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Led by the Institute of Geology and Geophysics, Chinese Academy of Sciences, we have built a brand‐new modular active digital phased array, with all solid‐state transmission and digital receiving incoherent scatter radar (ISR) in Sanya (18.3°N, 109.6°E), a station in low latitude China called Sanya ISR (SYISR) since 2015. The development of SYISR involved the indoor design and development of key components, an outdoor prototype test, and the production and debugging of the entire array. The unique features of SYISR include a single‐channel directly connected T/R unit and antenna, a radar array monitoring and calibration network, environmental adaptability design and open architecture. The entire radar has 4,096 channels and 5,930 modules in total. All the technical indices, mainly including a >2 MW peak power, a 43 dBi antenna gain and a <120 K noise temperature, either meet or are superior to the designed value through an independent evaluation. Waveforms of single pulse, linear frequency modulation, Barker code, long pulse and alternating code have been implemented to meet multiple purposes. Four observational modes for ionospheric experiments, including zenith stare, perpendicular to geomagnetic field, meridian scan, and all sky scan, have been developed. We have implemented time domain decoding, frequency domain decoding, and statistical inversion methods in calculating the autocorrelation function and power spectra in signal processing. The preliminary experimental results on ionospheric parameters, plasma lines, irregularities and hard targets are reasonable and encouraging, which greatly enhances our confidence in achieving our scientific goals in the future. Plain Language Summary: The Earth's ionosphere (∼60–1,000 km above the sea level) is composed of dense plasma, which shows complicated variations versus altitude, geographic location, and solar activity level. When a ground‐based radio wave is transmitted onto these plasmas, it will generate stimulated radiation, with a small portion of them radiating back to the ground. If this backscattered radiation is collected by a ground‐based radar, a variety of ionospheric parameters, including plasma density, temperature, and movement speed, can be derived through a complicated signal processing algorithm. An ionospheric monitoring method called incoherent scatter radar (ISR) was therefore developed. However, since this backscattered radiation is very weak, the ISR should be built with sufficiently large power (usually megawatts) and apertures (usually hundreds of square meters). This leads to the high cost of building an ISR, with only a few having been built in the world thus far. In this paper, we describe a newly built ISR, called Sanya ISR (SYISR), in low latitude China by applying modern radar technologies, such as modular active digital phased arrays, all solid‐state transmitting and digital receiving. The development of SYISR followed the principle of a gradual and orderly progress, including indoor design, outdoor prototype testing and the entire radar construction and debugging. The radar technical indices have been evaluated, and all meet the designed value. We also developed a signal processing algorithm and a variety of observational modes. Finally, preliminary experimental results are shown, which are reasonable and encouraging. In the future, we will gradually achieve our scientific goals via effective data accumulation and analysis. Key Points: Sanya incoherent scatter radar (SYISR) is a newly built incoherent scatter radar in low latitude China with modular active electronic scanning, digital phased array, solid‐state transmitting, and digital receivingThis paper details the hardware, key parameters, data processing and preliminary experimental results of SYISRThe key parameters meet or are superior to the original design, and the preliminary observations are encouraging [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]

This paper investigates the influence of Earth's dipole tilt angle on the reverse convection cells (sometimes referred to as lobe cells) in the Northern Hemisphere ionosphere during northward IMF, which we relate to high‐latitude reconnection. Super Dual Auroral Radar Network plasma drift observations in 2010–2016 are used to quantify the ionospheric convection. A novel technique based on Spherical Elementary Convection Systems (SECS) that was presented in our companion paper (Reistad et al., 2019, https://doi.org/10.1029/2019JA026634) is used to isolate and quantify the reverse convection cells. We find that the dipole tilt angle has a linear influence on the reverse cell potential. In the Northern Hemisphere the reverse cell potential is typically two times higher in summer than in winter. This change is interpreted as the change in interplanetary magnetic field‐lobe reconnection rate due to the orientation of the dipole tilt. Hence, the dipole tilt influence on reverse ionospheric convection can be a significant modification of the more known influence from vswBz. These results could be adopted by the scientific community as key input parameters for lobe reconnection coupling functions. Key Points: For purely northward IMF the reverse convection potential difference is typically two times higher in summer than in winterThe reverse convection potential difference has a linear dependence on the Earth's dipole tilt angleThe Earth's dipole tilt angle is a secondary important controlling parameter of the lobe reconnection rate [ABSTRACT FROM AUTHOR]

The ionosphere of Titan hosts a complex ion chemistry leading to the formation of organic dust below 1,200 km. Current models cannot fully explain the observed electron temperature in this dusty environment. To achieve new insight, we have re‐analyzed the data taken in the ionosphere of Titan by the Cassini Langmuir probe (LP), part of the Radio and Plasma Wave Science package. A first paper (Chatain et al., 2021) introduces the new analysis method and discusses the identification of four electron populations produced by different ionization mechanisms. In this second paper, we present a statistical study of the whole LP dataset below 1,200 km which gives clues on the origin of the four populations. One small population is attributed to photo‐ or secondary electrons emitted from the surface of the probe boom. A second population is systematically observed, at a constant density (∼500 cm−3), and is attributed to background thermalized electrons from the ionization process of precipitating particles from the surrounding magnetosphere. The two last populations increase in density with pressure, solar illumination and Extreme ultraviolet flux. The third population is observed with varying densities at all altitudes and solar zenith angles (SZA) except on the far nightside (SZA > ∼140°), with a maximum density of 2,700 cm−3. It is therefore certainly related to the photo‐ionization of the atmospheric molecules. Finally, a fourth population detected only on the dayside and below 1,200 km reaching up to 2000 cm−3 could be photo‐ or thermo‐emitted from dust grains. Key Points: Four cold electron populations are detected by the Cassini Langmuir probe in Titan's ionosphere below 1,200 km altitudeThree populations are likely produced by particle precipitation ionization and photo‐ionization in the atmosphere and on the probe boomThe fourth electron population, observed on dayside below 1,200 km, could be photo‐ or thermo‐emitted from dust grains [ABSTRACT FROM AUTHOR]

The Floating Potential Measurement Unit onboard the International Space Station includes a Wide sweeping Langmuir Probe that has been operating in the F‐region of the ionosphere at ∼400 km since 2006. While traditional Langmuir probe estimates include critical plasma parameters like electron density and temperature, we have also extracted the O+ percentage from the total ion constituents. This O+ composition data set from the recent minimum in the Solar Cycle 24 reveals orbits with dropouts in O+ to below 80% of the total background ion density at International Space Station orbital altitudes. The observed O+ percentages during these dropouts are much lower than the values predicted by the International Reference Ionosphere 2016 (IRI2016) empirical model. In this paper, we present the climatology of these O+ dropouts with their dependency on season, local time and geographical location. The results show that the lowered O+ percentages are more significant in the winter hemispheres and are routinely observed for orbits in the pre‐sunrise periods. The patterns in O+ dropouts can be explained in part from the lowering of the O+/H+ transition height during solar minimum along with patterns in neutral wind variation. Plain Language Summary: The plasma environment experienced by the International Space Station (ISS) is of key importance to the space physics community. A suite of instruments on the ISS has been operating since 2006 that consist of a spherical Langmuir probe, called the Wide sweeping Langmuir Probe, which is a sensor that periodically collects information about the density and energy of the ambient electrons. Recently, we presented a technique to also infer the composition of O+ ions relative to the total ion density which is an important parameter in understanding the interaction between the terrestrial atmosphere with the exosphere. Results from the most recent period of solar minimum reveal dropouts in O+ densities compared to predictions from empirical models. We show global climatology plots from 2018 to 2019 and observe that these large dropouts in O+ are more prevalent in the post‐midnight region of the winter hemisphere. Past studies have discussed increased presence of lighter H+ ions at higher orbital altitudes, but the fact that O+ dropouts are observed even at ISS altitudes is striking and one of the prominent features of solar minimum. Key Points: O+ percentages often drop below 80% at International Space Station altitudes during solar minimumThe dropouts in O+ abundance are common in post‐midnight sector in winter hemisphereThe climatology of the dropouts is governed by neutral wind variations and lowered O+/H+ transition height [ABSTRACT FROM AUTHOR]

Normally, auroral kilometric radiation (AKR) which is emitted in the auroral zones escapes from the Earth. But since a few decades very similar radiations are observed by ground‐based receivers and by satellites at altitudes below the AKR generation area. They are called leaked AKR or AKR‐like emissions because it is expected that there are linked to AKR. This paper deals with observations of such AKR‐like emissions observed in the auroral zones (in the North and in the South) by the low‐altitude satellite DEMETER. In total, 2,526 events have been recorded during 6.5 years. These events are not very rare as they occur at least 2% of the time. Although this data set has a severe flaw due to a latitudinal constraint, it was possible to draw interesting properties of these emissions. In fact they are very similar to usual AKR observed at much higher altitudes during auroral activities (the same frequency range, magnetic local time (MLT) sector, and invariant latitude). The main difference concerns a strong asymmetry between the Northern and the Southern hemispheres: (a) the number of AKR‐like emissions in the Northern hemisphere is 32% larger than in the Southern hemisphere but this percentage decreases when the auroral activity increases, and (b) there is an important seasonal effect because the number of events decreases during the winter season both in the North and in the South. Plain Language Summary: Auroral kilometric radiation (AKR) is the strongest terrestrial radio emission generated between 30 and 800 kHz along the auroral field lines and associated with a discrete auroral arc. This emission is generated by superthermal electrons (several keV) which are injected from the magnetotail when the solar activity increases. Typical location of the source is ∼22 hr of magnetic local time (MLT), ∼70° of invariant latitude, and (2–10)103 km of altitude. AKR propagates from the Earth and should not be able to reach low altitudes because of the ionospheric frequency cutoff. But a few decades ago, AKR‐like emissions have been recorded by low orbiting satellites and even by ground‐based receivers. It was explained by a mode conversion of the wave propagating in inhomogeneous plasma. Up to now, only studies of some AKR‐like events have been reported, and this paper presents the statistical properties of such emissions recorded by the low altitude satellite DEMETER during 6.5 years. AKR‐like emissions display similar characteristics of AKR in terms of frequency, MLT, and invariant latitude. It is shown that more events are observed in the North than in the South, and that there is a seasonal effect (the number of events decreases in winter hemispheres). Key Points: Numerous auroral kilometric radiation‐like events have been recorded by the low‐altitude satellite DEMETER during moderate and high magnetic activityThere are more events in the Northern than in the Southern hemisphere but this difference decreases when the auroral activity increasesThe number of events decreases in winter in each hemisphere [ABSTRACT FROM AUTHOR]

