar jet to lower latitudes often is indeed observed over Alaska. Our observation of abrupt stalling during quiet solar and geomagnetic conditions is a very significant difference from the model predictions, with potentially important ramifications‐ which is the motivation for the present study. Plain Language Summary: We have analyzed ground‐based remote sensing measurements of thermospheric neutral winds above Alaska, at 240 km altitude. In particular, we have studied how space weather affects the well‐known large‐scale flow that carries winds from the sunlit dayside of the Earth across the polar cap into the night side. This powerful flow feature is typically expected to emerge from the polar cap in the midnight sector and continue blowing equatorward well into sub‐auroral latitudes. However, our data show instances in which the equatorward flow instead stalls over Alaska in an unexpectedly abrupt manner. We have explored the conditions under which this abrupt stalling occurs, and commented on the consequences that this behavior might have on technological systems that depend on accurate models of thermospheric wind fields. Key Points: Localized stalling of the thermospheric cross‐polar jet was observed equatorward of the auroral zoneStalling occurs over short meridional distances of ∼100–200 kmAbrupt stalling was found to occur more frequently during quiet solar and geomagnetic conditions [ABSTRACT FROM AUTHOR]

Accurate representation of ensemble members of coupled Ionosphere‐Thermosphere (I‐T) is crucial to the ionosphere forecast in the ensemble Kalman filter (EnKF) data assimilation system. In this paper, besides the model drivers used in previous studies, including the solar 10.7 cm radio flux (F10.7), auroral hemispheric power (HP) and cross‐tail potential drop (CP), the eddy diffusion coefficient (ED), and nighttime vertical O+ flux at the top boundary are also perturbed to generate ensemble members. Based on our developed EnKF data assimilation system, the impact of perturbing different model external forcing parameters on ensemble generation and forecast capability of ionosphere and thermosphere has been investigated in detail through a series of sensitivity tests and data assimilation experiments. This system uses the National Center for Atmospheric Research (NCAR) Thermosphere Ionosphere Electrodynamics General Circulation Model (TIEGCM) as a background model. The findings are summarized as follows: (a) Associated with perturbing two additional model forcing parameters, better ensemble members of ionosphere and thermosphere states of the background model can be generated during both daytime and nighttime. (b) The improvement of the forecast capability of the ionosphere and thermosphere variables can be further enhanced. This study can provide a reference for ensemble generating strategy for the coupled I‐T EnKF data assimilation in the future. Plain Language Summary: The ensemble member of EnKF data assimilation considering as much as possible variabilities existed in the coupled I‐T system should represent model uncertainties better. In the previous researches on EnKF ionosphere forecast, ensemble members are mainly generated by perturbing solar and geomagnetic activity index common model drivers (e.g., F10.7, KP). However, solely perturbing those indices is not sufficient to distribute the ionosphere and thermosphere states at all local times. In this study, in addition to perturbing those indices, the key model forcing parameters of the lower and upper boundary of the TIEGCM are also perturbed to generate the ensemble member of the coupled I‐T system. We found that an improved ensemble spread can be obtained at all local times in this way, which can further contribute to the forecast capability of the ionosphere and thermosphere states. Key Points: The eddy diffusion and upper boundary are additionally perturbed to generate ensembles in EnKF I‐T assimilation systemEnsemble members can be better obtained at no matter daytime or nighttimeThe forecasting capability of I‐T states can be further improved [ABSTRACT FROM AUTHOR]

We describe a long‐term data set of global average thermospheric mass density derived from orbit data on ∼7,700 objects in low Earth orbit, via the effect of atmospheric drag. The data cover the years 1967–2019 and altitudes 250–575 km, and the temporal resolution is 3–4 days for most years. The data set is an extension and revision of a previous version. The most important change is the use of more precise orbit data: special perturbation state vectors are now used starting in 2001, instead of mean Keplerian orbital elements. The data are suitable for climatological studies of thermospheric variations and trends, and for space weather studies on time scales longer than 3 days. Plain Language Summary: The region of Earth's atmosphere between about 90 and 500 km altitude, known as the thermosphere, exerts drag on the thousands of satellites and debris objects that orbit in this region. It is therefore important to understand and predict thermospheric variations in order to predict the future positions of these objects, for example, for collision avoidance and reentry prediction purposes. For the past five decades, the orbital trajectories of many of these objects have been routinely calculated from ground‐based radar measurements. The orbits change in large part due to atmospheric drag, and this effect can be used to infer the density of the part of the atmosphere traversed by an object. This paper describes a long‐term data set generated by applying the method to ∼7,700 objects to accurately estimate the global average atmospheric density as a function of time and altitude. The density data set covers the years 1967–2019 and altitudes 250–575 km, and it is suitable for climatological studies of thermospheric variations and trends, and for space weather studies on time scales longer than 3 days. The data set is a revision and extension of a previous version; the new version ingests more precise orbital trajectory data. Key Points: Extension and revision of orbit‐derived thermospheric mass density data setData is suitable for thermospheric climate studies and space weather on time scales longer than 3 daysNew technique for deriving density from orbital state vectors [ABSTRACT FROM AUTHOR]

A method for retrieving temperature and composition from 150 to 350 km in Earth's thermosphere using total number density measurements made via extreme ultraviolet (EUV) solar occultations by the Project for OnBoard Autonomy 2/Large Yield Radiometer (PROBA2/LYRA) instrument is presented. Systematic and random uncertainties are calculated and found to be less than 5% for the temperature measurements and 5%–20% for the composition measurements. Regression coefficients relating both temperature and the [O]/[N2] abundance ratio with EUV irradiance at 150, 275, and 350 km are reported. Additionally, it is shown that the altitude where [O] equals [N2] decreases with increasing solar EUV irradiance, an effect attributed to thermal expansion. Temperatures from 2010 to 2017 are compared with estimates from the MSIS empirical model and show good agreement at the dawn terminator but LYRA is markedly cooler at the dusk terminator, with the MSIS‐LYRA temperature difference increasing with solar activity. Anthropogenic cooling can explain this discrepancy at periods of lower solar activity, but the divergence of temperature with increasing solar activity remains unexplained. LYRA measurements of the exospheric temperature sensitivity to EUV irradiance are compared with contemporaneous measurements made at Mars, showing that the exospheric temperature at Mars is approximately half as sensitive to EUV variability as that of Earth. Plain Language Summary: The Large Yield Radiometer (LYRA) instrument onboard the Project for OnBoard Autonomy 2 (PROBA2) satellite measures the structure of the upper atmosphere by tracking the absorption of sunlight as it passes through the atmosphere as the Sun sets or rises as viewed from the satellite. This paper presents a method for determining atmospheric temperature to a high degree of accuracy from 150 to 350 km from the PROBA2/LYRA measurements. Additionally, a method for determining the relative abundance of atomic Oxygen and molecular Nitrogen is presented. Both temperature and relative abundance are known to change with changing extreme ultraviolet (EUV) radiation from the Sun. The dependence of these values on solar EUV radiation is reported, and is shown to be different from predictions of the MSIS model, frequently used to estimate atmospheric temperature and composition. Additionally, these results for Earth are compared with similar measurements made at Mars and show that the Earth's atmosphere heats twice as efficiently from solar EUV radiation as the Mars atmosphere. Key Points: Temperature and composition from 150 to 350 km from 2010 to 2017 are presentedDusk temperatures are markedly cooler than MSIS predictionsMars exospheric temperature is half as sensitive to extreme ultraviolet variability as that of Earth [ABSTRACT FROM AUTHOR]

