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To improve Thermosphere–Ionosphere modeling during disturbed conditions, data assimilation schemes that can account for the large and fast-moving gradients moving through the modeled domain are necessary. We argue that this requires a physics based background model with a non-stationary covariance. An added benefit of using physics-based models would be improved forecasting capability over largely persistence-based forecasts of empirical models. As a reference implementation, we have developed an ensemble Kalman Filter (enKF) software called Thermosphere Ionosphere Data Assimilation (TIDA) using the physics-based Coupled Thermosphere Ionosphere Plasmasphere electrodynamics (CTIPe) model as the background. In this paper, we present detailed results from experiments during the 2003 Halloween Storm, 27–31 October 2003, under very disturbed (Kp = 9) conditions while assimilating GRACE-A and B, and CHAMP neutral density measurements. TIDA simulates this disturbed period without using the L1 solar wind measurements, which were contaminated by solar energetic protons, by estimating the model drivers from the density measurements. We also briefly present statistical results for two additional storms: September 27 – October 2, 2002, and July 26 – 30, 2004, to show that the improvement in assimilated neutral density specification is not an artifact of the corrupted forcing observations during the 2003 Halloween Storm. By showing statistical results from assimilating one satellite at a time, we show that TIDA produces a coherent global specification for neutral density throughout the storm – a critical capability in calculating satellite drag and debris collision avoidance for space traffic management.

To understand the global response of thermospheric‐ionospheric (TI) parameters to variations in solar irradiance measurements from the Global‐Scale Observations of the Limb and Disk (GOLD) ultraviolet imaging spectrograph, solar radio flux F10.7, predictions from the Coupled Thermosphere Ionosphere Plasmasphere electrodynamics (CTIPe) model, and International Global Navigation Satellite System Service total electron content maps (TEC) have been used. Various parameters such as GOLD O/N2, O2, and the nighttime peak electron density (Nmax) have been compared with the CTIPe model simulations. The GOLD observed Nmax shows a number of significant features including a winter anomaly and an equatorial ionization anomaly. The comparison with solar proxies showed that the GOLD QEUV correlates very well with the EUV observations compared to the F10.7 index. The study also examined the relationship between the solar proxies and Nmax on different time scales and found that Nmax responded significantly to QEUV at both medium‐ and long‐term timescales. Furthermore, a low correlation between Nmax in the equatorial region and solar proxies was found. A delayed ionospheric TEC response against solar flux variations within the 27‐day solar rotation was investigated. This ionospheric delay of TEC with respect to solar flux was observed to be less than 1 day, which was reproduced in model simulations. The current study has shown that the GOLD observations can be used to investigate the delayed ionospheric response and to gain a better understanding of the influence of solar activity on the TI system. Key Points: Ionospheric‐thermospheric parameters observed by Global‐Scale Observations of the Limb and Disk (GOLD) ultraviolet imaging spectrograph are compared with Coupled Thermosphere Ionosphere Plasmasphere electrodynamics model simulationsThe delayed ionospheric response against the solar flux is less than 1 day at the time scale of the 27 days solar rotation periodThe integrated 1–45 nm solar energy flux values observed by GOLD correlate well with the F10.7 and EUV [ABSTRACT FROM AUTHOR]