In this paper we present an algorithm which identifies ion energy dispersion events in low altitude cusp observations. Such an algorithm is suitable for implementation to prioritize data for downlink, or to be run on the ground for data mining. The energy dispersion events, sometimes known as dispersed particle precipitation, have been interpreted as due to time‐of‐flight effects of energized ions from magnetopause reconnection sites. Our algorithm utilizes a novel method of integrating a scoring function across moving time windows to capture the dispersion structure. The algorithm is implemented and prototyped with Defense Meteorological Satellite Program data but specified in terms that apply to other missions for both ions and electrons. Two new types of dispersion events are uncovered with the algorithm, consisting of a cold inverse double dispersion event where one population dominates, and a V‐type dispersion event during northward interplanetary magnetic field. Plain Language Summary: Ion energy dispersion events occur in the low altitude cusp region, originating from reconnection at the magnetopause. This paper presents an algorithm for automatically detecting these events in data from the Defense Meteorological Satellite Program (DMSP) satellites. Though the algorithm is demonstrated with DMSP, the algorithm can work with other satellites. The algorithm is also used to search for new types of dispersion, and two new types of dispersion events are presented in this paper. Key Points: A novel method for capturing the energy dispersion structure is developed and applied to the DMSP dataThe algorithm is applied and validated against a G2‐class Moderate StormNew types of ion dispersion events are uncovered with the algorithm and their relevance is stated [ABSTRACT FROM AUTHOR]

A paper by Jiang et al. (2019, https://doi.org/10.1029/2018JA026040) apparently tries to find common ground between observations of the F3 layer and TIDs by referring to the later when observed near the equatorial anomaly peak as a new type 1 and TIDs by using ray tracing models with arbitrarily adjusted TID parameters. Further confusion in conflating the two separate phenomena is created by referring to both as "stratification." This comment points out that the F3 is a daytime temporary layer maintained by E × B uplift at the magnetic equator, normally occurs consistently within the peaks of the equatorial ionization anomaly in F2 electron density and may last for hours. Such a layer can be described as a stratification. How this effect dies out beyond the anomaly peak has yet to be experimentally ascertained. A cusp or inflection type TID is a manifestation of a gravity wave with no consistency in location and local time, of short duration measured in minutes, involving a continuous descent in height and should not be described as stratification. The theory of both is well known, and attempting to explain a possible F3 layer as a new type of layer when seen in the vicinity of an equatorial anomaly peak misunderstands the theory of the phenomenon. Attempts to characterize such a phenomenon as a TID manifestation are not defensible. Key Points: F3 layer and TID can usually be distinguished on ionograms by their time histories, consistency, location, and durationAn effort to reproduce the appearance of the F3 using a TID model is neither justified nor required [ABSTRACT FROM AUTHOR]

We associate new data from icebear, a coherent scatter radar located in Saskatchewan, Canada, with scale‐dependent physics in the ionosphere. We subject the large‐scale icebear 3D echo patterns (treated as 2D point clouds) to a data analysis technique hitherto never applied to the ionosphere, a technique that is widely applied in cosmological red‐shift surveys to characterize the spatial clustering of galaxies. The technique results in a novel method to calculate the spatial power spectral density of the greater ionospheric irregularity field. We compare results from this method to in‐situ plasma density and magnetic field observations from the Swarm mission. We show that there is a remarkable similarity between echo clustering spectra in the E‐region and the field‐aligned current structuring spectrum observed in the F‐region: a clear and characteristic preferred scale (5 km) both in the E‐ and F‐region spectra. We discuss the possibility that this represents evidence of an energy injection into the ionospheric irregularity field via energetic particle precipitation, but offer alternative interpretations with wider connotations for the ionosphere‐magnetosphere system. These findings open new and promising avenues of research for the study of the location of ionospheric scatter echoes with 3D information. It constitutes a novel way to consider the pattern of ionospheric irregularities over wide fields of view when there is an abundance of radar echoes, which allows for the analysis of radar data as point clouds. Plain Language Summary: With the advent of precision radar data from the aurora containing detailed information about radar echo locations, we are in a position to apply statistical methods from cosmology to extract information about plasma turbulence in the ionosphere. This interdisciplinary paper presents some birds‐eye view discussions about the use of correlation functions in physics, showing how it can be used to understand physics on virtually all spatial scales in the observable universe. We present results pertinent to the ionosphere, shedding light on plasma turbulence and its relation to energetic particles in the aurora. Here, we use observations both from space, by the orbiting satellites in the European Space Agency's Swarm mission, and using an experimental auroral radar in Saskatchewan, Canada. Key Points: Clustering Statistics are used to extract large scale turbulence information from 3‐m structures in the E‐regionE‐region clustering spectra are remarkably similar to in‐situ spectra from field‐aligned currents in the F‐regionA preferential 5‐km scale is simultaneously observed in the E‐region and in the F‐region field‐aligned currents [ABSTRACT FROM AUTHOR]

Geomagnetic conjugate mid‐latitude nighttime ionospheres are frequently simultaneously populated with electrified nighttime medium‐scale traveling ionospheric disturbances (MSTIDs). Earlier observations and theoretical analysis have underscored the ionosphere E‐F coupling and the postulation of coupled conjugate hemispheres, playing a pivotal role in the formation of electrified MSTIDs. In this paper, the conjugate MSTIDs are studied to elucidate the causes and effects of E‐F coupling in the interhemispheric coupled ionosphere. The hemisphere‐coupled ionospheres over Japan and Australia are observed and analyzed using total electron content (TEC) measurements, supplemented with multi‐source observations from ionosondes, Ionospheric Connection Explorer (neutral wind), Constellation Observing System for Meteorology, Ionosphere, and Climate (electron density), and Swarm (magnetic field). A double‐thin‐shell model is introduced to analyze the ionospheric responses in E and F regions during the coupling process. For the first time, observation results provide the evidence that F‐region geomagnetic conjugate irregularities in both hemispheres are mainly driven by the Es layers in the summer hemisphere. The Es layer in the summer hemisphere subsequently triggers local E‐F coupling and inter‐hemispheric coupling. In the winter hemisphere, Es layers show amplitude reduction or even dissipation during the interhemispheric coupling process. Furthermore, thermospheric winds, non‐equipotential magnetic field lines, and background TEC are presumed candidates for the inter‐hemispheric asymmetry in MSTIDs amplitudes and growth rates. Key Points: The F‐region geomagnetic conjugate medium‐scale traveling ionospheric disturbances (MSTIDs) in both hemispheres are mainly driven by the Es layers in the summer hemispherePreexisting Es layers in the winter hemisphere show intensity reduction or even dissipation after the interhemispheric couplingThe amplitude and growth rate of hemispheric conjugate MSTIDs are smaller in the winter hemisphere [ABSTRACT FROM AUTHOR]

Surface waves on Earth's magnetopause have a controlling effect upon global magnetospheric dynamics. Since spacecraft provide sparse in situ observation points, remote sensing these modes using ground‐based instruments in the polar regions is desirable. However, many open conceptual questions on the expected signatures remain. Therefore, we provide predictions of key qualitative features expected in auroral, ionospheric, and ground magnetic observations through both magnetohydrodynamic theory and a global coupled magnetosphere‐ionosphere simulation of a magnetopause surface eigenmode. These show monochromatic oscillatory field‐aligned currents (FACs), due to both the surface mode and its non‐resonant Alfvén coupling, are present throughout the magnetosphere. The currents peak in amplitude at the equatorward edge of the magnetopause boundary layer, not the open‐closed boundary as previously thought. They also exhibit slow poleward phase motion rather than being purely evanescent. We suggest the upward FAC perturbations may result in periodic auroral brightenings. In the ionosphere, convection vortices circulate the poleward moving FAC structures. Finally, surface mode signals are predicted in the ground magnetic field, with ionospheric Hall currents rotating perturbations by approximately (but not exactly) 90° compared to the magnetosphere. Thus typical dayside magnetopause surface modes should be strongest in the East‐West ground magnetic field component. Overall, all ground‐based signatures of the magnetopause surface mode are predicted to have the same frequency across L‐shells, amplitudes that maximize near the magnetopause's equatorward edge, and larger latitudinal scales than for field line resonance. Implications in terms of ionospheric Joule heating and geomagnetically induced currents are discussed. Plain Language Summary: Waves on the boundary of the magnetosphere, the magnetic shield established by the interplay of the solar wind with Earth's magnetic field, play a controlling role on energy flow into our space environment. While these waves can be observed as they pass over satellites in orbit, due to the small number of suitable satellites available it would be helpful to be able to detect these waves from the surface of the Earth with instruments that measure the northern/southern lights, motion of the top of our atmosphere, or magnetic field on the ground. However, we do not currently understand what the signs of these waves should look like in such instruments. In this paper we develop theory and use computer simulations of these boundary waves to predict key features one might expect to measure from the ground. Based on these predictions, we also discuss how the waves might contribute to the hazards of space weather. Key Points: Theory and global simulations of magnetopause surface waves' effects on the aurorae, ionosphere, and ground magnetic field are investigatedWe predict poleward‐moving periodic aurora, convection vortices, and ground pulsations, with larger latitudinal scales than Alfvén modesAmplitudes of all signals peak near the projection of the inner/equatorward edge of the magnetopause rather than the open–closed boundary [ABSTRACT FROM AUTHOR]

Releasing diffuse artificial clouds into the space environment using a rocket or spacecraft can modify the natural plasma state. This uses space itself as a laboratory for studying plasma phenomena that cannot be reproduced on the ground. The Space Measurement of A Rocket‐released Turbulence (SMART) mission will inject a beam of barium neutral vapor into the ionosphere and perpendicular to the Earth's magnetic field. Barium atoms will be photoionized, forming an ion ring distribution that is unstable to lower hybrid waves. Large amplitude lower hybrid waves nonlinearly scatter to whistler and magnetosonic waves that can propagate out to the magnetosphere. This paper details the theory, modeling, and simulation we used to design this release experiment to optimize the energy in the plasma waves under a variety of constraints. A product of this analysis is quantitative predictions for some in situ and remote measurements. Hydrocode simulations of barium vapourization and acceleration via shaped charge provide the initial state of the neutral beam. We optimize the apogee, orientation, and position of the payload and instruments. Cloud dynamics are simulated with a direct simulation Monte Carlo technique, which includes photoionization with metastable barium states, collisions with the neutral background atmosphere, optical line emission, gravity, and electromagnetic forces. We predict the plasma density measured by the instrument payload and the remotely measured optical intensity. We also examine how the plasma wave growth differs at the measurement location compared to the bulk of the barium cloud. Key Points: A variety of models and simulations are combined to optimize plasma wave energy in a neutral beam injection experimentModeling provides quantitative predictions of measurements (e.g., plasma density, beam velocity, total barium produced, optical intensity)We discuss the many details that arose between concept and implementation of the experiment, and how these affected mission design [ABSTRACT FROM AUTHOR]