Recent studies have reported that thermospheric effects due to solar wind driving can be observed poleward of auroral latitudes. In these papers, the measured neutral mass density perturbations appear as narrow, localized maxima in the cusp and polar cap. They conclude that Joule heating below the spacecraft is the cause of the mass density increases, which are sometimes associated with local field-aligned current structures, but not always. In this paper we investigate neutral mass densities measured by accelerometers on the CHAllenging Minisatellite Payload (CHAMP) and Gravity Recovery and Climate Experiment (GRACE) spacecraft from launch until years 2010 (CHAMP) and 2012 (GRACE), approximately 10 years of observations from each satellite. We extract local maxima in neutral mass densities over the background using a smoothing window with size of one quarter of the orbit. The maxima have been analyzed for each year and also for the duration of each set of satellite observations. We show where they occur, under what solar wind conditions, and their relation to magnetic activity. The region with the highest frequency of occurrence coincides approximately with the cusp and mantle, with little direct evidence of an auroral zone source. Our conclusions agree with the 'hot polar cap' observations that have been reported and studied in the past. [ABSTRACT FROM AUTHOR]

Strong zonal flows greater than 1,000 m/s have recently been identified by Swarm in the midnight sector of the winter hemisphere auroral zone. These flows are typically observed between upward and downward field aligned current pairs and therefore are referred to as Birkeland current boundary flows (BCBFs). The Swarm observations also identify ion heating and upflows associated with the fast zonal flows. BCBFs and the resulting ion heating and upflows play an important role in thermosphere-ionosphere-magnetosphere coupling given their high occurrence rate in the quiet time nightside auroral zone. This paper utilizes Super Dual Auroral Radar Network (SuperDARN) highresolution fits to line-of-sight data to investigate two known BCBF events. The large (>2,000 m/s) BCBFs observed with Swarm A and B are found to be collocated with the location of field line resonances (FLRs) with periods of approximately 20–30 min. The SuperDARN high-resolution fits determine FLR peak flow velocities of 400–700 m/s. When the FLR velocities are superimposed upon the background ionospheric convection of 400–600 m/s, this yields total east-west velocities of 800–1,300 m/s near the time and location of the BCBF events. The peak flows of the field line resonances are shown to be bounded by pairs of upward and downward field aligned current similar to the BCBFs. The observations suggest a causal relationship between BCBFs and FLRs. [ABSTRACT FROM AUTHOR]

This paper reports, for the "first time," the delayed response of O¹D 630.0 nm dayglow emissions over Trivandrum (8.5°N, 77°E, 0.5°N dip lat.), a geomagnetic dip equatorial station in India, to the noontime X-class solar flare event of July 30, 2005. The dayglow measurements were made using a unique dayglow photometer operating at three wavelengths. The Equatorial Electrojet induced magnetic field, measured using a proton precession magnetometer, showed a magnetic spike having ~90 nT enhancement during this flare with a time delay of ~7.2 min. A noteworthy observation is that unlike to the conventional belief, the O¹D 630.0 nm dayglow over the dip equator exhibited a fourfold enhancement during the noontime flare after a time delay of ~45 min. Analysis of satellite measured electron density and modeling simulations indicate that the thermospheric O¹D 630.0 nm dayglow emission over the dip equatorial region during a solar flare is primarily driven by the electrodynamics, rather than the direct solar control. This finding is new, unique and very important for the studies related to plasma-neutral coupling and also for modeling studies on the equatorial thermosphere-ionosphere region is concerned. [ABSTRACT FROM AUTHOR]

Here we test the precursory enhancement in ionospheric total electron content (TEC) which has been reported by Heki (2011, https://doi.org/10.1029/2011GL047908) and numerous Global Navigation Satellite System (GNSS) TEC observational studies before the 2011 Mw9.0 Tohoku‐Oki and many great earthquakes. We verify the frequency of this TEC enhancement via analysis of a 2‐month vertical TEC (VTEC) time series that includes the Tohoku‐Oki earthquake using the procedure, based on Akaike's information criterion, and threshold of Heki and Enomoto (2015, https://doi.org/10.1002/2015JA021353). The averaged occurrence rate of the TEC enhancement is much larger than that reported by Heki and Enomoto (2015, https://doi.org/10.1002/2015JA021353) when all of the visible GPS satellites at a given station are taken into account. The frequency assessment by Heki and Enomoto (2015, https://doi.org/10.1002/2015JA021353) using only one satellite underestimates the frequency of the TEC enhancement. In fact, the TEC enhancement is sufficiently frequent to explain all the reported enhancements prior to the great earthquakes as a product of chance. We also analyze the spatial distribution of the preseismic TEC enhancement and coseismic TEC depletion for the Tohoku‐Oki earthquake with the data after reducing intertrace biases. We observe significant postseismic depletion that lasted at least 2 h after the earthquake and extended at least 500 km around the center of the large‐slip area. This means that evaluation of the enhancements using reference curves which was adopted by Heki (2011, https://doi.org/10.1029/2011GL047908) and even by the recent papers (e.g., He & Heki 2016, https://doi.org/10.1002/2016GL069863; 2017, https://doi.org/10.1002/2017JA024012; 2018, https://doi.org/10.1029/2017JA024871) is in danger of mistaking a large and long‐lasting postseismic TEC depletion for a preseismic enhancement. Key Points: Ionospheric TEC enhancement occurs so often without earthquakes, which can explain the "precursors" as just a product of chanceA large TEC depletion spreading 500 km in diameter and lasting at least 120 min after the Tohoku‐Oki earthquakeExcluding window is in danger of mistaking the postseismic depletion for a preseismic enhancement [ABSTRACT FROM AUTHOR]

