Using ICON and GOLD satellite observations, the response of the thermospheric daytime horizontal winds and neutral temperature to the 2020/2021 major sudden stratospheric warming (SSW) is studied at low- to middle latitudes (0° - 40°N). Comparison with observations during the non-SSW winter of 2019/2020 and the pre-SSW period (December 2020) clearly demonstrates the SSW-induced changes. The northward and westward thermospheric winds are enhanced during the warming event, while temperature around 150 km drops by up about 50 K compared to the pre-SSW phase. Changes in the horizontal circulation during the SSW can generate upwelling at low-latitudes, which can contribute to the adiabatic cooling of the low-latitude thermosphere. The observed changes during the major SSW are a manifestation of long-range vertical coupling in the atmosphere.

We simulated the Nov 4, 2021 geomagnetic storm event penetrating electric field using the Multiscale Atmosphere-Geospace Environment (MAGE) and compared with the NASA ICON observation. The ICON observation showed enhancement of the vertical ion drift when the penetrating electric field arrived at the equatorial region. The simulated vertical ion drifts are consistent with ICON observation. Hence, we are able to verify the MAGE simulation with ICON observation. On the dusk side, the MAGE simulation showed strong pre-reversal enhancement (PRE), whereas the ICON observation did not display any sign of the PRE. The MAGE simulation did show that PRE amplitude decreases as altitude increase. Because the ICON orbital height is above the model upper boundary, it could be a factor for the discrepancy. Instrumental issue cannot be ruled out at this moment. GOLD UV image at the same time exhibits multiple plasma bubbles, which seem to suggest the existence of the PRE.

Solar eruptions cause geomagnetic storms in the near-Earth environment, creating spectacular aurorae visible to the human eye and invisible dynamic changes permeating all of geospace. Just equatorward of the aurora, radars and satellites often observe intense westward plasma flows called subauroral polarization streams (SAPS) in the dusk-to-midnight ionosphere. SAPS occur across a narrow latitudinal range and lead to intense frictional heating of the ionospheric plasma and atmospheric neutral gas. SAPS also generate small-scale plasma waves and density irregularities that interfere with radio communications. As opposed to the commonly observed duskside SAPS, intense eastward subauroral plasma flows in the morning sector were recently discovered to have occurred during a super storm on 20 November 2003. However, the origin of these flows termed “dawnside SAPS” could not be explained by the same mechanism that causes SAPS on the duskside and has remained a mystery. Through real-event global geospace simulations, here we demonstrate that dawnside SAPS can only occur during major storm conditions. During these times the magnetospheric plasma convection is so strong as to effectively transport ions to the dawnside, whereas they are typically deflected to the dusk by the energy-dependent drifts. Ring current pressure then builds up on the dawnside and drives field-aligned currents that connect to the subauroral ionosphere, where eastward SAPS are generated. The origin of dawnside SAPS explicated in this study advances our understanding of how the geospace system responds to strongly disturbed solar wind driving conditions that can have severe detrimental impacts on human society and infrastructure.

We report on a new method to calibrate the FORMOSAT-7/COSMIC-2 (F7/C2) ion velocity meter (IVM) in-situ ion density data using the Tri GNSS (Global Navigation Satellite System) Radio-occultation System (TGRS) differential total electron content (TEC) data. This calibration is made using collocated measurements from the IVM and TGRS instruments on the same satellite. We found that the IVM ion density is about 8-15% lower than the TGRS derived density at the insertion orbit (~ 710 km) and 5% higher at the mission operation orbit (~ 540 km). Using a linear correction specific to orbit altitude and satellite, an adjustment is implemented for the IVM density data. These corrections remove the offsets between the IVM and TGRS density measurements. We believe the corrected densities eliminate any inter-spacecraft discrepancy in the IVM density data, making it suitable for use in multi-satellite scientific investigations of longitudinal and local time variations and space weather operational applications.

Using the newly developed, Multiscale Atmosphere-Geospace Environment (MAGE) model, we simulated the penetrating electric field in the equatorial region under different interplanetary magnetic field (IMF) Bz conditions during September 2020. Two intervals were selected for detailed analysis with the vertical ion drift data from the NASA ICON. The MAGE simulations show that in southward IMF (S-IMF) cases, the dawn-dusk electric potential drop at the equator is about 14% of the cross polar cap potential difference. The dawn-dusk potential drop at the equator varies instantaneously with the changes in the IMF Bz or interplanetary electric field, which in turn alters the vertical ion drift. The daytime changes of the equatorial vertical ion drift in response to the penetrating electric field related to the IMF Bz are only half of that during the nighttime, due mostly to the E-region dynamo. MAGE simulation shows pre-reversal enhancement (PRE) during southward IMF cases, but the PRE was absent in the ICON IVM observations. Further observations and modeling are needed to resolve this discrepancy.