In this paper, the internal structuring of Polarization Jet (PJ)/Subauroral Ion Drift (SAID) is studied. It is shown that PJ/SAID consists of several Polarization Jet Strata (PJS) of electron density and temperature. A new type of phenomena has been discovered called Stratified Subauroral Ion Drift (SSAID), which is a PJ/SAID containing small‐scale PJS. Internal small‐scale structure of SSAIDs is measured during various geomagnetic events using data from NorSat‐1, DMSP F‐17 and F‐18 satellites. High resolution measurements on the Norsat‐1 satellite make it possible to detect and study small‐scale structures inside the PJ/SAID, that is, SSAID. The data from DMSP satellites is used to determine explicitly the presence of PJ/SAID during the considered geomagnetic events. Results show that SSAID consist of 2–4 large PJS and from a few units to several tens of small PJS. Large PJS have a size of 0.2°–0.3° latitude and small PJS have a size of <0.1°. Several suggestions regarding possible physical mechanisms of production of SSAID and PJS are presented. Plain Language Summary: In the subauroral zone, narrow fluxes of fast subauroral ion drifts are observed near the projection of the plasmapause at heights of the F‐layer of the ionosphere, which are most noticeable during geomagnetic activity against the background of large‐scale plasma convection, called polarization jet (PJ) or subauroral ion drifts (SAID). Despite the importance of using a variety of ground‐based observation facilities for studying and analyzing PJ/SAID properties, as well as developing analytical models and numerical modeling, in situ observations are most valuable. These observations can be obtained only during the satellite flyby through the subauroral zone during the PJ/SAID development. Small‐scale processes inside PJ/SAID were practically not studied and many open questions remain here due to lack of high‐resolution satellite measurements. In this work, using NorSat‐1 satellite high sampling rate data, we report that there are Stratified Subauroral Ion Drifts (SSAIDs) which consist of small‐scale Polarization Jet Strata (PJS) of electron density and temperature. Key Points: We explore the internal structuring of subauroral polarization jets/ion drifts (SSAID ‐ Stratified Subauroral Ion Drift)SSAIDs consist of 2–4 large Polarization Jet Strata (PJS) and from a few units to several tens of small PJSLarge PJS have a size of 0.2°‐0.3° latitude and small PJS have a size of <0.1° [ABSTRACT FROM AUTHOR]

This paper presents a new algorithm for detecting high‐speed flow channels in the polar cap. The algorithm was applied to Super Dual Auroral Radar Network data, specifically to data from the new Longyearbyen radar. This radar is located at 78.2°N, 16.0°E geographical coordinates looking north‐east, and is therefore at an ideal location to measure flow channels in the high‐latitude polar cap. The algorithm detected >500 events over 1 year of observations, and within this paper two case studies are considered in more detail. A flow channel on "old‐open field lines" located on the dawn flank was directly driven under quiet conditions over 13 min. This flow channel contributed to a significant fraction (60%) of the cross polar cap potential and was located on the edge of a polar cap arc. Another case study follows the development of a flow channel on newly opened field lines within the cusp. This flow channel is a spontaneously driven event forming under strong solar wind driving and is intermittently excited over the course of almost an hour. As they provide a high fraction of the cross polar cap potential, these small‐scale structures are vital for understanding the transport of magnetic flux over the polar cap. Key Points: Polar cap flow channels can account for a substantial amount (40–60%) of the cross polar cap potentialFlow channels can form due to dayside reconnection or appear on the edge of polar cap arcsMagnetic field lines that opened 25 min ago can still cause fast flow channels deep inside the polar cap [ABSTRACT FROM AUTHOR]

This paper describes a novel technique that allows separation and quantification of different sources of convection in the high‐latitude ionosphere. To represent the ionospheric convection electric field, we use the Spherical Elementary Convection Systems representation. We demonstrate how this technique can separate and quantify the contributions from different magnetospheric source regions to the overall ionospheric convection pattern. The technique is in particular useful for distinguishing the contributions of high‐latitude reconnection associated with lobe cells from the low‐latitude reconnection associated with Dungey two‐cell circulation. The results from the current paper are utilized in a companion paper (Reistad et al., 2019, https://doi.org/10.1029/2019JA026641) to quantify how the dipole tilt angle influences lobe convection cells. We also describe a relation bridging other representations of the ionospheric convection electric field or potential to the Spherical Elementary Convection Systems description, enabling a similar separation of convection sources from existing models. Key Points: Spherical Elementary Convection Systems (SECS) are used to represent the high‐latitude convection electric fieldA novel technique is presented that allows separation and quantification of different sources of ionospheric convectionThe convection pattern can be separated to quantify the magnetic flux transport associated with different reconnection sites [ABSTRACT FROM AUTHOR]

The magnetosheath and near‐Earth solar wind emit X‐rays due to charge‐exchange between the extended atmosphere and highly ionized particles in the solar wind. These emissions can be used to remotely sense the dynamic processes in this region. The Solar Wind Magnetosphere Ionosphere Link Explorer mission will carry out these measurements. In a previous paper, we looked at the effect of photon counting statistics on determining the location of the magnetopause and bow shock. In this paper we explore, through simulations, the more challenging question of orbital viewing geometry bias when the model and the emissions do not match each other exactly. Our simulations conclude that while care must be taken to avoid false minima in the fitting, there is very little to no orbital bias in extracting the position and large‐scale shape of the magnetopause and bow shocks from 2‐D X‐ray images from the future Solar Wind Magnetosphere Ionosphere Link Explorer mission. Key Points: A 3‐D model fit to 2‐D images can extract magnetopause and bow shock boundaries even when the model is not an exact fit to the observationsOrbital bias produced model/observation differences is within the science goal of the SMILE mission for large part of the orbitWhen using the simplex method, the initial guess must be chosen carefully to avoid false minima [ABSTRACT FROM AUTHOR]

This paper reports a large‐scale magnetospheric line radiation (MLR) event during a moderate geomagnetic storm on 11 September 2018, which was well recorded by the China‐Seismo‐Electromagnetic Satellite (CSES) in the upper ionosphere. The event shows a symmetrical propagation feature at the conjugated locations between the two hemispheres, exhibiting a large spatial extension roughly from the latitudes 54°N to 53°S. The parallel structures are visible both in the electric and magnetic fields at a frequency band ranging from the local proton cyclotron frequency to ∼1.6 kHz. The wave intensity of parallel spectral lines was primarily enhanced in high latitude regions, gradually weakening at mid‐low latitudes, and then got absorbed in the equatorial region, presenting a distinct V‐shaped structure. The frequency spacings between neighboring spectral lines roughly vary from ∼80 to 110 Hz at the high latitudes and ∼80–130 Hz at the low latitudes, suggesting a slight variation feature with latitude. The parallel spectral structures of MLR drift between ∼0.39 and 0.57 Hz/s at high latitudes and ∼0.18–0.19 Hz/s at low latitudes. The wave vector analysis shows that the MLR waves are right‐hand polarized, obliquely propagating toward the Earth and in the azimuthal direction, where the Poynting flux is primarily oriented perpendicular to the ambient magnetic field. The other large‐scale MLR events all exhibit similar parallel structures and polarization characteristics, suggesting the universality of such a phenomenon. However, the azimuthal angles differ among different events, showing complex features. Plain Language Summary: Magnetospheric line radiation (MLR) is a unique electromagnetic wave distinguished by parallel spectral lines. This study reports a large‐scale MLR event that occurred in the dayside ionosphere. The event shows a symmetrical propagation feature, with a large spatial extension between latitudes 54°N and 53°S. The parallel structures are visible both in the electric and magnetic spectrogram, ranging from the local proton cyclotron frequency to ∼1.6 kHz. The MLR structures were primarily enhanced in high latitude regions, gradually weakening at mid‐low latitudes, and then got absorbed in the equatorial region, presenting a distinct V‐shaped structure. The frequency spacings of MLR roughly vary from ∼80 to 110 Hz in the high latitudes and from ∼80 to 130 Hz in the mid‐low latitude region, slightly varying with latitude. The MLR structures drift between ∼0.39 and 0.57 Hz/s at high latitudes and ∼0.18–0.19 Hz/s at low latitudes. This MLR event is right‐hand polarized, obliquely propagating toward the Earth and in the azimuthal direction, and the Poynting flux is primarily oriented perpendicular to the ambient magnetic field. However, the azimuthal angles differ among different events, indicating the complexity of the wave propagation feature. Key Points: A large‐scale magnetospheric line radiation (MLR) event shows a symmetrical propagation feature in two hemispheres, presenting a distinct V‐shaped structureBoth the frequencies of the parallel spectral lines and their frequency spacings slightly drift with latitudesThe MLR waves are right‐hand polarized, obliquely propagating toward the Earth and azimuthal direction [ABSTRACT FROM AUTHOR]

This paper reports the first high‐resolution 4‐dimensional ionospheric disturbance caused by hurricane Matthew on 7 October 2016. The temporal as well as vertical and horizontal variations of the concentric traveling ionospheric disturbances (CTIDs) were reconstructed using 3‐dimensional computerized ionospheric tomography (3DCIT) technology, based upon the Global Navigation Satellite System data from the dense receiver network over North America. The frequency range of disturbances was determined by spectrum analysis, and a Butterworth band‐pass filter was used to de‐trend the total electron content (TEC) sequences to determine TIDs. A remarkable CTID segment was detected at a distance of 1,000–1,500 km from the hurricane eye at ∼5:40–6:10 UT on 7 October 2016, moving westward with the horizontal phase velocity of ∼153.4 m/s, the period of ∼30 min and the horizontal wavelength of ∼276.1 km. The positive and negative wavefronts dominated the CITD at different times during the event. From 4:00 to 8:00 UT, the altitudinal variation of the CTIDs in electron density exhibited clear downward phase progression predominately in the range of 150–400 km altitudes; however, the percentage of electron density disturbances was larger below 250 km. The inverted cone‐like geometry of CTID wavefronts was presented. The vertical phase velocities of the CTIDs ∼1,100 km away from the hurricane eye in the northwest direction near 88°W, 34°N were ∼203.7–277.8 m/s, and at the same location, the horizontal phase velocities at 300 km altitude were ∼149.1–181.5 m/s, slightly larger than those at 200 km altitude (∼145.1–178.5 m/s). Key Points: The propagation features of concentric traveling ionospheric disturbances (CTID) caused by Hurricane Matthew on 7 October 2016 were analyzedFirst 4‐dimensional CTIDs caused by hurricane were reconstructed using ionospheric tomographyThe horizontal phase velocities at different altitudes and the vertical phase velocities of the ionospheric disturbances were estimated [ABSTRACT FROM AUTHOR]