We present long‐term density trends of the Earth's upper atmosphere at altitudes between 71 and 116 km, based on atmospheric occultations of the Crab Nebula observed with X‐ray astronomy satellites, ASCA, RXTE, Suzaku, NuSTAR, and Hitomi. The combination of the five satellites provides a time period of 28 years from 1994 to 2022. To suppress seasonal and latitudinal variations, we concentrate on the data taken in autumn (49 < doy < 111) and spring (235 < doy < 297) in the northern hemisphere with latitudes of 0°–40°. With this constraint, local times are automatically limited either around noon or midnight. We obtain four sets (two seasons × two local times) of density trends at each altitude layer. We take into account variations due to a linear trend and the 11‐year solar cycle using linear regression techniques. Because we do not see significant differences among the four trends, we combine them to provide a single vertical profile of trend slopes. We find a negative density trend of roughly −5%/decade at every altitude. This is in reasonable agreement with inferences from settling rate of the upper atmosphere. In the 100–110‐km altitude, we found an exceptionally high density decline of about −12%/decade. This peak may be the first observational evidence for strong cooling due to water vapor and ozone near 110 km, which was first identified in a numerical simulation by Akmaev et al. (2006, https://doi.org/10.1016/j.jastp.2006.03.008). Further observations and numerical simulations with suitable input parameters are needed to establish this feature. Plain Language Summary: Numerical simulations have shown that, while an increase of greenhouse gases such as CO2 in the atmosphere causes heating of the troposphere (near surface), it causes cooling of the middle and upper atmosphere, which is the so‐called "greenhouse cooling." The greenhouse cooling should result in atmospheric contraction and consequently a temporal density decrease at a fixed height. However, observational evidence for the density decrease has been scarce in the mesosphere and lower thermosphere (MLT: 80–110 km), owing to difficulty in measuring the density in this region. Here, we present the first direct measurements of long‐term variations for combined N and O atom number density in the MLT, based on atmospheric occultations of the Crab Nebula observed with X‐ray astronomy satellites. The combination of five X‐ray astronomy satellites, ASCA, RXTE, Suzaku, NuSTAR, and Hitomi, allows us to explore density trends for a long period from 1994 to 2022. We take into account variations due to a temporal linear trend and the 11‐year solar cycle, using linear regression techniques. As a result, we find a negative density trend of roughly −5%/decade at every altitude, with a local minimum of −12%/decade near 105 km. This is in reasonable agreement with the state‐of‐the‐art numerical simulations. Key Points: Time series of combined O and N densities are measured in the MLT, based on atmospheric occultations of the Crab Nebula using X‐ray astronomy satellitesThe density is decreasing everywhere, with a local minimum of −12%/decade near 105 kmThe local minimum in density trends may be due to strong cooling by water vapor and ozone, as was first predicted by Akmaev et al. (2006, https://doi.org/10.1016/j.jastp.2006.03.008) [ABSTRACT FROM AUTHOR]

Optical measurements from three selected wavelengths have been combined with modeling of emissions from an auroral event to estimate the magnitude and direction of small‐scale electric fields on either side of an auroral arc. The temporal resolution of the estimates is 0.1 s, which is much higher resolution than measurements from Super Dual Auroral Radar Network (SuperDARN) in the same region, with which we compare our estimates. Additionally, we have used the Scanning Doppler Imager instrument to measure the neutral wind during the event in order to calculate the height integrated Joule heating. Joule heating obtained from the small scale electric fields gives larger values (17 ± 11 and 6 ± 9 mWm−2 on average on each side of the arc) than the Joule heating obtained from more conventionally used SuperDARN data (5 mWm−2). This result is significant, because Joule heating will cause changes in the thermosphere from thermal expansion and thermal conductivity, and may also affect the acceleration of the neutral wind. Our result indicates that high spatial and temporal resolution electric fields may play an important role in the dynamics of the magnetosphere‐ionosphere‐thermosphere system. Key Points: Small scale electric fields are estimated on both sides of an auroral arc using optical techniquesJoule heating is estimated, by combining optical data with neutral wind observationsLocal, high resolution Joule heating shows much higher values than the Joule heating obtained from more averaged radar measurements [ABSTRACT FROM AUTHOR]

This paper examines the response of the upper atmosphere to equatorial Kelvin waves with a period of ∼3 days, also known as ultrafast Kelvin waves (UFKWs). The whole atmosphere model Ground‐to‐topside model of Atmosphere and Ionosphere for Aeronomy (GAIA) is used to simulate the UFKW events in the late summer of 2010 and 2011 as well as in the boreal winter of 2012/2013. When the lower layers of the model below 30‐km altitude are constrained with meteorological data, GAIA is able to reproduce salient features of the UFKW in the mesosphere and lower thermosphere as observed by the Aura Microwave Limb Sounder. The model also reproduces ionospheric response, as validated through comparisons with total electron content data from the Gravity field and steady‐state Ocean Circulation Explorer satellite as well as with earlier observations. Model results suggest that the UFKW produces eastward‐propagating ∼3‐day variations with zonal wavenumber 1 in the equatorial zonal electric field and F region plasma density. Model results also suggest that for a ground observer, identifying ionospheric signatures of the UFKW is a challenge because of ∼3‐day variations due to other sources. This issue can be overcome by combining ground‐based measurements from different longitudes. As a demonstration, we analyze ground‐based magnetometer data from equatorial stations during the 2011 event. It is shown that wavelet spectra of the magnetic data at different longitudes are only in partial agreement, with or without a ∼3‐day peak, but a spectrum analysis based on multipoint observations reveals the presence of the UFKW. Key Points: GAIA simulations are performed for ultrafast Kelvin wave (UFKW) in the middle and upper atmosphereUFKW produces eastward‐propagating ∼3‐day ionospheric variations with zonal wavenumber 1First magnetic detection of UFKW in the equatorial electrojet is also presented [ABSTRACT FROM AUTHOR]

The Global‐scale Observations of the Limb and Disk (GOLD) instrument was launched on 25 January 2018 onboard the SES‐14 commercial communications satellite and began nominal science operations in October 2018. Operating from geostationary orbit at 47.5°W longitude, GOLD images the Earth's thermosphere and ionosphere in the far‐ultraviolet (132–162 nm), measuring critical geophysical parameters by continuously scanning the Earth's disk and limb 18 hours per day. GOLD also performs stellar occultation measurements using bright type O and B stars. Up to 10 occultations per day are obtained at latitudes ranging from 60°S to 45°N and two fixed longitudes, ~33°E and ~128°W. The occultation data provide a direct measurement of atmospheric absorption in the O2 Schumann Runge continuum, which is used to retrieve the O2 density profile from 130‐ to 200‐km altitude. This paper describes the GOLD occultation measurement technique and the operational retrieval algorithm used to derive the Level 2 O2DEN data product. The spatial, temporal, and local time sampling of the GOLD O2 data product is summarized, and the data quality, retrieval errors, and validation plans are discussed. Occultations are observed under both daytime and nighttime conditions, providing the O2 density over a complete range of local times. GOLD retrievals have an estimated precision of 10% and a vertical resolution of 5–10 km, depending on altitude. The data have sufficient accuracy and resolution to provide scientifically useful constraints on the O2 abundance and variability. Initial comparisons show qualitative agreement, but also notable differences, between the GOLD O2 data and the Naval Research Laboratory Mass Spectrometer Incoherent Scatter Radar (NRLMSISE‐00) empirical model predictions. Key Points: Thermospheric O2 density profiles derived from stellar occultation measurements by the GOLD instrument are describedThe GOLD O2 data set covers altitudes from 130 to 200 km, all local times and latitudes from 60°S to 45°N, with 10% precision and 5‐ to 10‐km vertical resolutionPreliminary validation shows overall agreement with empirical models but also significant differences in magnitude and local time dependence [ABSTRACT FROM AUTHOR]