Strong subauroral plasma flows were observed in the dawnside ionosphere during the 20 November 2003 super geomagnetic storm. They are identified as dawnside subauroral polarization streams (SAPS) in which plasma drift direction is eastward and opposite to the westward SAPS typically found in the dusk sector. Both dawnside and duskside SAPS are driven by the enhanced meridional electric field in the low latitude portion of Region-2 field-aligned currents (FACs) in the subauroral region where ionospheric conductance is relatively low. However, dawnside eastward SAPS were only observed in the main and recovery phases while duskside westward SAPS were found much earlier before the sudden storm commencement. Simulations with the Multiscale Atmosphere-Geospace Environment (MAGE) model demonstrate that the eastward SAPS are associated with dawnside ring current build-up. Unlike the duskside where ring current build-up and SAPS formation can occur under moderate driving conditions, strong magnetospheric convection is required for plasmasheet ions to overcome their energy-dependent drifts to effectively build up the dawnside ring current and upward Region-2 FACs. We further used test particle simulations to show the characteristic drift pattern of energetic protons under strong convection conditions and how they are related to the dawnside SAPS occurrence. This study demonstrates the connection between the level of solar wind driving condition and a rare ionospheric structure, eastward SAPS on the dawnside, which only occur under strong convection typically associated with intense or super storms. Dawnside SAPS are suggested as a unique feature of major geomagnetic storms.

Using the horizontal neutral wind observations from the MIGHTI instrument onboard NASA’s ICON (Ionospheric Connection Explorer) spacecraft with continuous coverage, we determine the climatology of the mean zonal and meridional winds and the associated mean circulation at low- to middle latitudes (10S-45N) for Northern Hemisphere solstice conditions between 90 km and 200 km altitudes, specifically on 20 June 2020 solstice as well as for a one-month period from 8 June-7 July 2020. The data are averaged within appropriate altitude, longitude, latitude, solar zenith angle, and local time bins to produce mean wind distributions. The geographical distributions and local time variations of the mean horizontal circulation are evaluated. The instantaneous horizontal winds exhibit a significant degree of spatiotemporal variability often exceeding ~150 m/s. The daily averaged zonal mean winds demonstrate day-to-day variability. Eastward zonal winds and northward (winter-to-summer) meridional winds are prevalent in the lower thermosphere, which provides indirect observational evidence of the eastward momentum deposition by small-scale gravity waves. The mean neutral winds and circulation exhibit smaller scale structures in the lower thermosphere (90-120 km), while they are more homogeneous in the upper thermosphere, indicating the increasingly dissipative nature of the thermosphere. The mean wind and circulation patterns inferred from ICON/MIGHTI measurements can be used to constrain and validate general circulation models, as well as input for numerical wave models.

This paper examined the variability of equatorial thermospheric meridional and zonal wind speeds at night-time using an optical Fabry–Perot interferometer (FPI) located in Abuja, Nigeria (Geographic: 8.99°N, 7.39°E; Geomagnetic latitude: -1.60). The study period covered 9 months with useable data of 139 nights between March 2016 and January 2018. The hourly zonal wind speed is between 19.33 and 250 ms-1 and that of the meridional wind ranged between 0 and 200 ms-1. These speeds are greater than those reported in other longitudinal sectors, and this could be one of the reasons responsible for reduced EXB drift in this region compared to other regions. Comparison of FPI ground-based measurements with estimates from the Horizontal Wind Model (HWM-14) accurately reproduced the meridional component, but for some departure of ~45 ms-1 in May and June 2016, and January 2018. A very good agreement is observed between the predicted and measured zonal winds speed in the months of 2017. However, the HWM-14 overestimated the zonal wind speed in the early evening values by ~30 ms-1 and underestimated the post-midnight values by a larger factor in December 2017. Hence, this necessitates a call for improvement of the HWM-14 by using newly observed data in order to better characterize the West African sector. The varying zonal winds showed modal periods of 25.9 and 133.5 days, which are quasi 27-days and quasi-terannual periodic variations, respectively. On the meridional wind, oscillatory periods of 133.5 and 23.1 days are seen in year 2016 and 2017, respectively.