In this paper, the ionospheric quiet‐time disturbances otherwise known as Pre‐Magnetic Storm Signatures (PMS) have been studied using the F2‐layer peak electron density (NmF2) data obtained from 12 Digisonde/ionosonde stations distributed across equatorial, low and mid‐latitudes. The datasets used spans the years 2010–2012. Results from this study reveals strong PMS in NmF2 with percentage deviations (ΔNmF2) ranging from −91% to 500% at the equatorial, low and mid‐latitudes, with maxima occurring at the equatorial region. Significant effects on the peak height of the F2‐layer (hmF2) were also observed, and they are correlated with the variations in NmF2 particularly at the equatorial station during the PMS. The duration of a PMS is found to be 12–48 h. Although, it was difficult to state clearly the connection between the PMS and the geomagnetic storm that usually follows within 24–48 h; but the NmF2 and hmF2 responses during the PMS were quite similar to those observed during geomagnetic storms. A slight increase in the Solar‐wind‐plasma speed (>20 km/s) was also observed during PMS. The PMS occur under a southward IMF‐Bz, moderate aurora activity (AE ranging from 114 to 560 nT) and quiet ring current (Dst >−10 nT). Therefore, it is pertinent to consider a certain threshold of the aurora indices (AE, AL, and AU) in addition to the Dst, ap, and Kp in the definition of a geomagnetically quiet day. This may eliminate the ambiguity in explaining the ionospheric variability that occurs few days before Sudden Storm Commencement (SSC). Key Points: Pre‐Magnetic Storm Signatures (PMS) on NmF2 were observed to be maximum at the equatorial ionosphereIonospheric Responses during PMS were observed to be similar to the effect of geomagnetic stormsThere is a need to consider a certain threshold of the aurora indices (AE, AL, and AU) in the definition of a geomagnetic quiet day [ABSTRACT FROM AUTHOR]

Current models of Titan's ionosphere have difficulties in explaining the observed electron density and/or temperature. In order to get new insights, we re‐analyzed the data taken in the ionosphere of Titan by the Cassini Langmuir probe (LP), part of the Radio and Plasma Wave Science (RPWS) instrument. This is the first of two papers that present the new analysis method (current paper) and statistics on the whole data set. We suggest that between two and four electron populations are necessary to fit the data. Each population is defined by a potential, an electron density and an electron temperature and is easily visualized by a distinct peak in the second derivative of the electron current, which is physically related to the electron energy distribution function (Druyvesteyn method). The detected populations vary with solar illumination and altitude. We suggest that the four electron populations are due to photo‐ionization, magnetospheric particles, dusty plasma and electron emission from the probe boom, respectively. Key Points: The Cassini Langmuir probe dataset in Titan's ionosphere is re‐analyzed with a specific interest on the electron density and temperature2 to 4 cold electron populations with distinct potentials are observedElectron populations vary with altitude and solar illumination suggesting origins linked to solar photons, magnetospheric particles and dust [ABSTRACT FROM AUTHOR]

Inside the magnetosheath, the IBEX‐Hi energetic neutral atom (ENA) imager measures a distinct background count rate that is more than 10 times the typical heliospheric ENA emissions observed when IBEX is outside the magnetosheath. The source of this enhancement is magnetosheath ions of solar wind (SW) origin that deflect around the Earth's magnetopause (MP), scatter and neutralize from the anti‐sunward part of the IBEX‐Hi sunshade, and continue into the instrument as neutral atoms, behaving indistinguishably from ENAs emitted from distant plasma sources. While this background pollutes observations of outer heliospheric ENAs, it provides a clear signature of IBEX crossings over the magnetospheric boundaries. In this study, we investigate IBEX encounters with the magnetosheath boundaries using ∼8 yr of orbital data, and we determine the MP and bow shock (BS) locations derived from this background signal. We find 280 BS crossings from XGSE ∼ 11 Re to XGSE ∼ −36 Re and 241 MP crossings from XGSE ∼ 6 Re to XGSE ∼ −48 Re. We compare IBEX BS and MP crossing locations to those from IMP‐8, Geotail, Cluster, Magion‐4, ISEE, and Magnetospheric Multiscale Mission, and we find that IBEX crossing locations overlap with the BS and MP locations inferred from these other data sets. In this paper, we demonstrate how IBEX can be used to identify magnetosheath crossings, and extend boundary observations well past the terminator, thus further constraining future models of magnetosheath boundaries. Furthermore, we use the IBEX data set to show observational evidence of near‐Earth magnetotail squeezing during periods of strong interplanetary magnetic field By. Plain Language Summary: The Interstellar Boundary Explorer (IBEX) spacecraft is designed to use remote‐sensing to examine the outer reaches of the solar system, yet it has been shown to have many magnetospheric applications as well. The primary interaction between the SW and the Earth's magnetosphere is the creation of a comet‐tail like structure known as the magnetosheath. The magnetosheath is highly dynamic, and its size and shape changes in response to changing SW conditions, with the exact details presently unknown. In this paper, we demonstrate how the IBEX's highly eccentric orbit allows for hundreds of encounters with both the inner and outer magnetosheath boundaries, and we use these crossings to create a new magnetosheath boundary data set. We then compare our new data set with data sets from previous magnetospheric missions. Finally, we use our new data set to provide observational evidence for a poorly understood phenomenon known as magnetotail squeezing. Key Points: We demonstrate a technique for producing both Bow Shock and Magnetopause data sets using the Interstellar Boundary ExplorerWe introduce new boundary crossings further tailward than previously used data sets into underexplored magnetosheath regionsWe provide observational evidence of magnetotail squeezing during periods when the interplanetary magnetic field has a strong y‐component [ABSTRACT FROM AUTHOR]

Jupiter has a strong magnetic field, and a huge magnetosphere is formed through the solar wind-Jupiter interaction. The generated magnetosphere-ionosphere system is reproduced based on the 9-component Magnetohydrodynamics (MHD) and the current conservation in the ionosphere. Assuming Io plasma emission rate 1.4 t/sec, this paper reproduces self-consistently global magnetic configuration, generations of the field-aligned current (FAC) and aurora, formation of the Io plasma disk at 8-20 RJ, plasma corotation, instability in the plasma disk, transition from the Io plasma disk to the plasma sheet at 20-150 RJ, and the plasmoid ejection. The rotating Io plasma in the disk forms instabilities that promotes radial diffusion. H+ is supplied from the ionosphere along high-latitude magnetic field lines and mixed with heavy ions around 15-20 RJ. Beyond 20 RJ, mixed plasma diffuses further outward by the centrifugal force that can exceed magnetic tension. In the ionosphere, the main oval occurs at 13.7°-15.5° colatitude. The Io disk is inner side of magnetic field lines traced from the low-latitude edge of the main oval. Along magnetic field lines, the main oval is mapped from the outer edge of the Io disk to the entire plasma sheet accompanying rotation delay. Due to the corotation limit, convection is accompanied by plasmoid ejection. Back reaction of plasmoid ejection affects even transport process in the Io disk. The downward FAC occurs in the polar cap showing variability. The region of externally driven Dungey convection seems quite narrow. [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]

The work is devoted to modeling and verification of the dynamics of charged components of the Earth's lower ionosphere, obtained using different values of the background neutral atmosphere parameters. Long‐term experimental data on temperature and concentration of neutral components from the Aura satellite and Mass Spectrometer Incoherent Scatter Radar (MSIS) model were used as input data. The average relative difference between the electron concentration values obtained using Aura and MSIS data (11 June 2014, 6–18 UT, h = 50–90 km, 41°N 10°E) was 34%. Obtained results were verified with use of very low frequency (VLF) signals propagation data, which were collected on Mikhnevo Geophysical Observatory (GPO) and A118 station from GBZ, GQD, DHO, TBB, and ICV transmitters. Based on the results of the verification, it was found that the response of the lower ionosphere to X‐ray flares, that occurred on 9–11 June 2014, is better described by the Aura satellite data in 4 out of 5 cases. Consequently, they are more preferable for predicting the state of ionospheric parameters below the reflection height of signals of the considered frequencies (∼75 km). It is also shown that taking into account the ion concentration for calculation the amplitude‐phase characteristics of the VLF‐signals makes it possible to describe the dynamics of parameters better. Plain Language Summary: Due to the small amount of direct experimental measurements, the lower ionosphere still remains the least studied. At the same time, it is responsible for the distortion of the amplitude and phase of the very low frequency (VLF) signals and the absorption of high frequency radio waves during strong disturbances. The correctness of the existing plasma‐chemical models of the lower ionosphere depends on the values of the input parameters, many of which are known with low accuracy. In this paper, we compare and verify results of the plasma‐chemical model with different values of the background neutral atmosphere parameters—experimental data on the temperature and concentration of neutral components from the Aura satellite and the Mass Spectrometer Incoherent Scatter Radar (MSIS) model results. These two sets of neutral atmospheric input data were used to simulate the response of the lower ionosphere to solar flares that occurred on 9–11 June 2014. Verification is carried out according to the VLF‐signals propagation from GBZ, GQD, DHO, TBB, and ICV transmitters to the Mikhnevo Geophysical Observatory and A118 station. It is shown that in 80% of cases the measurements of the Aura satellite make it possible to describe the state and dynamics of the medium more correctly than the results of the MSIS model. Key Points: Simulation of the ionospheric response to X‐ray flares using Aura satellite data is more accurate than using the Mass Spectrometer Incoherent Scatter Radar model resultsAccounting for the ion concentration when calculating very low frequency (VLF)‐signals propagation improved agreement with experiment in 38% of the casesVerification of the model results was carried out on 10 different VLF‐paths with use of data from the A118 station and Mikhnevo Geophysical Observatory [ABSTRACT FROM AUTHOR]

The geospace plume, referring to the combined processes of the plasmaspheric and the ionospheric storm‐enhanced density (SED)/total electron content (TEC) plumes, is one of the unique features of geomagnetic storms. The apparent spatial overlap and joint temporal evolution between the plasmaspheric plume and the equatorial mapping of the SED/TEC plume indicate strong magnetospheric‐ionospheric coupling. However, a systematic modeling study of the factors contributing to geospace plume development has not yet been performed due to the lack of a sufficiently comprehensive model including all the relevant physical processes. In this paper, we present a numerical simulation of the geospace plume in the 31 March 2001 storm using the Multiscale Atmosphere‐Geospace Environment model. The simulation reproduces the observed linkage of the two plumes, which, we interpret as a result of both being driven by the electric field that maps between the magnetosphere and the ionosphere. The model predicts two velocity channels of sunward plasma drift at different latitudes in the dusk sector during the storm main phase, which are identified as the sub‐auroral polarization stream (subauroral polarization streams (SAPS)) and the convection return flow, respectively. The SAPS is responsible for the erosion of the plasmasphere plume and contributes to the ionospheric TEC depletion in the midlatitude trough region. We further find the spatial distributions of the magnetospheric ring current ions and electrons, determined by a delicate balance of the energy‐dependent gradient/curvature drifts and the E × B drifts, are crucial to sustain the SAPS electric field that shapes the geospace plume throughout the storm main phase. Key Points: The first whole geospace simulation to demonstrate coherent storm‐time evolution of plasmaspheric and total electron content (TEC) plumesThe model demonstrates plasmasphere erosion and TEC depletion by the subauroral polarization streams (SAPS)SAPS is sustained by magnetospheric ion and electron distributions formed by a delicate balance of energy‐dependent and E × B drifts [ABSTRACT FROM AUTHOR]

