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.

Themospheric conditions during a minor geomagnetic event of 3 and 4 February 2022 has been investigated using disk temperature (T$_{disk}$) observations from Global-scale Observations of the Limb and Disk (GOLD) mission and model simulations. GOLD observed that the T$_{disk}$ increases by more than 60 K during the storm event when compared with pre-storm quiet days. A comparison of the T$_{disk}$ with effective temperatures (i.e., a weighted average based on airglow emission layer) from Mass Spectrometer Incoherent Scatter radar version 2 (MSIS2) and Multiscale Atmosphere-Geospace Environment (MAGE) models shows that MAGE outperforms MSIS2 during this particular event. MAGE underestimates the T$_{eff}$ by about 2\%, whereas MSIS2 underestimates it by 7\%. As temperature enhancements lead to an expansion of the thermosphere and resulting density changes, the value of the temperature enhancement observed by GOLD can be utilized to find a GOLD equivalent MSIS2 (GOLD-MSIS) simulation $\textendash$ from a set of MSIS2 runs obtained by varying geomagnetic ap index values. From the MSIS2 runs we find that an ap value of 116 nT produces a T$_{eff}$ perturbation that matches with the GOLD T$_{disk}$ enhancement. Note that during this storm the highest value of the 3 hr cadence ap was 56 nT. From the MSIS-GOLD run we found that the thermospheric density enhancement varies with altitude from 15\% (at 150 km) to 80\% (at 500 km). Independent simulations from the MAGE model also show a comparable enhancement in neutral density. These results suggest that even a modest storm could impact the thermospheric densities significantly.

Global-scale Observations of Limb and Disk (GOLD) disk measurements of far ultraviolet molecular nitrogen band emissions are used to retrieve temperatures (T$_{disk}$), which are representative of lower thermospheric altitudes. The present investigation studies the response of lower thermospheric temperatures to geomagnetic activities of varying magnitudes. In this study, it has been observed that T$_{disk}$ increases over all latitudes in response to enhanced geomagnetic activity. The increase in temperature is proportional to the strength of the geomagnetic activity and is greater at higher latitudes. Temperature enhancements vary from 10s to 100s of Kelvins from low- to mid-latitudes. Local time behavior shows that pre-noon enhancements in temperatures, during relatively stronger geomagnetic activities, are greater compared to afternoon, which can be attributed to the combined action of daytime dynamics and geomagnetic forcing. This study thus demonstrates the utility of GOLD T$_{disk}$ measurements investigating the effects of dynamical and external forcings in the thermosphere.

The unambiguous 2-dimensional (2D) maps of OI 135.6 nm radiance retrieved from the Global Observation of Limb and Disk (GOLD) after sunset are compared with the total electron content (TEC) maps measured by GPS receivers in the America sector. The OI 135.6 nm radiance observed by GOLD is an indicator of the peak electron density of the ionosphere, while the TEC depends on the total electron density in the column. The comparisons show that both of them are able to capture the large structures in the equatorial ionization anomaly (EIA) well, and sometimes they both also capture bubbles. Both show that the ionosphere after sunset is quite dynamic and has strong day-to-day variability. A statistical study has also been carried out to check the occurrence rate of bubbles and the apparent EIA structure between Oct 17, 2018 to May 31, 2019. GOLD is able to image the areas where it is difficult to situate GPS receivers such as the ocean, while TEC data covers the full-diurnal cycle. In all, the GOLD and TEC have valuable synergy to allow us to gain a better understanding of the equatorial ionosphere.

We examine characteristics of the seasonal variation of thermospheric composition using column number density ratio ∑O/N2 observed by the NASA Global Observations of Limb and Disk (GOLD) mission from low-mid to mid-high latitudes. We found that the ∑O/N2 seasonal variation is hemispherically asymmetric: in the southern hemisphere, it exhibits the well-known annual and seminal pattern, with highs near the equinoxes, and primary and secondary lows near the solstices. In the northern hemisphere, it is dominated by an annual variation, with a minor semiannual component with the highs shifting towards the wintertime. We also found that the durations of the December and June solstice seasons in terms of ∑O/N2 are highly variable with longitude. Our hypothesis is that ion-neutral collisional heating in the equatorial ionization anomaly region, ion drag, and auroral Joule heating play substantial roles in this longitudinal dependency. Finally, the rate of change in ∑O/N2 from one solstice season to the other is dependent on latitude, with more dramatic changes at higher latitudes.

Satellite in-situ electron density observations of the storm enhanced density 2 and the polar Tongue of Ionization on the noon meridional plane in the F 3 region during the  The first report on satellite in-situ electron density measurements of the storm enhanced 15 density at the noon meridian plane 16  The lifecycle of ionospheric storm enhanced densities is mainly controlled by variations 17 of the dayside prompt penetration electric fields 18  The key methodologies include a comparison of TIEGCM modeling with satellite in-situ 19 electron density observations and a correlation analysis 20 21 22 Abstract 23 Ionospheric storm enhanced density (SED) has been extensively investigated using Total 24 Electron Content (TEC) deduced from GPS ground and satellite-borne receivers. However, in-25 situ electron density measurements have not been reported for SEDs yet. We report in-situ 26 electron density measurements of a SED event and its associated polar tongue of ionization 27 (TOI) at the noon meridian plane measured by the CHAMP polar-orbiting satellite at about 390 28 km altitude during the 20 November 2003 magnetic storm. The measurements provided rare 29 evidence about the SED’s life cycle at a fixed magnetic local time. CHAMP detected the SED 30 onset right after the arrival of an interplanetary coronal mass ejection shock front. The SED 31 electron density enhancement extended from the equatorial ionization anomaly to the noon cusp, 32 through which plasmas entered into the polar cap as polar plasma clouds/TOI. For several 33 satellite-ground conjunction passes, CHAMP measured the electron density of plasma clouds 34 comparable to the TOI density measured by the Tromso ISR, establishing that the plasma clouds 35 were related to the TOI. The SED plume in the NH retreated gradually to lower latitudes six 36 hours after the SED onset. We conducted TIEGCM modeling to demonstrate that the SED 37 density enhancement was likely due to the vertical transport of plasmas. The observed mid-38 latitude electron density varied with the cross-polar cap electric fields, suggesting that prompt 39 penetration electric fields (PPEFs) in the zonal direction played a dominant role. The 40 implication is that variations of the dayside PPEFs largely control the SED lifecycle. 41 42 Plain Language Summary 43 Ground radar and GPS stations have frequently detected enhancement of ionospheric electron 44 density at mid-latitudes and in the polar cap during the magnetic storm recovery phase. We 45 report in-situ satellite observations near 400 km at the noon meridian plane during an intense 46 magnetic storm. It provides for the first time clear evidence about the life cycle of ionospheric 47 electron density enhancement, starting from its onset at mid-latitudes, entry into the polar cap, 48 and retreat to lower latitudes. The mid-latitude ionospheric electron density was mainly 49 enhanced in the northern hemisphere, triggered by the passage of a solar wind dynamic pressure 50 shock front. Global circulation modeling suggests that the vertical transport of ionospheric 51 plasmas probably produced the enhancement. The dayside prompt-penetration electric fields in 52 the zonal direction likely drove the vertical plasma uplift. Thus, it appears that the SED lifecycle 53 is mainly controlled by variations of the dayside prompt electric field. 54