On 3 February 2022, at 18:13 UTC, SpaceX launched and a short time later deployed 49 Starlink satellites at an orbit altitude between 210 and 320 km. The satellites were meant to be further raised to 550 km. However, the deployment took place during the main phase of a moderate geomagnetic storm, and another moderate storm occurred on the next day. The resulting increase in atmospheric drag led to 38 out of the 49 satellites reentering the atmosphere in the following days. In this work, we use both observations and simulations to perform a detailed investigation of the thermospheric conditions during this storm. Observations at higher altitudes, by Swarm‐A (∼438 km, 09/21 Local Time [LT]) and the Gravity Recovery and Climate Experiment Follow‐On (∼505 km, 06/18 LT) missions show that during the main phase of the storms the neutral mass density increased by 110% and 120%, respectively. The storm‐time enhancement extended to middle and low latitudes and was stronger in the northern hemisphere. To further investigate the thermospheric variations, we used six empirical and first‐principle numerical models. We found the models captured the upper and lower thermosphere changes, however, their simulated density enhancements differ by up to 70%. Further, the models showed that at the low orbital altitudes of the Starlink satellites (i.e., 200–300 km) the global averaged storm‐time density enhancement reached up to ∼35%–60%. Although such storm effects are far from the largest, they seem to be responsible for the reentry of the 38 satellites. Plain Language Summary: On 3 February 2022, at 18:13 UTC, the 38 out of the 49 Starlink satellites reentered the atmosphere during a moderate geomagnetic storm that occurred on current and the next day. The storm‐time enhanced thermospheric density is the cause of an enhanced atmospheric drag effect on the LEO satellites and space debris. In this work, the whole thermosphere neutral mass density conditions are investigated in detail by a combination of the observations and the model simulations. The results show that ∼110%–120% storm‐time increase in the upper thermospheric density (e.g., ∼400–500 km), and the storm‐time density increase reached ∼35%–60% at the orbit altitude of the Starlink satellites, that is, 200–300 km. Although such storm effects are far from the largest, they seem to be responsible for the reentry of the 38 satellites. Key Points: Observations by Swarm‐A (∼438 km) and Gravity Recovery and Climate Experiment Follow‐On (∼505 km) missions show 110% and 120% storm‐time increase in the thermospheric densityBoth empirical and numerical models captured the observed storm‐time density enhancements, however simulated results differ by up to 70%Simulations show that the storm‐time density increase reached 35%–60% at the orbit altitude of the Starlink satellites, that is, 200–300 km [ABSTRACT FROM AUTHOR]

The magnetosphere is a major source of energy for the Earth’s ionosphere and thermosphere (IT) system. Current IT models drive the upper atmosphere using empirically calculated magnetospheric energy input. Thus, they do not sufficiently capture the storm-time dynamics, particularly at high latitudes. To improve the prediction capability of IT models, a physics-based magnetospheric input is necessary. Here, we use the Open Global General Circulation Model (OpenGGCM) coupled with the Coupled Thermosphere Ionosphere Model (CTIM). OpenGGCM calculates a three-dimensional global magnetosphere and a two-dimensional high-latitude ionosphere by solving resistive magnetohydrodynamic (MHD) equations with solar wind input. CTIM calculates a global thermosphere and a high-latitude ionosphere in three dimensions using realistic magnetospheric inputs from the OpenGGCM. We investigate whether the coupled model improves the storm-time IT responses by simulating a geomagnetic storm that is preceded by a strong solar wind pressure front on August 24, 2005. We compare the OpenGGCM-CTIM results with low-earth-orbit satellite observations and with the model results of Coupled Thermosphere-Ionosphere-Plasmasphere electrodynamics (CTIPe). CTIPe is an up-to-date version of CTIM that incorporates more IT dynamics such as a low-latitude ionosphere and a plasmasphere, but uses empirical magnetospheric input. OpenGGCM-CTIM reproduces localized neutral density peaks at ~ 400 km altitude in the high-latitude dayside regions in agreement with in situ observations during the pressure shock and the early phase of the storm. Although CTIPe is in some sense a much superior model than CTIM, it misses these localized enhancements. Unlike the CTIPe empirical input models, OpenGGCM-CTIM more faithfully produces localized increases of both auroral precipitation and ionospheric electric fields near the high-latitude dayside region after the pressure shock and after the storm onset, which in turn effectively heats the thermosphere and causes the neutral density increase at 400 km altitude.

McMurdo station (77.8°S, 166.7°E), locating at the poleward edge of the auroral oval, provides great opportunities for researchers to study the interactions among neutral atmosphere, ionosphere and magnetosphere. More than four years of valuable data have been collected, leading to several new discoveries from the McMurdo lidar campaign. Presented here are the winter temperature tides and their responses to the magnetospheric sources. Winter temperature structures from the lidar observations are also presented for this high southern latitude.