Earthquake (EQ) prediction is an essential topic in recent geophysics and disaster management. In order to explain EQ precursory phenomena, especially the ionospheric perturbations, a few hypotheses have been proposed as the mechanism of lithosphere‐atmosphere‐ionosphere coupling. The most reliable seems to be the atmospheric gravity wave (AGW) hypothesis, because a lot of subionospheric very low frequency (VLF) observations and also ground surface measurements support this hypothesis. However, no direct signature of AGW activity in the middle atmosphere has been yet obtained. So in this study, we have used the ERA5 reanalysis data to investigate the tempo‐spatial evolution of stratospheric AGW activity before a recent huge seismic event of the 2016 Kumamoto EQ on 15 April (Mw7.2). It was found that the AGW potential energy is enhanced significantly during about 1 week before the EQ. The enhancement was mainly distributed around the EQ epicenter and expanded toward eastern Japan. The AGW activity was highest around 11 April, then it calmed down and returned to the background level right before the EQ. These results were compared with the subionospheric VLF observations during the same period, and then the tempo‐spatial evolutions of the lower ionospheric perturbation are found to be very consistent with the stratospheric behavior. The present paper reports on the first finding that the abnormal AGW activity in the stratosphere is detected before a major EQ, and the coincidence of stratospheric AGW activity with VLF subionospheric perturbations provides further support to the AGW hypothesis of the lithosphere‐atmosphere‐ionosphere coupling process. Key Points: This paper reports on the first investigation on the stratospheric AGW activity before a major earthquake (EQ)The stratospheric AGW activity was significantly enhanced around the EQ epicenter during about 1 week prior to the EQThe tempo‐spatial coincidence of AGW activity and VLF subionospheric perturbations further supports the AGW hypothesis of LAIC process [ABSTRACT FROM AUTHOR]

Geomagnetic disturbances caused by solar storms have the potential to create large‐scale geomagnetically induced currents in long conductors at the Earth's surface. These may disrupt the operation of electric power grids and cause blackouts. Ocean‐continent boundary regions are of particular concern because of the sharp contrast between the higher ocean electrical conductivity compared to the lower continental conductivity. This contrast may generate high‐amplitude geoelectric fields and cause power grids in coastal regions to be more vulnerable to space weather hazards. Previously, analytical calculations were used to estimate geomagnetically induced currents at ocean‐continent boundaries. However, for the analytical equations to be solvable, the physics and geometries of the problem were simplified. As a result of these simplifications and due to a lack of published measurements examining this issue in coastal regions, it is difficult to know for sure whether there are unique hazards to electric power grids at ocean‐continent boundaries. In this paper, a grid‐based, time domain modeling approach is used to solve the complete Maxwell's equations, which permits accommodation of (1) the complete physics of the propagating electromagnetic fields from disturbed ionospheric currents through the air and into the lithosphere and even into the ocean via the skin effect and (2) more realistic coastal geometries. Using this more robust approach, in the variety of scenarios studied in this paper, only a local enhancement of the electric fields was observed, which are expected to only pose potential hazards to power grids only a local scale near ocean‐continent boundaries. Key Points: Electromagnetic fields across ocean‐continent boundaries are solved via FDTD while accounting for a finite‐depth ocean and topographyAnalytical approaches are found to provide a close approximation to the more realistic FDTD results for the range of studied scenariosSurface electric field enhancements are highly localized along the coast, even for low frequencies and shallow oceans [ABSTRACT FROM AUTHOR]

We present the first observational determination of statistical limits on the rise of equatorial plasma bubbles as a function of solar flux. We analyzed in situ electron density data collected onboard the Communications/Navigations Outage Forecasting System (C/NOFS) satellite to characterize the distribution of peak altitudes of equatorial ionospheric plasma bubbles. We first describe our algorithm for detecting ionospheric irregularities within the observations and then use a series of statistical simulations to identify and compensate for the sampling biases inherent in observations from a single satellite in a low‐inclination elliptical orbit. The simulations also confirmed that space‐based orbital platforms such as the C/NOFS satellite undersample the existing irregularities in the ionosphere and provide a measure of the satellite's inefficiency in observing those naturally occurring irregularities. In deducing the variation of the peak‐height distributions of the irregularities with solar activity, we find that the median maximum height of the bubbles increases linearly from about 490 km at the solar minimum (2008) to 740 km during the (2014) solar maximum in the longitude sector 80°W–10°E. The results will be valuable for the development of improved scintillation mapping models for both real‐time and postprocessing applications. We also confirm our observational findings with modeling results from a physics‐based model, allowing us to identify field‐line‐integrated Pedersen conductance as the key determinant of terminal bubble altitude: a bubble will cease to rise further when the conductance inside the bubble is equal to that of the background ionosphere. Plain Language Summary: In this study, we take a new approach of data analysis in determining the peak heights at the magnetic equator, or the peak apex heights, of the equatorial ionospheric irregularities from the ion‐density observations made by a sensor onboard Communications/Navigations Outage Forecasting System (C/NOFS) satellite. The variation of these peak‐height distributions with solar activity is studied over the years of operation of C/NOFS mission (2008–2014). We use numerical simulations to confirm the approach used in determining the peak‐height distributions from the observed height distributions and, in the process, discovered that the efficiency of C/NOFS for detecting equatorial irregularities was less than 50% throughout the solar cycle. We find that the median maximum height of the equatorial ionospheric irregularities increases linearly from about 490 km at the solar minimum to 740 km during solar maximum in the longitude sector 80°W–10°E. The results will be valuable for the development of space‐weather forecasting models. We also confirm the observational findings presented in this paper with modeling results from a physics‐based model. In the model, we identify conductance as the key parameter controlling the peak heights of the equatorial irregularities. Key Points: Quantitative distributions of equatorial plasma bubble peak apex heights as a function of solar fluxField‐line‐integrated Pedersen conductance as the key determinant of terminal bubble altitude of the equatorial plasma bubbles using the physics‐based modelEstimation of the meridional extent of scintillation activity as a function of solar flux [ABSTRACT FROM AUTHOR]

The ionosphere exhibits enhanced semidiurnal lunitidal (M2) perturbations during sudden stratospheric warming (SSW) events, of which the manifestation and mechanism are not well documented and understood. This paper studies the latitudinal and inter‐hemispheric variations of the ionospheric M2 perturbations during the 2021 major Arctic SSW with total electron content (TEC) data in the American, Asian‐Australian, and African‐European sectors. Results show that the M2 perturbations in the three sectors all enhanced during the SSW, with the strongest M2 perturbations in the Northern and Southern Hemispheres at about 10°∼15°N and 25°∼30°S geomagnetic latitudes, respectively. Also, clear M2 perturbations in the three sectors all extended to middle latitudes during the SSW only in the Southern Hemisphere. Comparatively, the inter‐hemispheric asymmetry of the magnitude of the M2 perturbations during the SSW was more complex. The inter‐hemispheric asymmetry of the crest locations of the low‐latitude M2 perturbations in the three sectors during the SSW was probably due to the F‐region meridional wind effect on the diffusion process of the low‐latitude plasma fountain. Meanwhile, the longitudinal differences were also obvious. The TEC M2 amplitude in the American sector was obviously larger than those in the Asian‐Australian and African‐European sectors during the SSW. The M2 characteristics in the southern middle latitude in the American sector were significantly different from those in the other two sectors during the SSW, which may be related to the differences of the combined effects of the local wind and geomagnetic configuration. In addition, a moderate geomagnetic activity occurred during the 2021 SSW, which brings difficulties to the analysis of the SSW‐related TEC variation. Key Points: The latitudinal differences of the ionospheric semidiurnal lunitidal (M2) perturbations during the 2021 sudden stratospheric warming were studied with total electron contentThe low‐latitude M2 perturbations exhibited similar latitudinal and inter‐hemispheric variations in the American, Asian and African sectorsThe M2 perturbation in the southern middle latitude was obviously different in the American from those in the Asian and African sectors [ABSTRACT FROM AUTHOR]

This paper uses a regional simulation of plasma convective instability in the postsunset equatorial ionosphere together with a global atmosphere/ionosphere/plasmasphere GCM (WAM‐IPE) to forecast irregularities associated with equatorial spread F (ESF) for 1–2 hr after sunset. First, the regional simulation is initialized and forced using ionosphere state parameters derived from campaign data from the Jicamarca Radio Observatory and from empirical models. The irregularities produced by these simulations are found to be quantitatively similar to those observed. Next, the aforementioned state parameters are replaced with parameters from WAM‐IPE, and the resulting departures between the simulated and observed irregularities are noted. In one of five cases, the forecast failed to accurately predict ESF irregularities due to the late reversal of the zonal thermospheric winds. In four of five cases, significant differences between the observed and predicted prereversal enhancement (PRE) of the background vertical drifts resulted in degraded forecast accuracy. This highlights the need for improved PRE forecasting in the global‐scale model. Key Points: Equatorial ionospheric F‐region stability can be reproduced quantitatively using a regional data‐driven direct numerical simulation (DNS)DNS model runs become forecasts when initialized and driven with parameters from a comprehensive GCM such as Whole Atmosphere Model with Ionosphere, Plasmasphere, ElectrodynamicThe post‐sunset background vertical plasma drift and zonal thermospheric wind profile are the most critical forecast parameters [ABSTRACT FROM AUTHOR]

