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Based on the observations from the balloon‐borne instrument High‐altitude Interferometer WIND experiment (HIWIND) and the simulations from the Thermosphere Ionosphere Electrodynamics General Circulation Model (TIEGCM), the Grid Agnostic MHD Environment for Research Applications (GAMERA)‐TIEGCM (GT), and the GAMERA‐TIEGCM‐RCM (GTR), we investigate the variations of summer high‐latitude thermospheric winds and their physical mechanisms from 25 to 30 June, 2018. HIWIND observations show that the meridional winds were the largest at midnight and exhibited strong day‐to‐day variations during the 6‐day period, which were generally reproduced by those three models. The day‐to‐day variations of winds were mainly associated with the interplanetary magnetic field (IMF)perturbations, while the magnetic latitude variations also contributed to the large day‐to‐day variations of the winds seen in the observations. Meanwhile, the zonal winds were mostly westward during the daytime, and the wind speed became large, especially in the afternoon, which is related to the westward ion drift velocity. The observed meridional winds tend to turn equatorward during the daytime on some days, while the simulated winds blow mostly poleward except for simulations by the GTR model on 26 June. The GTR model revealed that the equatorward meridional winds on 26 June were associated with strong and negative IMFconditions, which tilts the convection pattern to the prenoon sector. The simulations also revealed that the ring current could contribute to affecting the neutral wind variations, especially under geomagnetically active conditions.

The storm‐time ionospheric‐thermospheric (IT) state is of great interest, since the IT dynamics change dramatically as energy is input and dissipated in the upper atmosphere. Lagrangian coherent structures (LCSs), which are objective ridges in time‐evolving flows that describe the tendency of neighboring fluid elements to separate, provides a unique opportunity to infer the dynamics in the IT system. In this work, we model IT flows with the Thermosphere‐Ionosphere‐Electrodynamics General Circulation Model and identify the LCSs. We compare the LCSs in the neutral winds and plasma drifts during quiet times versus during active times. We find that LCSs are largely aligned in the modeled IT flows, with a dawn‐dusk asymmetry in their latitudinal position. During a geomagnetic storm, the thermospheric LCSs (T‐LCSs) and ionospheric LCSs (I‐LCSs) shift equatorward, align more closely with each other, and maintain a dawn‐dusk asymmetry. The collocation of T‐LCSs and I‐LCSs and their analogous response to the geomagnetic storm provide evidence of energy input into the thermosphere and ionosphere simultaneously, and the ion drag is the dominant effect causing LCS alignment during a geomagnetic storm.

Nitric oxide (NO) infrared radiation is an essential cooling source for the thermosphere, especially during and after geomagnetic storms. An accurate representation of the three‐dimension (3‐D) morphology of NO emission in models is critical for predicting the thermosphere state. Recently, the deep‐learning neural network has been widely used in space weather prediction and forecast. Given that the 3‐D image of NO emission from the Sounding of the Atmosphere using Broadband Emission Radiometry (SABER) onboard the Thermosphere Ionosphere Energetics and Dynamics satellite contains a large amount of missing data which is unobserved, a context loss function is applied to extract the features from the incomplete SABER NO emission images. A 3‐D NO emission model (referred to as NOE3D) that is based on the convolutional neural network with a context loss function is developed to estimate the 3‐D distribution of NO emission. NOE3D can effectively extract features from incomplete SABER 3‐D images. Additionally, NOE3D has excellent performance not only for the training datasets but also for the test datasets. The NO emission climate variations associated with solar activities have been well reproduced by NOE3D. The comparison results suggest that NOE3D has better capability in predicting the NO emission than the Thermosphere‐Ionosphere Electrodynamics General Circulation Model. More importantly, NOE3D is capable of providing the variations of NO emission during extremely disturbed times.

In this work, we carry out a comprehensive modeling study, using the Thermosphere–Ionosphere–Electrodynamics General Circulation Model, to explore the physical processes by which the longitude‐dependent geomagnetic field drives the longitudinal variations of the sunrise enhancement of the zonal electric fields at the dip equator near the June solstice. Numerical experiments and diagnostic analyses of the electrodynamics equation show that the longitudinal differences of the equatorial zonal electric fields near sunrise are primarily associated with the longitudinal variations in the zonal wind dynamo, with those from the meridional wind dynamo contributing secondarily. Furthermore, the longitudinal differences of the wind dynamo near sunrise are mainly related to the longitudinal variations ofand conductance, which are caused primarily by the direct influence of the longitudinal structures of magnetic field declination and strength. Meanwhile, the longitudinal variations of neutral winds, which also result in moderatelongitudinal variations, play a secondary role in the longitudinal variations of the neutral wind dynamo, while plasma density, which has minor longitudinal differences near sunrise, contributes slightly by modifying the conductance. Overall, the sunrise enhancement in June is more significant at the longitudes where the magnetic field strength and distortion are larger or the magnetic field declination is smaller in the Northern Hemisphere.