Simulations of the ionospheric response to solar flux changes driven by the 27 d solar rotation have been performed using the global 3-D Coupled Thermosphere Ionosphere Plasmasphere electrodynamics (CTIPe) physics-based numerical model. Using the F10.7 index as a proxy for solar extreme ultraviolet (EUV) variations in the model, the ionospheric delay at the solar rotation period is well reproduced and amounts to about 1 d, which is consistent with satellite and in situ measurements. From mechanistic CTIPe studies with reduced and increased eddy diffusion, we conclude that the eddy diffusion is an important factor that influences the delay of the ionospheric total electron content (TEC). We observed that the peak response time of the atomic oxygen to molecular nitrogen ratio to the solar EUV flux changes quickly during the increased eddy diffusion compared with weaker eddy diffusion. These results suggest that an increase in the eddy diffusion leads to faster transport processes and an increased loss rate, resulting in a decrease in the ionospheric time delay. Furthermore, we found that an increase in solar activity leads to an enhanced ionospheric delay. At low latitudes, the influence of solar activity is stronger because EUV radiation drives ionization processes that lead to compositional changes. Therefore, the combined effect of eddy diffusion and solar activity leads to a longer delay in the low-latitude and midlatitude region. [ABSTRACT FROM AUTHOR]

The ionospheric total electron content (TEC) provided by the International GNSS Service (IGS) and the TEC simulated by the Coupled Thermosphere Ionosphere Plasmasphere Electrodynamics (CTIPe) model have been used to investigate the delayed ionospheric response against solar flux and its trend during the years 2011 to 2013. The analysis of the distinct low-latitude and midlatitude TEC response over 15∘ E shows a better correlation of observed TEC and the solar radio flux index F10.7 in the Southern Hemisphere compared to the Northern Hemisphere. Thus, a significant hemispheric asymmetry is observed. The ionospheric delay estimated using model-simulated TEC is in good agreement with the delay estimated for observed TEC against the flux measured by the Solar Dynamics Observatory (SDO) extreme ultraviolet (EUV) Variability Experiment (EVE). The average delay for the observed (modeled) TEC is 17(16) h. The average delay calculated for observed and modeled TEC is 1 and 2 h longer in the Southern Hemisphere compared to the Northern Hemisphere. Furthermore, the observed TEC is compared with the modeled TEC simulated using the SOLAR2000 and EUVAC flux models within CTIPe over northern and southern hemispheric grid points. The analysis suggests that TEC simulated using the SOLAR2000 flux model overestimates the observed TEC, which is not the case when using the EUVAC flux model. [ABSTRACT FROM AUTHOR]

The delayed ionospheric response is analyzed for two well-defined 27-day solar rotation periods in the year 2019 with solar radio flux index F10.7 and Global-scale Observations of the Limb and Disk (GOLD) data, like solar extreme ultraviolet (EUV) flux proxy, O/N2 column density ratio and peak electron density, as well as International Global Navigation Satellite System Service rapid high-rate total electron content (TEC) map data. Although the correlation between GOLD solar EUV flux proxy and TEC is similar to the correlation between F10.7 and TEC, it is shown that the estimated delays based on GOLD data are in much better agreement with recent studies using EUV measurements compared to the delays based on F10.7 data. The GOLD peak electron density correlates well with TEC and allows insight to a local time interval when the ionosphere is not controlled by solar activity changes (17:00 LT to 21:00 LT). The present study investigates the impact of the solar activity (F10.7, GOLD EUV flux proxy) and O/N2 column density ratio on the ionospheric delay for two representative solar rotation periods. The capabilities of GOLD data for future research on the ionospheric response to the 27-day solar rotation period are demonstrated and discussed. These results are crucial information for precise ionospheric models and forecasts. [ABSTRACT FROM AUTHOR]