The morphology of the auroral oval is an important geophysical parameter that helps to further understand the solar wind‐magnetosphere‐ionosphere coupling process. However, it is still a challenging task to automatically obtain auroral poleward and equatorward boundaries completely and accurately. In this paper, a new model based on the deep feature and dual level set method is proposed to extract the auroral oval boundaries in the images acquired by the Ultraviolet Imager (UVI) onboard the Polar spacecraft. With the deep feature extracted by the convolutional neural network (CNN), the corresponding deep feature energy functional is constructed and incorporated into the variational segmentation framework. The dual level set method is implemented to extract the accurate poleward and equatorward boundaries with the gradient descent flow. The experimental results on the test data set demonstrate that this model can extract complete auroral oval contours that are consistent well with annotations and owns higher accuracy compared with the previously proposed methods. Comparison between the extracted auroral boundaries and the precipitating boundaries determined by Defense Meteorological Satellite Program (DMSP) SSJ precipitating particle data validates that the proposed method is trustworthy to capture the global morphology of the auroral ovals. Key Points: We adopt the convolutional neural network to extract the deep features from Polar/UVI auroral imagesThe deep feature energy functional is constructed and incorporated into the variational framework to determine the auroral boundariesThe extracted boundaries agree well with expert annotations and conform to the DMSP precipitating particle data [ABSTRACT FROM AUTHOR]

A stable auroral red (SAR) arc is an aurora with a dominant 630 nm emission at subauroral latitudes. SAR arcs have been considered to occur due to the spatial overlap between the plasmasphere and the ring‐current ions. In the overlap region, plasmaspheric electrons are heated by ring‐current ions or plasma waves, and their energy is then transferred down to the ionosphere where it causes oxygen red emission. However, there have been no study conducted so far that quantitatively examined plasma and electromagnetic fields in the magnetosphere associated with SAR arc. In this paper, we report the first quantitative evaluation of conjugate measurements of a SAR arc observed at 2204 UT on 28 March 2017 and investigate its source region using an all‐sky imager at Nyrölä (magnetic latitude: 59.4°N), Finland, and the Arase satellite. The Arase observation shows that the SAR arc appeared in the overlap region between a plasmaspheric plume and the ring‐current ions and that electromagnetic ion cyclotron waves and kinetic Alfven waves were not observed above the SAR arc. The SAR arc was located at the ionospheric trough minimum identified from a total electron content map obtained by the GNSS receiver network. The Swarm satellite flying in the ionosphere also passed the SAR arc at ~2320 UT and observed a decrease in electron density and an increase in electron temperature during the SAR‐arc crossing. These observations suggest that the heating of plasmaspheric electrons via Coulomb collision with ring‐current ions is the most plausible mechanism for the SAR‐arc generation. Plain Language Summary: A stable auroral red (SAR) arc is an aurora with an optical red emission at latitudes slightly lower than the auroral zone. SAR arcs have been considered to occur due to the spatial overlap between the low‐energy plasmaspheric electrons and the high‐energy ring‐current ions. In the overlap region, plasmaspheric electrons are heated by ring‐current ions or plasma waves, and their energy is then transferred down to the upper atmosphere to cause the red emission. However, there have been no study conducted so far that quantitatively examined plasma and electromagnetic fields in the magnetospheric source region of SAR arcs. In this paper, we report the first quantitative evaluation of a SAR arc using an all‐sky imager at Nyrölä, Finland, and the Arase satellite. The Arase observation shows that the SAR arc appeared in the overlap region between a plasmaspheric plume and the ring‐current ions in the inner magnetosphere. The electromagnetic waves associated with the SAR arc were not observed. These observations suggest that the heating of plasmaspheric electrons by ring‐current ions is the most plausible mechanism for the SAR‐arc generation. This result provides direct evidence of the previous theoretical expectation on the generation mechanism of red aurora at lower latitudes. Key Points: The conjugate measurements of a detached SAR arc on 28 March 2017 are analyzed using observations from the Arase satelliteSwarm and GNSS‐TEC data show that the electron density decreased and the electron temperature increased in the ionosphere above the SAR arcThe observed plasmas and electromagnetic fields suggest that Coulomb collision is the most plausible mechanism for the SAR‐arc generation [ABSTRACT FROM AUTHOR]

In this paper, we first investigate the effects of a postevening Weddell Sea Anomaly (WSA)‐like plasma patch on a southwestward propagating medium‐scale traveling ionospheric disturbance (MSTID) over Xinglong, China (40.4°N, 117.6°E; Mlat. ~30.4°N). We found that some of this kind of WSA‐like plasma patches moved northward from the equatorial ionization anomaly (EIA) regions to midlatitudes, as they traveled westward into China. During 2012–2016, these plasma patches frequently occurred during the period of May–August with a monthly occurrence rate of over 20%, causing nighttime plasma density enhancements (NPDEs) in midlatitudes of China. Over 50% of these structures were accompanied by concurrent MSTID and Es. We propose that an intense polarization electric field (PEF) associated with an MSTID/Es from the more northern regions of EIA could frequently drive these plasma patches poleward. On the night of 27 June 2014 (Kp < 2−), either an MSTID and an intense Es layer (the maximum foEs >12 MHz) occurred at the onset of the poleward motion of a WSA‐like plasma patch, lending credence to this possibility. The observed plasma patch was pushed poleward to interact with the observed MSTID, causing some poleward extending C‐shaped airglow depletions/enhancements of the MSTID in a transition region where the ionosphere changed from a collapse region to an uplifted one. We attributed those poleward extending airglow depletions to the interaction between the MSTID and the plasma patch because of the secondary gradient drift instability and those of airglow enhancements to the redistribution of plasma. Plain Language Summary: This study found that postevening Weddell Sea Anomaly (WSA)‐like patchy plasma structures frequently occurred during May–August of 2012–2016 in the East Asian regions. Among these plasma patches, a small portion moved northward from the equatorial ionization anomaly (EIA) regions to midlatitudes, as they moved westward into China. On the magnetically quiet night of 27 June 2014 (Kp < 2−), such a plasma patch already moved westward and poleward to the midlatitude region of China, affecting the evolution/propagation of a southwestward propagating medium‐scale traveling ionospheric disturbance (MSTID). This paper, therefore, investigates the interaction between a midlatitude MSTID and a poleward moving WSA‐like plasma patch. We mainly investigate the possible physical mechanisms behind this kind of plasma patches and how they could affect the propagation/evolution of MSTIDs over midlatitudes of China. Key Points: Poleward extending C‐shaped airglow depletions/enhancements of MSTID were caused by interaction between an MSTID and a postevening WSA‐like plasma patchWestward and northward moving WSA‐like plasma patches frequently caused nighttime midlatitude plasma enhancements over Xinglong, ChinaAn intense PEF associated with an MSTID/Es from the more northern regions of EIA could frequently cause poleward motion of the WSA‐like plasma patch [ABSTRACT FROM AUTHOR]

This paper evaluates the role of magnetic reconnection in the dayside ionosphere of Mars in the collisional regime and presents some relevant data from the MAVEN (Mars Atmosphere and Volatile EvolutioN) mission. Magnetic reconnection is an important process operating in the solar corona, planetary magnetospheres, and astrophysical plasmas, but most of the literature focuses on collisionless plasmas. However, at Mars the reconnection should often occur in regions where collisions are important. Mars does not have a large-scale global magnetic field; however, Mars has locally large magnetic fields associated with remnant crustal magnetization, particularly in the highland regions of the southern hemisphere. The crustal fields provide a "target" for reconnection both in the ionosphere and in the magnetotail of Mars. The current paper emphasizes the dayside ionosphere. Magnetic reconnection is associated with topological changes in the magnetic field and is also a source of energy for the plasma and can thus affect the ionospheric dynamics and ion loss at Mars. Both theoretical concepts and MAVEN particle and field data from several instruments are considered in this paper. [ABSTRACT FROM AUTHOR]

Electron injections are critical processes associated with magnetospheric substorms, which deposit significant electron energy into the ionosphere. Although wave scattering of <10 keV electrons during injections has been well studied, the link between magnetotail electron injections and energetic (≥100 keV) electron precipitation remains elusive. Using conjugate observations between the Electron Loss and Fields Investigation (ELFIN) and Magnetospheric Multiscale (MMS) missions, we present evidence of tens to hundreds of keV electron precipitation to the ionosphere potentially driven by kinetic Alfvén waves (KAWs) associated with magnetotail electron injections and magnetic field gradients. Test particle simulations adapted to observations show that dipolarization‐front magnetic field gradients and associated ∇B drifts allow Doppler‐shifted Landau resonances between the injected electrons and KAWs, producing electron spatial scattering across the front which results in pitch‐angle decreases and subsequent precipitation. Test particle results show that such KAW‐driven precipitation can account for ELFIN observations below ∼300 keV. Plain Language Summary: Energetic electron precipitation from magnetospheric injections has a major impact on magnetosphere‐ionosphere coupling. This energy deposition is largely in the form of electron precipitation driven by wave‐particle interactions in the magnetotail. Although wave‐driven precipitation with energies less than approximately ∼10 keV has been studied extensively, the link between energetic electron precipitation (≥∼100 keV) and electron injections remains elusive. Combining observations and simulations, this paper provides evidence of such precipitation driven by kinetic Alfvén waves, which have been previously observed to be ubiquitously associated with magnetospheric electron injections but have not been considered as an important driver for precipitation of such electrons. Key Points: Conjugate Electron Loss and Fields Investigationand Magnetospheric Multiscale observations reveal evidence of tens to hundreds of keV electron precipitation associated with kinetic Alfvén wavesDipolarized magnetic field gradients and perpendicular magnetic drifts allow Landau resonance between waves and injected electronsAdiabatic transport and spatially varying E × B and grad‐B drifts induce perpendicular momentum scattering and electron losses [ABSTRACT FROM AUTHOR]

Ground‐based facilities make an important contribution to our understanding of the magnetospheric system. Coordination with ground‐based facilities has been embedded within the Cluster mission since before launch, and this has given rise to a large number of studies which have exploited data from both Cluster and one or more of the diverse range of ground‐based instrumentation that have been available during the mission timespan. In this paper, we review the advances that have been made to our understanding of the magnetosphere‐ionosphere system, in which insight has been enhanced by combining ground‐based observations with observations from Cluster. Topics covered span from the bow shock to the magnetotail and down to the aurora. Key Points: Space‐ and ground‐based measurements strongly complement each other, and combining the two further enhances the insight they provideStudies exploiting ground‐based observations have been a key aspect of the Cluster mission, yielding approximately 100 papersWe review these studies, covering the full range of solar wind/magnetosphere/ionosphere coupling and dynamics [ABSTRACT FROM AUTHOR]