We have studied atmospheric gravity waves (AGWs) and nighttime medium‐scale traveling ionospheric disturbances (MSTIDs) by applying three‐dimensional spectral analysis technique to 557.7‐ and 630.0‐nm airglow images at Shigaraki (SGK) (35°N, 136°E, 1999–2017) and Rikubetsu (RIK) (44°N, 144°E, 1999–2017), Japan, Athabasca (ATH), Canada, (55°N, 247°E, 2005–2017), and Magadan (MGD), Russia (60°N, 151°E, 2008–2017), focusing on their horizontal wavenumber spectra. For the AGWs in 557.7‐nm images, the power spectra in summer are stronger than in other seasons, probably due to stronger tropospheric convection. The highest energy content of the waves are mostly at wavelengths between 20 and 300 km at MGD, ATH, and RIK, while it is above 200 km at SGK. The largest power spectral density is obtained at RIK at wavelengths of 30–100 km and then ATH. The slopes of the horizontal wavenumber spectra varies from −2.77 to −3.22. From the MSTIDs in 630.0‐nm images, the power spectra in summer at RIK and SGK are stronger than those in other seasons regardless of solar activity. The power spectra in solar quiet time are stronger than those in solar active time at all four stations. These features can be explained by the Perkins instability with coupling between sporadic E and F layers. The spectral slope decreases with increasing latitudes. Weak positive correlations were obtained between the daily wave power of AGWs in 557.7‐nm images and MSTIDs in 630.0‐nm images, suggesting that the MSTIDs in the thermosphere may be partially generated by the AGWs from the mesopause region. Plain Language Summary: In this paper we study atmospheric gravity waves (AGWs) and nighttime medium‐scale traveling ionospheric disturbances (MSTIDs) observed at four stations in Japan, Canada, and Russia, in 557.7‐ and 630.0‐nm airglow images over more than 10 years. The 557.7‐nm airglow has an emission layer at altitudes of 90–100 km (mesopause region). The waves seen in the 557.7‐nm airglow images mainly indicates AGWs. The 630.0‐nm airglow has an emission layer at altitudes of 200–300 km (bottomside ionosphere). The waves seen in the 630.0‐nm airglow images mainly indicates MSTIDs in the ionosphere. The AGWs in the mesopause region are the main driver of global atmospheric circulation in the middle atmosphere. The MSTIDs in the bottomside ionosphere are one of the causes of the satellite positioning error. We show typical energy content, propagation direction, and wavelengths of these waves at these two altitudes and discuss possible reason of the observed characteristics. These results contribute to our understanding of generation and propagation of AGWs and MSTIDs in the upper atmosphere. Key Points: We conducted the statistical analysis of the horizontal wavenumber spectra of waves in airglow images in Japan, Canada, and RussiaDifferences of horizontal wavenumber spectra at different locations, seasons, and solar activities are quantitatively obtainedWeak positive correlation of the power between atmospheric gravity waves and medium‐scale traveling ionospheric disturbances was observed [ABSTRACT FROM AUTHOR]

This special issue reports the latest results on long‐term variations and trends in the middle and upper atmosphere, in particular, in the mesosphere and thermosphere. The issue is based on selected papers from the 10th IAGA/ICMA/SCOSTEP workshop on Long‐term changes and trends in the atmosphere, held in Hefei, China, 14–18 May 2018. This introduction highlights recent progress in key science areas of long‐term changes in the upper atmosphere. Key Points: Long‐term change in the middle and upper atmosphere is caused by increasing carbon dioxide and ozone changeLong‐term decrease in thermosphere density increases risks of space trafficThis introduction highlights of recent progress on long‐term changes in the middle and upper atmosphere [ABSTRACT FROM AUTHOR]

Ground‐based hydrogen Balmer‐α observations from Northern midlatitudes span multiple solar cycles, facilitating investigation of decadal scale variations, including natural variability in the hydrogen response to solar geophysical changes. Here we present a reanalysis of ground‐based hydrogen emission observations from the early 1990s and their comparison with observations obtained in 2000–2001 in the context of the extended Northern Hemisphere midlatitude geocoronal hydrogen emission data set. This work suggests an increase in hydrogen emission intensity between the solar‐maximum period of 1990–1991 (Solar Cycle 22) and the near‐solar‐maximum period of 2000–2001 (Solar Cycle 23), with the caveat that this is a limited data set and that there are calibration uncertainties discussed in this paper. Solar activity was higher during the earlier solar maximum period. Thus, the apparent increase in intensity is counter to previous observations from midlatitudes in which the observed intensity increases with higher solar activity. This increase was also not seen in comparison of intensities from three solar minima periods. Further, the apparent increase in intensity is also likely of larger magnitude than model simulations would predict due to increases in methane and carbon dioxide. We will discuss the reanalysis and recalibration of the 1990–1991 observations using current analysis approaches and the interpretation of these observations in the context of observations and modeling of hydrogen variation over different time scales. The detailed review of the calibration procedures has also provided insights to guide design of future observations. Plain Language Summary: Atomic hydrogen is a key constituent of the thermosphere and exosphere, the uppermost region of the Earth's atmosphere. Hydrogen in this region is a by‐product of molecules at lower altitudes that contain hydrogen such as water vapor and methane, two of the greenhouse gases most important to the energy balance of the Earth. The 11‐year solar cycle is a major source of natural variability in the upper atmosphere. Here we present observations of geocoronal hydrogen emissions (~400 km and above) taken by ground‐based Fabry‐Perot instruments at Wisconsin and at the Kitt Peak, Observatory in Arizona. These observations span two solar cycles and suggest an increase in the geocoronal hydrogen emission intensity between the 1990–1991 solar maximum (Solar Cycle 22) and the 2000–2001 near‐solar‐maximum (Solar Cycle 23) periods, with the caveat that this is a limited data set and there are calibration uncertainties. Solar activity was higher during the earlier solar maximum period. The apparent increase in intensity is counter to previous observations from midlatitudes in which the observed intensity increases with higher solar activity. Further, the apparent increase in intensity is also likely of larger magnitude than model simulations would predict due to increases in methane and carbon dioxide. Key Points: There is a suggestion of an increase in hydrogen emission intensity between two solar maxima, with a caveat that it is a limited data setThe apparent increase in intensity is counter to previous observations of the response of hydrogen emissions to changes in solar activityThe apparent increase in intensity is likely of larger magnitude than would be predicted due to greenhouse gas increases [ABSTRACT FROM AUTHOR]