The recently launched Global‐scale Observations of the Limb and Disk (GOLD) Mission observed prominent quasi‐16‐day oscillations (Q16DOs) in the OI 135.6‐nm nightglow from the equatorial ionization anomaly (EIA) region during 26 November 2018 to 25 January 2019. Meanwhile, the enhanced and concurrent quasi‐16‐day wave (Q16DW) activity was observed in the middle atmosphere by the Sounding of the Atmosphere using Broadband Emission Radiometry (SABER) instrument. This implies that the F‐region ionosphere couples with the lower atmosphere through large‐scale waves. A noteworthy phenomenon was that the Q16DOs displayed an asymmetric behavior between the northern and southern EIA crests. This asymmetry indicates that the ionospheric Q16DOs might be driven by the Q16DW‐modulated tides, which not only modulated the E‐region dynamo winds but also modulated the F‐region field‐aligned neutral winds via vertical penetration, producing the oscillations in plasmas densities. Plain Language Summary: Driving of the short‐term variability in the ionosphere is a crucial scientific question of space physics, which connects the thermosphere and ionosphere to the solar and magnetospheric forcing, as well as terrestrial atmosphere weather. It is also the most challenge part of space weather forecast. Traveling planetary waves (PWs), including the eastward propagating Kelvin waves and westward propagating Rossby waves, are responsible for a large amount of day‐to‐day variability in ionospheric parameters. Our study showed an evidence of the Rossby waves coupling with the nighttime F‐region ionosphere observed by GOLD. More importantly, the observations suggested a physical pathway accounting for the PW coupling in the atmosphere‐ionosphere system; that is, PW‐modulated tides penetrate into the F‐region ionosphere and produce the field‐aligned motion of the plasmas, leading to the oscillations in electron density at the PW periods. Key Points: Significant quasi‐16‐day oscillations (Q16DOs) were observed in the OI 135.6‐nm nightglow by GOLDThe Q16DOs in nightglow are closely correlated with quasi‐16‐day wave (Q16DW) activities in the middle atmosphereThe EIA crests showed antisymmetric Q16DOs between the two hemispheres likely driven by Q16DW‐modulated tides [ABSTRACT FROM AUTHOR]

The Global‐scale Observations of the Limb and Disk (GOLD) mission provides, for the first time, a synoptic view of the thermospheric response to a moderate geomagnetic storm at a 30‐min cadence. The observations showed that the OI 135.6‐nm emission substantially decreased at middle and high latitudes, but it increased at low latitudes. The depletion pattern between 4 and 6 November 2018 was likely due to the strong upwelling of the high‐latitude thermosphere. The deepest depletion in brightness that occurred near local noon was of ~300–400 R (~35% of the quiet time background level) and displayed a striking westward movement. First‐principles modeling reasonably reproduced the overall morphology of brightness response to the geomagnetic storm despite the depletion magnitude in brightness being underestimated. The postprocessing of the simulation results revealed that the westward movement of the deepest depletion was attributable to the horizontal wind changes driven by the geomagnetic storm. Plain Language Summary: Geomagnetic storms are triggered by enhanced solar wind emitted from the Sun, and a large portion of the absorbed solar energy dissipates into the high latitudes of the Earth's atmosphere through Joule heating and ion drag. Geomagnetic storms usually bring tremendous disturbances into the thermosphere‐ionosphere (e.g., neutral and plasma density), which significantly impact human activities (e.g., satellite orbit determination, navigation, and telecommunications). Accurate prediction of the response to geomagnetic storms in the thermosphere‐ionosphere is one of the core scientific objectives of space weather. The newly launched Global‐scale Observations of the Limb and Disk mission provides unprecedented synoptic measurements of ionosphere‐thermosphere properties. This study reports a large response of the OI 135.6‐nm emission to a moderate geomagnetic storm that occurred on 8–9 November 2018 observed by Global‐scale Observations of the Limb and Disk, which demonstrates a huge potential to advance our knowledge of the thermosphere‐ionosphere system and ability to simulate and forecast space weather. Key Points: GOLD provided, for the first time, synoptic observations of the thermospheric response to a geomagnetic storm with a 30‐min cadenceDayglow brightness depletions occurred at high latitudes and were extended into midlatitudes with evident interhemispheric asymmetryThe deepest depletion occurred near local noon and moved westward during and after the geomagnetic storm [ABSTRACT FROM AUTHOR]