Measurement of the dense cold thermal plasma in planetary ionospheres via orbiting spacecraft is challenging because ion energies are small (0–4 eV), densities can vary by four orders of magnitude, composition varies with altitude, spacecraft charging varies in time and must be measured very accurately, and instrumental effects (e.g., detector dead‐time and background) can be significant. The SupraThermal And Thermal Ion Composition instrument team has recently released a new set of data products that contain density moments of the primary ion species at Mars, including those derived at periapsis, subject to the full suite of calibration factors required. This article discusses the challenges associated with deriving these densities and provides examples of the key caveats that users of the data should be aware of. A preliminary statistical study of this new data set focuses on the structure and variability of Mars' ionosphere, demonstrating that solar zenith angle effects, the crustal magnetic fields, and electron precipitation on the nightside, drive the strongest structural features, consistent with photochemical theory and previous studies. Dayside ionospheric density profiles are highly repeatable below altitudes of 200 km, marking the region where photochemistry and collisions dominate. In the upper dayside ionosphere (altitudes >300–400 km) changes in the solar wind dynamic pressure on timescales of Mars Atmosphere and Volatile EvolutioN's orbit (hr) drive the largest (factors of 1–3) variability in ionospheric density. In contrast variability in ionospheric density peaks between 150 and 250 km altitude on the nightside (factors of 1–2), consistent with electron precipitation driving ionization in this region. Plain Language Summary: Making accurate measurements of the ions present in planetary atmospheres via orbiting spacecraft is difficult due to the large changes in conditions encountered throughout spacecraft orbits. This paper describes the techniques implemented to address the full array of difficulties associated with measuring planetary ions at Mars using measurements made by the SupraThermal And Thermal Ion Composition instrument. These techniques are used to calculate ion densities in Mars' atmosphere and we perform a preliminary investigation of this new data set, revealing several important characteristics of how ions behave in Mars' atmosphere. In particular, we find that on the dayside of the planet, there is a transition region between the upper and lower atmosphere where planetary ions behave differently. Above this transition region planetary ions are influenced by forces that arise in the space environment about Mars, while below this transition region, planetary ions are well shielded from these effects. Our results provide insight into the processes that shape Mars' atmosphere and tell us how energy is transported through the Mars system. Key Points: The calibration challenges and caveats of ion densities derived from SupraThermal And Thermal Ion Composition observations are discussedSolar zenith angle effects and the crustal magnetic fields drive the strongest structural features in ionospheric densityA variety of processes drive ionospheric variability throughout different regions of the Martian ionosphere [ABSTRACT FROM AUTHOR]

The ionosphere experiences strong diurnal and seasonal changes. The longitudinal variations of electron density (Ne) in the ionosphere at the middle latitudes also show strong diurnal and seasonal changes. In this paper, we use in situ Ne measurements from the DEMETER satellite and electron density profiles retrieved from the COSMIC data to study the local time (LT) and seasonal dependence of the longitudinal variations of topside Ne at middle latitudes during 2007–2009. With regard to the diurnal trend, the reversal phase of longitudinal peaks/valleys of topside Ne with a 12 hr interval occurred in less than half of the cases, and there were less cases with eastward phase shift of the longitudinal variations of topside Ne with LT in winter than those in other seasons. The seasonal trends of transition longitudes of topside Ne might be westward from winter to summer and eastward from summer to winter in the daytime and in the opposite direction at night in both hemispheres in some cases and sometimes they were located within 20° of longitude at 52°N in other cases. The longitudinal peaks/valleys of hmF2 and/or NmF2 and the longitudinal peaks/valleys of topside Ne were within 30° of longitude in most cases at all local times, in all seasons, and in both hemispheres. Exceptions to this were independent of season or LT. Key Points: Reversal phase of longitudinal peaks/valleys of topside Ne with a 12 hr interval occurred in less than half of the casesLess cases with eastern phase shift of longitudinal variations of topside Ne during a day in winter than those in other seasonsSeasonal trends of the transition longitudes of topside Ne in the daytime and at night were in the opposite direction in both hemispheres [ABSTRACT FROM AUTHOR]

Based on the data of magnetic field and electronic density of SWARM, and Total Electron Content, this paper screened six typhoons that died out on the sea. We studied how typhoons affected the ionosphere when they died out without the influence of topography and other factors. The statistical results show that during the extinction of typhoons, the magnetic disturbances all appeared, and the electron density disturbances were observed in five typhoons. The frequency of the electronic density disturbance is slightly higher than the magnetic disturbance. The range of the electronic density disturbance is mostly five or six hundred kilometers and relatively concentrated. The range of the magnetic disturbance can reach about two thousand kilometers and is relatively scattered. Our results indicate that the extinction of typhoon may affect the ionosphere through acoustic gravity waves (AGWs). The AGWs is not generated by friction, but may be from the typhoon itself or a new phenomenon when the typhoon dies out. Key Points: Our statistics show that the extinction process of typhoon on the sea can cause ionospheric disturbanceThe amplitude of magnetic disturbance is more obvious than disturbance of electron densityThe magnetic disturbance and disturbance of electron density in ionosphere may be caused by acoustic gravity waves [ABSTRACT FROM AUTHOR]

The ionospheric variabilities are influenced by various drivers, and quantifying their relative contributions is beneficial to ionospheric modeling and ionospheric forecasting, but this is still a significant challenge in the ionospheric community. In present paper, the global ionosphere maps, solar indices, solar wind and interplanetary parameters, geomagnetic indices, and lower atmospheric parameters from 2001 to 2021 are collected, and the Liang‐Kleeman information flow is used to measure the amount of information exchanges from external drivers to ionospheric TEC at different geographical regions, and the normalized Liang‐Kleeman information flow is adopted to quantify their relative information contributions. The results show that the geographical distribution of information flow has prominent hemispheric asymmetry, and overall, the information flows from solar indices (solar wind and interplanetary parameters, geomagnetic indices) to ionospheric TEC in the northern hemisphere are larger than those in the southern hemisphere. Ionospheric self‐influence (i.e., internal processes of the ionosphere) has the largest information contribution to ionospheric TEC, accounting for about 46.78%. Solar activity manifested as P index has the second largest contribution (nearly 35.29%). Solar wind and interplanetary activity represented as IMF magnitude contributes around 2.94%, geomagnetic activity characterized as ASYH index accounts for about 1.52%, lower atmospheric forcing contributes nearly 7.75%, and noise accounts for approximately 5.73%. What's more, the performances of the IRI‐2020 model are evaluated from the perspective of information flow. Key Points: The Liang‐Kleeman information flow is used to measure the rates of information exchanges from external drivers to ionospheric TECThe geographical distribution of information flow shows a prominent hemispheric asymmetryIonospheric self‐influence has the largest information contribution to ionospheric TEC, followed by solar activity [ABSTRACT FROM AUTHOR]

This paper presents the response of the equatorial ionosphere to the noon time annular solar eclipse of 15 January 2010. The observation has been made using a Digisonde, Meteor Wind Radar, and Proton Precession Magnetometer over Trivandrum, (8.5oE; 77oN; dip lat 0.5oN), a geomagnetic dip equatorial station in India. It has been found that the E, F1, and F2 regions of the equatorial ionosphere respond to solar eclipse with different time delays, F1 being responding faster and the E and F2 regions slower. Though there have been studies on the delayed response of F2 region during the solar eclipses, the delayed response of E region is quite unexpected since this region is dominated by the recombination chemistry. The plausible reasons for this have been explored, and it is suggested that the downward diffusion of atomic oxygen plays a major role for the observed phenomenon. Key Points: This paper discusses about the altitudinally varying response of equatorial ionosphere to the annular solar eclipse of 15 January 2010It is intriguing to note that the E region electron densities were found to be responding to the solar flux changes later than those in F1 regionThe plausible reasons behind this are discussed in terms of prevailing neutral and electrodynamics and the diffusion chemistry [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]

The location of the polar cap boundary is typically determined by using low‐orbit satellite measurements in which the boundary is identified by its unique signature of a sharp decrease in energy and particle flux poleward of the auroral oval. In principle, this decrease in precipitating particles should appear as a concomitant sharp change in auroral luminosity. Based on a few events, Blanchard et al. (1995, https://doi.org/10.1029/94JA02631) suggested that a dramatic gradient in redline aurora may also be an indicator of the polar cap boundary. In recent years, advances in capabilities and the deployment of ground‐based all‐sky imagers have ushered in a new era of auroral measurements. Auroral imaging has moved well beyond the capabilities of the instrumentation in the previous study in terms of both spatial and temporal resolution. We now have access to decades of optical data from arrays spanning a huge spatial range, enabling a fresh examination of the relationship between redline aurora, particle precipitation, and the polar cap open closed boundary. In this study, we use data from the Defense Meteorological Satellite Program satellites in conjunction with the University of Calgary's Redline Emission Geospace Observatory (630.0 nm) data to assess the viability of automated detection of the 2‐dimensional polar cap boundary. Our results exhibit good agreement between the optical and particle polar cap boundary and suggest that a luminosity in redline emission could serve as a reasonable proxy for the location of the electron poleward boundary, while providing both high temporal and spatial resolution maps of the open‐closed boundary. Plain Language Summary: Named for its shape, the auroral oval is an oval‐shaped region located near the north and south poles where the aurora is formed. Studying the location of the equatorward and poleward limits of the auroral oval is fundamental to evaluate the dynamics and responses of the near‐Earth space environment. In particular, the poleward boundary corresponds to a region in which Earth's magnetic field transitions from closed (connected to Earth on both sides) to open field lines (connected to the solar wind on one side); because of this topological change in magnetic field, the location of this boundary is fundamental to understand mass and energy transport in the whole magnetosphere‐ionosphere system. While satellite data can determine the exact location of this boundary, they only provide point measurements. Auroral images are capable of detecting the luminosity around this boundary; however, many qualitative studies consider the region immediately poleward of this luminosity as the oval's poleward boundary. In this paper, we analyze the location of this boundary in a case study by using satellite data combined with auroral imaging data. Key Points: Polar cap boundary determined with Defense Meteorological Satellite Program particle data allows for examination of the boundary in optical redline dataWe find good agreement between the auroral boundary determined by auroral luminosity and those determined by particle fluxLuminosity method can be applied during active geomagnetic conditions, providing high spatial and temporal resolution polar cap boundary [ABSTRACT FROM AUTHOR]

Small‐scale magnetic structures have been observed in the induced Martian ionosphere by magnetometers onboard the Mars Global Surveyor (MGS) and the Mars Atmosphere and Volatile EvolutioN (MAVEN) spacecraft. The origin and evolution of these structures remain poorly understood and the goal of the current paper is to better characterize them and their distribution in the dayside ionosphere using Mars Atmosphere and Volatile EvolutioN (MAVEN) data. Mars Atmosphere and Volatile EvolutioN (MAVEN) Langmuir probe data is used to find thermal pressures in the ionosphere. The structures studied range in size from about 20 km up to a couple of hundred km. We constrain our investigation to the northern hemisphere, dayside Martian ionosphere in order to minimize crustal magnetic field interference. Magnetic pressure spikes (and/or field component variations), along with thermal pressure behavior can also sometimes characterize small‐scale structures. In addition to pressures and field components, minimum variance analyses are carried out for each structure in order to help classify them (e.g., horizontal slabs, ionopause‐like structures, flux tube and flux ropes). A "statistical" catalog of properties (pressure, field amplitude, ellipticity, width, etc.) is generated for about 1,000 structures. One conclusion we reached from this survey is that slab‐like features are more likely to be found than rope‐like features for altitudes above 250 km. Key Points: The magnetic barrier connection to the lower ionosphere is exploredWe characterize and examine the evolution of small‐scale magnetic structureMars Atmospheric and Volatile EvolutionN (MAVEN) data is analyzed for magnetic and thermal structure [ABSTRACT FROM AUTHOR]