Previous studies have shown that solar flares can significantly affect Earth's ionosphere and induce ion upflow with a magnitude of ∼110 m/s in the topside ionosphere (∼570 km) at Millstone Hill (42.61°N, 71.48°W). We use simulations from the Thermosphere‐Ionosphere‐Electrodynamics General Circulation Model (TIEGCM) and observations from Incoherent Scatter Radar (ISR) at Millstone Hill to reveal the mechanism of ionospheric ion upflow near the X9.3 flare peak (07:16 LT) on 6 September 2017. The ISR observed ionospheric upflow was captured by the TIEGCM in both magnitude and morphology. The term analysis of the F‐region ion continuity equation during the solar flare shows that the ambipolar diffusion enhancement is the main driver for the upflow in the topside ionosphere, while ion drifts caused by electric fields and neutral winds play a secondary role. Further decomposition of the ambipolar diffusive velocity illustrates that flare‐induced changes in the vertical plasma density gradient is responsible for ion upflow. The changes in the vertical plasma density gradient are mainly due to solar extreme ultraviolet (EUV, 15.5–79.8 nm) induced electron density and temperature enhancements at the F₂‐region ionosphere with a minor and indirectly contribution from X‐ray (0–15.5 nm) and ultraviolet (UV, 79.8–102.7 nm).

In this work, the Thermosphere‐Ionosphere‐Electrodynamics General Circulation Model is used to investigate the responses of ionospheric electrodynamic processes to the solar flares at the flare peaks and the underlying physical mechanisms on September 6 and 10, 2017. Simulations show that solar flares increased global daytime currents and reduced the eastward electric fields during the daytime from the equator to middle latitudes. Furthermore, westward equatorial electric fields and equatorial counter electrojets occurred in the early morning. At the flare peak, these electrodynamic responses are predominantly related to the enhanced E‐region conductivity by flares, as the responses of neutral winds and F‐region conductivity to flares are negligible. Specifically, the Cowling conductance enhancement is not the major process causing the reduction of zonal electric fields. This electric field reduction is primarily associated with the decrease of the ratio between the field line‐integrated wind‐driven currents and the conductance. The flare‐induced conductivity enhancement is larger but the background wind speed is smaller in the E‐region than in the F‐region, as a result, the increase of total integrated wind‐driven currents is less than the conductance enhancement.

The density cell structure in the high‐latitude thermosphere is referred to as the density enhancement or depletion with respect to the surrounding area. Previous simulation results showed that the density cells are only observed below about 350 km. In the present work, the global ionosphere‐thermosphere model is used to investigate the mechanism for the absence of the isolated density cell in the high‐altitude thermosphere during geomagnetic quiet time. The simulation results indicate that the ion convection tends to drive a neutral cyclonic flow on the dawnside of the Magnetic Pole in both the low‐ and high‐altitude thermosphere. Around the center of the cyclonic flow, a downward wind is formed as a consequence of mass conservation. It is interesting that under the influence of the downward flow, a density depletion relative to the background density with the same magnitude is generated inside the cyclone, which is independent of altitude. In the low‐altitude thermosphere, this density depletion is shown as an isolated low‐density cell. However, in the high‐altitude thermosphere, the ion drag‐driven density depletion turns to be an insignificant structure, as compared to the strong day‐night gradient of the background neutral density. Consequently, the isolated low‐density cell does not stand out in the high‐altitude thermosphere.

The high‐resolution thermosphere‐ionosphere‐electrodynamics general circulation model has been used to investigate the response ofF₂region electron density (Ne) at Millstone Hill (42.61°N, 71.48°W, maximum obscuration: 63%) to the Great American Solar Eclipse on 21 August 2017. Diagnostic analysis of model results shows that eclipse‐induced disturbance winds causeF₂region Ne changes directly by transporting plasma along field lines, indirectly by producing enhanced O/N₂ratio that contribute to the recovery of the ionosphere at and below theF₂peak after the maximum obscuration. Ambipolar diffusion reacts to plasma pressure gradient changes and modifies Ne profiles. Wind transport and ambipolar diffusion take effect from the early phase of the eclipse and show strong temporal and altitude variations. The recovery ofF₂region electron density above theF₂peak is dominated by the wind transport and ambipolar diffusion; both move the plasma to higher altitudes from below theF₂peak when more ions are produced in the lowerF₂region after the eclipse. As the moon shadow enters, maximizes, and leaves a particular observation site, the disturbance winds at the site change direction and their effects on theF₂region electron densities also vary, from pushing plasma downward during the eclipse to transporting it upward into the topside ionosphere after the eclipse. Chemical processes involving dimming solar radiation and changing composition, wind transport, and ambipolar diffusion together cause the time delay and asymmetric characteristic (fast decrease of Ne and slow recovery of the eclipse effects) of the topside ionospheric response seen in Millstone Hill incoherent scatter radar observations.