The study investigates previously unrecognized wind reversals of the daytime thermosphere in the Southern Hemisphere at latitudes of 50°S–70°S employing observations of O(1D) airglow volume emission rates (VER), temperature, and neutral winds at 170–300‐km height by the Wind Imaging Interferometer. Atomic oxygen densities derived from the Wind Imaging Interferometer O+ (732–733 nm) emission observations were also considered together with contemporaneous observations of total electron content by the TOPEX Poseidon satellite mission. The O(1D) VER for fall equinox (March/April 1994) and summer solstice (January 1995) revealed a peak in the VER over the longitude range of 50°E–150°E, for both seasons and daytime local times. The Doppler temperatures also exhibited a maximum at the same location and times. The co‐located zonal wind field shows a westward trough at 50°E–150°E with wind speeds reaching −450 m/s and an eastward peak at 200°E–300°E. Meridional winds show a "peak and trough" structure over the 50°E–150°E region, with a distinct eastern boundary at ~150°E. During summer solstice the atomic oxygen is depleted at the region of the O(1D) VER enhancement in the presence of westward zonal wind. However, the total electron content observations showed a peak that coincided with the O(1D) VER and temperature enhancements. Plain Language Summary: The study investigates unusual perturbations in upper atmospheric winds that occur at a very specific location in the high‐latitude Southern Hemisphere. These include wind reversals, enhancements in the "red line" atomic oxygen airglow emission at 630.0 nm, temperature, depletion of atomic oxygen, and enhancements of ionospheric electron density. The location is 50°E–150°E longitude, 50°S–70°S latitude. The atmospheric observations were made by the Wind Imaging Interferometer, launched on NASA's Upper Atmosphere Research Satellite in 1991; these continued until 2003. The wind reversals involved westward winds of as strong as 450 m/s. The electron densities were of total electron content integrated along the line of sight from the satellite to the Earth and were obtained from the TOPEX satellite for the same time periods as the Wind Imaging Interferometer observations. Measurements were made at equinox and summer solstice and the atmospheric observations were the same for both. These new observations point the way to future studies of this highly localized phenomenon. Key Points: Previously unrecognized perturbations in thermospheric and ionospheric winds are observed at a specific high latitude location, 50°E–150°E and at 50°S–70°SThe neutral winds show a reversal to westward reaching −450 m/s, with a peak and trough structure of meridional wind, having an eastern boundary at ~150°EEnhancements occur in daytime O(1D) emission, temperature, and TEC; atomic oxygen is depleted [ABSTRACT FROM AUTHOR]

We investigate the ionospheric response to solar Extreme Ultraviolet (EUV) variations using different proxies, based on solar EUV spectra observed from the Solar Extreme Ultraviolet Experiment (SEE) onboard the Thermosphere Ionosphere Mesosphere Energetics and Dynamics (TIMED) satellite, the F10.7 index (solar irradiance at 10.7 cm), and the Bremen composite Mg-II index during January 2003 to December 2016. The daily mean solar proxies are compared with global mean Total Electron Content (GTEC) values calculated from global IGS TEC maps. The preliminary analysis shows a significant correlation between GTEC and both the integrated flux from SEE and the Mg II index, while F10.7 correlates less strongly with GTEC. The correlations of EUV proxies and GTEC at different time periods are presented. An ionospheric delay in GTEC is observed at the 27 days solar rotation period with the time scale of about ∼1 –2 days. An experiment with the physics based global 3-D Coupled Thermosphere/Ionosphere Plasmasphere electrodynamics (CTIPe) numerical model was performed to reproduce the ionospheric delay. Model simulations were performed for different values of the F10.7 index while keeping all the other model inputs constant. Preliminary results qualitatively reproduce the observed ∼1 –2 days delay in GTEC, which is might be due to vertical transport processes. [ABSTRACT FROM AUTHOR]