We present an empirical model of thermospheric winds (High‐latitude Thermospheric Wind Model [HL‐TWiM]) that specifies F region high‐latitude horizontal neutral winds as a function of day of year, latitude, longitude, local time, and geomagnetic activity. HL‐TWiM represents the large‐scale neutral wind circulation, in geomagnetic coordinates, for the given input conditions. The model synthesizes the most extensive collection to date of historical high‐latitude wind measurements; it is based on statistical analyses of several decades of F region thermospheric wind measurements from 21 ground‐based stations (Fabry‐Perot Interferometers and Scanning Doppler Imaging Fabry‐Perot Interferometers) located at various northern and southern high latitudes and two space‐based instruments (UARS WINDII and GOCE). The geomagnetic latitude and local time dependences in HL‐TWiM are represented using vector spherical harmonics, day of year and longitude variations are represented using simple harmonic functions, and the geomagnetic activity dependence is represented using quadratic B splines. In this paper, we describe the HL‐TWiM formulation and fitting procedures, and we verify the model against the neutral wind databases used in its formulation. HL‐TWiM provides a necessary benchmark for validating new wind observations and tuning our physical understanding of complex wind behaviors. Results show stronger Universal Time variation in winds at southern than northern high latitudes. Model‐data intra‐annual comparisons in this study show semiannual oscillation‐like behavior of GOCE winds, rarely observed before in wind data. Key Points: We developed a comprehensive empirical model of high‐latitude F region thermospheric winds (HL‐TWiM)Universal Time variations in high‐latitude winds are stronger in the Southern than Northern HemisphereHL‐TWiM provides a necessary benchmark for validating new high‐latitude wind observations and tuning first principal models [ABSTRACT FROM AUTHOR]

Conventionally, the minimum value of Dst (DstMin) and maximum values of Kp and AE (Kpmax and AEmax) representing the geomagnetic storm intensities have been used for investigating space weather in Earth's environment. The present paper uses the derived parameters (IpsDst, IpsKp, and IpsAE) giving the mean values of Dst, Kp, and AE during the main phase (MP) of Dst storms for investigating ionosphere‐thermosphere storms and low‐latitude (630 nm) aurora. The derived parameters (IpsDst, IpsKp, and IpsAE) representing the impulsive strength of geomagnetic storms seem to have more systematic dependence among themselves than among the intensities (DstMin, Kpmax, and AEmax). The ionosphere‐thermosphere storms observed by the CHAMP (Challenging Minisatellite Payload) satellite and low‐latitude auroras observed by optical imagers are much more intense during high impulsive storms than high intensity storms. In a statistical sense, over 175 positive ionospheric storms (△NmF2) observed in 1985–2005 and the intensity of 20 red auroras observed in 1989–2004 at midlatitudes correlate better with the impulsive parameters than the intensity parameters, with the best correlation being with IpsDst. The mechanism of the impulsive action (high‐energy input over a short duration) leading to large IpsDst arises from the impact of fast solar storms (interplanetary coronal mass ejections) with large IMF Bz southward at their front (or shock). The impulsive action results in bright low‐latitude auroras and strong ionosphere‐thermosphere storms. Key Points: IpsDst giving the mean value of Dst during storm main phase representsthe impulsive strength of geomagneticstormsIpsDst is better than other storm parameters (DstMin, Kpmax, AEmax, IpsKp, and IpsAE) for space weather applicationsGeomagnetic storms of large IpsDst occur due to the impact of fast solar storms with IMF Bz southward at their front (or shock) [ABSTRACT FROM AUTHOR]

This paper reports our simulations of the volume emission rate of the O(1D) redline nightglow perturbed by waves traveling across the thermosphere at around 250 km altitude. Waves perturb the electronic and neutral background densities and temperatures in the region and modify the O(1D) layer intensity as it is captured by ground‐based nightglow instruments. The changes in the integrated volume emission rate are calculated for various vertical wavelengths of the perturbations. We demonstrate that, as the solar activity intensifies, the vertical scales of most likely observable TID waves become larger. For high solar activity, we demonstrate that only waves presenting vertical wavelengths larger than 360 km are likely to be observed. The variation of the range of likely observable vertical wavelengths with the solar cycle offers a plausible explanation for the low occurrence rate of TID in measurements of the redline nightglow during high solar activity periods. We have compared our results with those of Negale et al. (2018; https://doi.org/10.1029/2017JA024876) and Paulino et al (2018; https://doi.org/10.5194/angeo-36-265-2018) to verify that observed vertical wavelengths distribute around 140–210 km, in good correspondence with our predicted threshold wavelength λzt∼160 km for very low solar cycle period. Key Points: In general conditions, traveling ionosphere disturbances (TID) of longer vertical wavelengths (λz>160 km) are more likely than shorter vertical wavelength TID to be observed from ground‐based imagery of the O(1D) nightglowFor high solar activity periods, the volume emission rate of the O(1D) nightglow layer is thicker (larger FWHM) and peaks at higher altitudes due to a larger concentration of O2 at higher altitudes in the thermosphereBecause the O(1D) layer is thicker under high solar activity conditions, only TID of λz> 360 km are likely to be detected from ground‐based observations. This explains the lower observation rate of TID in the O(1D) nightglow all‐sky imagery in high solar activity intervals [ABSTRACT FROM AUTHOR]