Super Dual Auroral Radar Network (SuperDARN) ionospheric convection maps are a powerful tool for the study of solar wind‐magnetosphere‐ionosphere interactions. SuperDARN data have high temporal (approximately minutes) and spatial (∼45 km) resolution, meaning that the convection can be mapped on fine time scales that show more detail than the large‐scale changes in the pattern. The Heppner‐Maynard boundary (HMB) defines the low‐latitude limit of the convection region, and its identification is an essential component of the standard SuperDARN convection mapping technique. However, the estimation of the latitude of this boundary is dependent on ionospheric scatter availability. Consequentially it is susceptible to nonphysical variations as areas of scatter in different latitude and local time regions appear and disappear, often due to changing propagation conditions. In this paper, the HMB is compared to an independent field‐aligned current data set from the Active Magnetosphere and Planetary Electrodynamics Response Experiment (AMPERE). A linear trend is found between the HMB and the boundary between the AMPERE Region 1 and Region 2 field‐aligned currents in the Northern Hemisphere, at both solar minimum and solar maximum. The use of this trend and the AMPERE current data set to predict the latitude position of the HMB is found to improve the interpretation of the SuperDARN measurements in convection mapping. Key Points: Scale sizes of the Northern Hemisphere convection (SuperDARN HMB) and FAC region (from AMPERE) are estimated for solar minimum and maximumThe location of the convection and FAC boundaries have a linear relationshipThe latitude of the FAC boundary can be used to improve the estimation of the SuperDARN HMB [ABSTRACT FROM AUTHOR]

This paper presents a detailed model‐data comparative study of the 17 March 2015 geomagnetic storm using the high‐resolution version of the thermosphere‐ionosphere‐electrodynamic general circulation model and the total electron content observations from a dense global navigation satellite system network. Driven by time‐dependent high‐latitude ionospheric convection and auroral precipitation inputs, together with an empirically defined subauroral plasma stream (SAPS) field, our simulation reproduce many observed storm‐related ionospheric phenomena, including large‐scale traveling ionospheric disturbances over Europe, the effects of prompt penetration electric field over South and Central America, and the formation of a storm‐enhanced density (SED) plume across the continental United States. Our simulation results reaffirm a number of important characteristics concerning the SED plume: (1) enhanced background ionospheric density is a necessary but not sufficient condition, and enhanced ion drift is required to form the SED plume; (2) the SAPS flow channel does not directly transport the plasma from midnight to postnoon via dusk to form the SED plume, instead, the SED plume is formed at the equatorward and westward edge of the SAPS channel; and (3) the SED plume appears to subcorotate with respect to the Earth. Plain Language Summary: Storm‐induced ionospheric density variations are a major concern of near‐Earth space environment as they could drastically disrupt satellite navigation and telecommunication systems. Although ionospheric disturbances such as traveling ionospheric disturbances (TIDs) and storm‐enhanced density (SED) are commonly observed during geomagnetic storms, accurate specification of these phenomena remains a great challenge for geospace models. This paper presents a detailed model‐data comparative study of the well‐known March 2015 St. Patrick's Day storm. The simulated TIDs and SED structures from the thermosphere‐ionosphere‐electrodynamic general circulation model (TIEGCM) are compared with the total electron content observations for a dense global navigation satellite system (GNSS) network over Europe, South and Central America, as well as North America. While the model reproduces many observed storm‐related ionospheric features, quantitative differences between the simulation results and the GNSS data indicate that further improvements to the TIEGCM (such as a more realistic, self‐consistent electrodynamic coupling of the ionosphere and magnetosphere) are required in order to meet the challenges of space weather specification and eventually forecast. This study not only serves a meaningful validation of our numerical model but also sheds some new lights on an apparent paradox concerning the formation of the SED plume. Key Points: We have carried out a detailed model‐data comparison as a way to validate the model outputsOur simulation results reproduce many observed features, including traveling ionospheric disturbances and the storm‐enhanced density plumeOur results shed new lights on the formation of the SED plume and its subcorotation motion [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]

The Rice Convection Model (RCM) is an established first‐principles physics model in which the field‐aligned current density is computed by the Vasyliunas equation. Its slow‐flow assumption neglects the inertial term in the MHD momentum equation, which is a major obstacle to using the RCM to model some of the fluid dynamics of bursty bulk flows in the plasma sheet. This paper describes the RCM‐I, which represents an effort to approximately add inertial effects in the RCM by correcting the expression for field‐aligned currents. That inertial current is calculated under the assumption that the mass on each field line is concentrated near the equatorial plane. RCM‐I results are presented with magnetic fields calculated in two different ways: A static statistics‐based model and an MHD code. In both cases, the bubble flow velocities are much smaller and more realistic than those calculated from the traditional RCM. An additional promising feature is that the injection of a low‐entropy bubble produces interchange (braking) oscillations and buoyancy waves that radiate away from the original bubble, forming multiple flow vortices. None of these features could be produced by traditional RCM simulations. Comparison simulations also suggest that gradient and curvature drifts have substantial effect on oscillation of bubbles and tend to damp buoyancy waves. We also note that substantial work will be needed in the future to further improve the pressure distribution by inertializing an anisotropic version of the RCM and to understand the effects of neglecting the magnetosphere‐ionosphere communication time. Plain Language Summary: The plasma sheet is the primary source of the aurora and hot particle population of Earth's magnetosphere. In the traditional theoretical picture of the plasma sheet, particles move slowly sunward in a large‐scale, coherent process called "magnetospheric convection." The Rice Convection Model (RCM) is a first‐principles computer code to describe that large‐scale motion. It has long included particle transport by E × B, gradient and curvature drifts along with high‐resolution coupling with the ionosphere, but its equations have assumed slow flow, and inertial terms have been neglected. However, in recent years, it has become clear that much of the transport in the plasma sheet takes the form of localized bursty bulk flows that move rapidly through a slow‐moving background. Accurate modeling of these flows has proved a huge challenge, because inertial effects must be included along with gradient/curvature drift and high‐resolution coupling to the ionosphere. This paper describes a first attempt to add inertial effects to the RCM in an approximate way. The equations of the new model are derived, and initial results are presented. The simulations now exhibit interchange oscillations and waves, which are often observed when flow bursts hit the inner plasma sheet. Key Points: The RCM is improved to include inertial effects by modifying the fundamental M‐I coupling equationThe new simulation results exhibit interchange oscillations of a plasma sheet bubble and buoyancy waves in the wake of the bubbleMultiple pairs of flow vortices may emerge associated with one bubble injection [ABSTRACT FROM AUTHOR]

Multifrequency scintillation observations show that strong amplitude scintillations on very high frequency (VHF) signals are accompanied by weak L‐band scintillations near the dip equator and strong L‐band scintillations in equatorial ionization anomaly (EIA) region. For several decades this has been attributed to higher ambient plasma density in the EIA region. Recent work suggests that occurrence of stronger L‐band scintillations in the EIA region requires that the intermediate‐scale (~100 m to few km) ionospheric irregularity spectrum in this region be significantly shallower than that in the equatorial region. However, this has not been established so far. Signal frequency dependence of amplitude scintillations is characterized by the frequency exponent that determines the dependence on signal frequency of the S4 index: the standard deviation of normalized intensity fluctuations. In this paper, theoretical calculations of the frequency exponent for VHF and L‐band signals are carried out using different irregularity spectra and compared with observations in order to characterize power‐law irregularity spectra in different latitude zones. Intermediate‐scale irregularities in the equatorial and low‐latitude region, which are aligned with the geomagnetic field, are described by a two‐dimensional model. Model simulations show that the frequency exponent derived from S4 indices for VHF and L‐band signals is determined only by the characteristics of the irregularity spectrum and hence can be utilized to identify the nature of the power‐law irregularity spectrum. Frequency exponents computed using VHF and L‐band observations near the dip equator and EIA regions show distinct patterns, which clearly indicate steep and shallow irregularity spectra, respectively, in these regions. Key Points: Simulated scintillations show that signal frequency dependence of S4 is determined by irregularity spectrum, not ambient plasma densityFrequency exponent for VHF and L‐band indicates steeper power‐law irregularity spectrum near dip equator than at 10°N geomagnetic latitudeShallow irregularity spectrum found in equatorial ionization anomaly region is critical for strong L‐band scintillations recorded there [ABSTRACT FROM AUTHOR]

The ionospheric electron temperature is important for determining the neutral/photochemical escape rate from the Martian atmosphere via the dissociative recombination of O2+. The Langmuir Probe and Waves instrument onboard MAVEN (Mars Atmosphere and Volatile EvolutioN) measures electron temperatures in the ionosphere. The current paper studies electron temperatures in the dayside for two regions where (1) crustal magnetic fields are dominant and (2) draped magnetic fields are dominant. Overall, the electron temperature is lower in the crustal‐field regions, namely, the strong magnetic field region, which is due to a transport of cold electrons along magnetic field lines from the lower to upper atmosphere. The electron temperature is also greater for high solar extreme ultraviolet conditions, which is associated with the local extreme ultraviolet energy deposition. The current models underestimate the electron temperature above 250‐km altitude in the crustal‐field region. Electron heat conduction associated with a photoelectron transport in the crustal‐field regions is altered due to kinetic effects, such the magnetic mirror and/or ambipolar electric field because the electron mean free path exceeds the relevant length scale for electron temperature. The mirror force can affect the electron and heat transport between low altitudes, where the neutral density and related electron cooling rates are the greatest, and high altitudes, while the ambipolar electric field decelerates the electron's upward motion. These effects have not been included in current models of the electron energetics, and consideration of such effects on the electron temperature in the crustal‐field region should be considered for future numerical simulations. Key Points: At high altitudes, the electron temperature is lower for the crustal‐field region than for the IMF‐dominated draped regionsThe low temperatures for the crustal‐field region are due to a heat transport along the field lines between lower and upper atmosphereKinetic effects such the magnetic mirror and the ambipolar electric field may affect the heat transport along crustal magnetic field lines [ABSTRACT FROM AUTHOR]