Martian lower thermospheric variations are complex due to internal surface dust storms and external solar activities. However, limited Martian measurement data are restricted to observe and understand its variations in the past. In this paper, multisatellite accelerometer‐derived densities and the Mars Climate Database are used to investigate seasonal variations, gravity waves, and coupling effects with the internal and external inputs, including Mars Global Surveyor, Mars Reconnaissance Orbiter, Mars Odyssey, and Mars Atmosphere and Volatile EvolutioN Mission. The diurnal and seasonal structures are reconstructed by the data, and the phase of the cycles is formed by solar heating/ionizing processes. Both amplitude and phase are impacted by surface dust activities during autumn and winter, for which density increases about 1.5–3.5 times compared to spring and summer seasons. A parameterized model that includes a newly introduced dust index is proposed to well fit and reinterpret the seasonal cycles. Furthermore, the coupling process between internal atmospheric gravity waves (IAGWs) and dust activities are investigated and explained. During dust storm times/seasons, the IAGWs exhibit both narrower amplitude peaks and deposit their energy at higher altitudes relative to "clear sky" times. The IAGWs could extend their energies into higher layers beyond exobase due to a thermospheric layer expansion(i.e., density increase) during dust seasons. Plain Language Summary: The new spacecraft Mars Atmosphere and Volatile EvolutioN Mission aims to observe Mars lower thermosphere from 100 to 200 km, namely, the lower thermosphere. The daily and yearly variations of the Mars thermospheric density are illustrated for the first time with this data. An improved fitting function is proposed for the yearly density variations. We found that the yearly variations are explained by seasonal solar illumination changes experienced by Mars during its orbit, which can be fit by a superposition of sine and cosine functions. The amplitude of the variations is impacted by surface dust storms during dust seasons(Mars northern autumn and winter). The behaviors of the vertically propagating internal atmospheric gravity waves (IAGWs) are investigated in different seasons. The interaction between these IAGWs and lower atmosphere dust storms is analogous to ocean waves hitting a beach, where the changes in the lower atmosphere dust function like the changing ocean depth close to shore that allow the ocean waves (i.e., IAGWs) to propagate more energy onto the shore (into the upper atmosphere). Key Points: Diurnal cycles of the Mars thermospheric density are obtained and illustrated from multisatellite accelerometersSeasonal cycles of the Mars thermospheric density are investigated with linking both solar‐heating and dust stormsCoupling processes between internal atmospheric gravity waves (IAGWs) and dust activities are presented [ABSTRACT FROM AUTHOR]

Far ultraviolet observations of Earth's dayglow from the National Aeronautics and Space Administration (NASA) Global‐scale Observations of the Limb and Disk (GOLD) mission presents an unparalleled opportunity for upper atmosphere radiance data assimilation. Assimilation of the Lyman‐Birge‐Hopfield (LBH) band emissions can be formulated in a similar fashion to lower atmosphere radiance data assimilation approaches. To provide a proof‐of‐concept for such an approach, this paper presents assimilation experiments of simulated LBH emission data using an ensemble filter measurement update step implemented with National Oceanic and Atmospheric Administration (NOAA)'s Whole Atmosphere Model (WAM) and National Center for Atmospheric Research (NCAR)'s Global Airglow (GLOW) model. Primary findings from observing system simulation experiments (OSSEs), wherein "truth" atmospheric conditions simulated by NCAR's Thermosphere Ionosphere Electrodynamic General Circulation Model (TIEGCM) are used to generate synthetic GOLD data, are as follows: (1) Assimilation of GOLD LBH disk emission data can reduce the bias in model temperature specification (ensemble mean) by 60% under both geomagnetically quiet conditions and disturbed conditions. (2) The reduction in model uncertainty (ensemble spread) as a result of assimilation is about 20% in the lower thermosphere and 30% in the upper thermosphere for both conditions. These OSSEs demonstrate the potential for far ultraviolet radiance data assimilation to dramatically reduce the model biases in thermospheric temperature specification and to extend the utility of GOLD observations by helping to resolve the altitude‐dependent global‐scale response of the thermosphere to geomagnetic storms. Key Points: A proof‐of‐concept study for assimilating satellite far ultraviolet observations of dayglow using an ensemble filter is presentedObserving system simulation experiments assess the impact of GOLD observations on thermospheric temperature specificationExperiments result in considerable reduction of modeled temperature bias under geomagnetically quiet and moderately disturbed conditions [ABSTRACT FROM AUTHOR]

We studied atmospheric gravity waves (AGWs) and nighttime medium‐scale traveling ionospheric disturbances (MSTIDs) using a three‐dimensional spectral analysis technique for airglow images at wavelengths of 557.7 nm (altitude: 90–100 km for AGWs) and 630.0 nm (200–300 km for MSTIDs), obtained from Athabasca (ATH), Canada (55°N, 247°E, 2005–2017), and Magadan (MGD), Russia (60°N, 151°E, 2008–2017), over 10–13 years. The AGW propagation direction in summer was from northwestward to northward in ATH and northeastward in MGD with phase speeds of 20–60 m/s. In winter at ATH, they are more omnidirectional with weak preference from northwestward to southward with a speed less than 40 m/s, while another weaker power exists from northeastward to southeastward from 70 to 120 m/s. In winter at MGD, there was no dominant direction in the phase‐velocity spectra with spectral power an order smaller than ATH. We suggest that these AGW characteristics were caused by wind filtering and intensity and locations of tropospheric sources. The MSTIDs at ATH propagated southwestward in spring and winter and northeastward in summer and fall. The MSTIDs at MGD propagated northeastward, eastward, and westward in spring, fall, and winter, respectively, with weaker power than that at ATH. The phase speeds are mostly less than 100 m/s except for fall. The propagation direction tends to change from south‐southwestward in the evening to north‐northeastward after the midnight at both ATH and MGD. We discuss possible reasons for these MSTID characteristics at high latitudes based on Perkins and E‐F coupling instabilities, high‐latitude plasma convection, and thermospheric neutral winds. Plain Language Summary: This paper focuses on propagation characteristics of atmospheric gravity waves (AGWs) in the mesopause region and nighttime medium‐scale traveling ionospheric disturbances (MSTIDs) in the ionosphere, using a three‐dimensional spectral analysis technique for airglow images at wavelengths of 557.7 nm (altitude: 90–100 km for AGWs) and 630.0 nm (200–300 km for MSTIDs), obtained from Athabasca (ATH), Canada (55°N, 247°E), and Magadan (MGD), Russia (60°N, 151°E), over 10–13 years. These waves are prevailing in the mesosphere and thermosphere and control global dynamics of the upper atmosphere. The MSTIDs affects accuracy of GNSS positioning. We show seasonal, local time, and yearly variations of propagation directions and phase speeds of AGWs and MSTIDs at high latitude Canada and Russia, and discuss their possible causes in relation to the wave sources in the lower atmosphere, their propagation in the middle and upper atmosphere, and ionospheric plasma instabilities. Key Points: We investigate spectral characteristics of atmospheric gravity waves (AGWs) and medium‐scale traveling ionospheric disturbances (MSTIDs)We found northward preference of AGWs in summer at both Magadan, Russia, and Athabasca, Canada, at high latitudesMSTID propagation direction changes from south‐southwestward in the evening to north‐northeastward after the midnight at both stations [ABSTRACT FROM AUTHOR]

Based on the neutral mass density measurements from the on-board accelerometer of Gravity field and steady-state Ocean Circulation Explorer (GOCE) satellite, the space-borne atmospheric density detectors on the Atmospheric density detection and Precise Orbit Determination (APOD) satellite and another Chinese satellite (CHN-sat) over almost a solar cycle, this paper presents the annual and semiannual variations of the dawn/dusk thermosphere mass density at 260 km, 460 km, and 630 km altitudes. Density maxima at dawn appear in the local summer at the southern high latitudes, and near equinoxes in the northern poles with the density maximum in March higher than that in October at solar minimum. Neutral density maxima near 630 km at dusk occur at high latitudes under low solar flux condition, and at low latitudes near the equator at medium solar activity. The latitude-seasonal density structure obtained from both the CHN-sat and APOD observations shows a distinct hemisphere asymmetry with a larger magnitude at high latitudes in the summer hemisphere at solar minimum. The semiannual variation dominates from low-middle latitudes at 260 km altitude and decreases with the increase of solar activity, while the annual variations dominate at 460 and 630 km altitudes in solar minimum. The growth of mass densities with increasing solar radiation gradually saturates in June. The amplitude of annual variation in mass densities at middle-high latitudes in the Northern Hemisphere (NH) decreases with increasing solar radiation. [ABSTRACT FROM AUTHOR]

This paper presents an analysis of Equatorial Plasma Bubble (EPB) events that occurred during pre- and post-sunrise hours in the South America sector on 18 March 2015, when the St. Patrick's Day geomagnetic storm was in the recovery phase. The data from a set of multi-diagnostic instruments, including Global Navigation Satellite Systems (GNSS) receivers, L-band GNSS ionospheric scintillation monitors, ionosondes, and a Fabry-Perot interferometer, are analyzed. Strong L-band amplitude scintillation was observed with spread-F and total electron content (TEC) fluctuations at times when the electron density and tTEC were minimum with respect to their diurnal variations. The EPBs remained observed for about 3:30 hr after sunrise on the east coast of the South American continent. The daytime ionospheric irregularities show a longitudinal dependency between the west and east coasts, separated by about 3,600 km. Our analysis of the observations suggests that a Disturbance Dynamo Electric Field effect caused by storm-time wind perturbations might have triggered the Rayleigh-Taylor instability near local dawn on the east and west coasts of the South American continent. The effects of ambient ionosphere-thermosphere conditions on the evolution of irregularities are assessed to understand the longitudinal differences. The assessment includes the effect of the E region that appears in the morning hours and the effects of geomagnetic declination and tilting the solar-terminator. [ABSTRACT FROM AUTHOR]

This paper reports a preliminary result for estimating thermospheric temperature around 155 km from the N2 Lyman‐Birge‐Hopfield bands observed by TIMED/Global Ultraviolet Imager (GUVI). Atmospheric Ultraviolet Radiance Integrated Code model (Strickland et al., 1999, https://doi.org/10.1016/S0022‐4073(98)00098‐3) calculations indicate that the intensity ratio in the N2 Lyman‐Birge‐Hopfield (0,0) and (1,0) bands at 144.5‐ to 145.5‐ and 141.0‐ to 142.0‐nm quasi‐linearly depend on N2 rotational temperature. The observed ratios and the Atmospheric Ultraviolet Radiance Integrated Code results are used together to specify the thermospheric temperature around 155 km under sunlit conditions. The estimated temperature agrees fairly well with the neutral temperature at 155 km from WACCM‐X model. The estimated temperature is also higher over the auroral oval and O/N2 depleted regions. Furthermore, meridional wave‐like structures were clearly seen in the derived temperature and were likely caused by traveling atmospheric disturbances. Key Points: Thermospheric temperature is derived using FUV dataTraveling atmospheric disturbance is detectedTemperature increases in O/N2 depleted region [ABSTRACT FROM AUTHOR]

In order to understand the characteristics of long‐lasting "C‐type" structure in the Sodium (Na) lidargram, six cases from different observational locations have been analyzed. The Na lidargram, collected from low‐, middle‐, and high‐latitude sites, show long lifetime of the C‐type structures which is believed to be the manifestation of Kelvin‐Helmholtz (KH) billows in the Mesosphere and Lower Thermosphere (MLT) region. In order to explore the characteristics of the long‐lasting C‐type structures, the altitude profile of square of Brunt‐Väisälä frequency in the MLT region has been derived using the temperature profile collected from the Na lidar instruments and the SABER instrument onboard TIMED satellite. It is found to be positive in the C‐type structure region for all the six cases which indicates that the regions are convectively stable. Simultaneous wind measurements, which allowed us to calculate the Richardson numbers and Reynolds numbers for three cases, suggest that the regions where the C‐type structure appeared were dynamically stable and nonturbulent. This paper brings out a hypothesis wherein the low temperature can increase the magnitude of the Prandtl number and convectively stable atmospheric region can cause the magnitude of Reynolds number to decrease. As a consequence, the remnant of previously generated KH billows in nearly "frozen‐in" condition can be advected through this conducive region to a different location by the background wind where they can sustain for a long time without much deformation. These long‐lived KH billows in the MLT region will eventually manifest the long‐lasting C‐type structures in the Na lidargram. Key Points: The stability of KH billows in the MLT region on Reynold number and Prandtl number on long‐lasting "C‐type" structure in the Na lidargramLow temperature can increase magnitude of Prandtl number and convectively stable atmosphere can decrease the magnitude of Reynolds numberThis conducive atmosphere is responsible for the long lifetime of the remnant of previously generated KH billows in the MLT region [ABSTRACT FROM AUTHOR]

This paper discusses the solar cycle variation of the DE3 and DE2 nonmigrating tides in the nitric oxide (NO) 5.3 μm and carbon dioxide (CO2) 15 μm infrared cooling between 100 and 150 km altitude and ±40° latitude. Tidal diagnostics of SABER NO and CO2 cooling rate data (2002–2013) indicate DE3 (DE2) amplitudes during solar maximum are on the order of 1 (0.5) nW/m3 in NO near 125 km, and on the order of 60 (30) nW/m3 in CO2 at 100 km, which translates into roughly 15–30% relative to the monthly zonal mean. The NO cooling shows a pronounced (factor of 10) solar cycle dependence (lower during solar minimum) while the CO2 cooling does not vary much from solar min to solar max. Photochemical modeling reproduces the observed solar cycle variability and allows one to delineate the physical reasons for the observed solar flux dependence of the tides in the infrared cooling, particularly in terms of warmer/colder background temperature versus smaller/larger tidal temperatures during solar max/min, in addition to cooling rate variations due to vertical tidal advection and tidal density variations. Our results suggest that (i) tides caused by tropospheric weather impose a substantial—and in the NO 5.3 μm case solar cycle dependent—modulation of the infrared cooling, mainly due to tidal temperature, and (ii) observed tides in the infrared cooling are a suitable proxy for tidal activity including its solar cycle dependence in a part of Earth's atmosphere where direct global temperature observations are lacking. Key Points: Large solar cycle effect is seen in NO cooling rate tide due to temperature, but tidal advection shows increasing contribution over solar minCO2 cooling rate tides are more important during solar min, but NO tides become more important during solar max above 125Tides in IR cooling rates are suitable proxy for height evolution of tidal spectrum in the thermosphere [ABSTRACT FROM AUTHOR]

The paper presents a detailed analysis of the density‐temperature (ρ‐T) synchrony in the thermosphere using a hydrostatic general circulation model. The numerical models in general offer not only great potential for forecasting the transient response of the thermosphere but also are excellent tools for understanding the driving mechanisms of various thermospheric trends and features. This study investigates and isolates the dependency of the ρ‐T synchrony on the season, altitude, space weather, high‐latitude electrodynamics, and the lower atmospheric tidal spectrum. The results demonstrate that the previously reported ρ‐T synchrony begins around 300‐km (350‐km) altitude at the equator (high latitudes). The effect of the lower atmospheric tidal spectrum on the ρ‐T synchrony patterns seems to be only marginal and more noticeable during the equinox months. The study demonstrates that the ρ‐T phase lag is larger in the high latitudes of the summer hemisphere and evolves through the day and is attributable to ion drag and temperature fluctuations via soft particle precipitation. The study provides physical insights into how the winds contribute to the ρ‐T synchrony. In addition, the results show that geomagnetic activity contributes significantly to the ρ‐T synchrony; the underlying mechanism may be related to temperature enhancements in the high latitudes via Joule heating and associated nonlinear interactions. While the ρ‐T phase lags attributable to different solar activity levels are modest, the solar heating is the primary source that maintains the ρ‐T synchrony in the low/middle latitudes via upward propagating thermal tides. Key Points: A detailed analysis of the neutral density‐temperature (ρ‐T) synchrony in the thermosphere is presentedThe ρ‐T synchrony begins around 300 km near the equator and varies significantly with latitude, height, and seasonThe ion drag and temperature fluctuations via electron precipitation alters the ρ‐T synchrony in the high latitudes [ABSTRACT FROM AUTHOR]

This paper presents measurements of the amplitudes and timings of the combined, annual, and semiannual variations of thermospheric neutral density, and a comparison of these density variations with measurements of the infrared emissions from carbon dioxide and nitric oxide in the thermosphere. The density values were obtained from measurements of the atmospheric drag experienced by the Challenging Minisatellite Payload, Gravity Recovery and Climate Experiment A, Gravity field and Ocean Circulation Explorer, and three Swarm satellites, while the optical emissions were measured with the Sounding of the Atmosphere using Broadband Emission Radiometry (SABER) instrument on the Thermosphere Ionosphere Mesosphere Energetics and Dynamics satellite. These data span a time period of 16 years. A database containing global average densities that were derived from the orbits of about 5,000 objects (Emmert, 2009, https://doi.org/10.1029/2009JA014102, 2015b, https://doi.org/10.1002/2015JA021047) was employed for calibrating these density data. A comparison with the NRLMSISE‐00 model was used to derive measurements of how much the density changes over time due to these seasonal variations. It is found that the seasonal density oscillations have significant variations in amplitude and timing. In order to test the practicality of using optical emissions as a monitoring tool, the SABER data were fit to the measured variations. Even the most simple fit that used only filtered carbon dioxide emissions had good correlations with the measured oscillations. However, the density oscillations were also well predicted by a simple Fourier series, contrary to original expectations. Nevertheless, measurements of the optical emissions from the thermosphere are expected to have a role in future understanding and prediction of the semiannual variations. Plain Language Summary: The uppermost atmosphere, known as the thermosphere, undergoes oscillations in the density of the neutral atoms and molecules, producing two peaks and valleys in the density in each year. The timing of of these "semiannual" variations or oscillations, as well as their amplitudes, tends to vary. Their unpredictability makes it harder to accurately model the amount of drag experienced by orbiting satellites. It had been noticed that the infrared light emitted by carbon dioxide molecules in the thermosphere has a tendency to follow the semiannual oscillations. Such emissions have been measured by an instrument on a NASA satellite for the past 16 years. We have compared these emissions with the variations in the semiannual oscillations that were derived from measurements of the drag seen by six different satellites flown by both NASA and the European Space Agency during the same time period, though not at the same time. The results of the comparison show how well the infrared emissions match the density oscillations, due to changes in both the composition and temperature of the thermosphere that influence both. Results show that further study will be needed to be able to accurately predict the density oscillations. Key Points: Four satellite missions are used to derive thermosphere density variations caused by annual and semiannual oscillations over a 16‐year periodVariations in thermospheric density are compared with emissions from carbon dioxide and nitric oxide measured with the SABER instrumentThe carbon dioxide emissions and simple Fourier series fits have good correlations with thermosphere density oscillations [ABSTRACT FROM AUTHOR]

This paper investigates and quantifies the longitudinal, solar cyclical, and diurnal variation of the ionosphere peak electron density observed by six ionosondes located between 18 and 151°E near 60°N. Embedded within this region is the Bering Sea anomaly (BSA) where the midnight peak electron density exceeds the midday peak electron density in summer. The BSA is a region West of Alaska extending from approximately 100° to 200° east geographic longitude and 55° to 70° north geographic latitude at its widest. By comparing a physical model with ionosonde data from the 1970s and 1980s, it is found that longitudinal changes in the neutral winds and neutral densities are the most likely explanation for the electron density variation between 18 and 151°E near 60°N. Longitudinal differences in magnetic declination and inclination are small and have a negligible effect on the electron density behavior. Our definition of and the behavior of the BSA are analogous to the Weddell Sea anomaly (WSA), a region in the Southern Hemisphere where the midnight peak electron density also exceeds the midday peak electron density in summer. Although the overall BSA electron density is a factor of 2 smaller than that in the WSA, the two anomalies have similar midnight to midday electron density ratios. It is found that the BSA gets stronger with increasing solar activity, while the WSA gets weaker. It is also demonstrated that including vibrationally excited N2 in an ionosphere model is crucial for producing the observed midnight to midday electron density ratios. Key Points: The longitudinal variation of the Bering Sea Anomaly (BSA) electron density is caused by variations in both the neutral densities and the neutral windsThe magnetic field configuration is only a minor factor in the longitudinal variation of the electron density in the BSAVibrationally excited N2 plays an important role in the midnight to midday electron density ratio [ABSTRACT FROM AUTHOR]