The Two‐Dimensional Evolution of Thermospheric ∑O/N₂ Response to Weak Geomagnetic Activity During Solar‐Minimum Observed by GOLD

Abstract

We conduct observational and modeling studies of thermospheric composition responses to weak geomagnetic activity (nongeomagnetic storms). We found that the thermospheric O and N 2 column density ratio (∑O/N 2) in part of the Northern Hemisphere measured by Global-scale Observations of the Limb and Disk (GOLD) exhibited large and long-lived depletions during weak geomagnetic activity in May and June 2019. The depletions reached 30% of quiet time values, extended equatorward to 10°N and lasted more than 10 hr. Furthermore, numerical simulation results are similar to these observations and indicate that the ∑O/N 2 depletions were pushed westward by zonal winds. The ∑O/N 2 evolution during weak geomagnetic activity suggests that the formation mechanism of the ∑O/N 2 depletions is similar to that during a geomagnetic storm. The effects of weak geomagnetic activity are often ignored but, in fact, are important for understanding thermosphere neutral composition variability and hence the state of the thermosphere-ionosphere system. Plain Language Summary The column density ratio of O and N 2 (∑O/N 2) has been used to monitor geomagnetic storm effects in the thermosphere, as well as providing valuable information about the ionosphere. This triggers an important question: Can weak geomagnetic activities cause changes in thermospheric composition too? Here, we conduct studies based on geostationary orbit observations and numerical simulations. Model outputs replicate the general morphology of this variability for the cases examined. This made it possible to understand the cause of the composition response to weak geomagnetic forcing. We found that the ∑O/N 2 depletion observed was pushed westward by the zonal wind. During weak geomagnetic activity, the ∑O/N 2 response is similar to the response during a geomagnetic storm, albeit it is weaker. In summary, our study suggests that weak geomagnetic activity can also generate strong and long-lived responses in thermosphere composition during solar minimum and that this response can be important to understanding the thermosphere and ionosphere variability during the so-called quiet times.

Variations in Thermosphere Composition and Ionosphere Total Electron Content Under “Geomagnetically Quiet” Conditions at Solar‐Minimum

“…10.1029/2021GL093300 6 of 9 the dominant process in generating the ∑O/N 2 and TEC variations, though it may still be a secondary or more minor process. Diagnostic analysis of the O continuity equation (Burns et al, 1989;Cai et al, 2020Cai et al, , 2021 has been performed to obtain insight into the variations of mid-latitude ∑O/N 2 in both cases. ∑O/N 2 occurs approximately on a constant pressure surface.…”

The Effect of the Thermosphere on Ionosphere Outflows

“…In addition, thermosphere observations (McDonald et al, 2015;Cai et al, 2020) suggest significant day-to-day variability. For example, satellite data can be used to estimate the globally averaged O density at altitude 400 km, where O is the dominant atom (Picone et al, 2005).…”

“…Because the polar wind and plasmasphere serve as a source and a sink, respectively, for geoeffective energetic ions, thermal (non-energized, Maxwellian) outflows are essential elements of space weather (Bortnik and Thorne, 2007;Millan and Thorne, 2007). Further, observations suggest significant day-to-day variability in thermosphere composition (Krall et al, 2016a;Cai et al, 2020) and winds (McDonald et al, 2015). In this brief review, we consider the effect of thermosphere composition and winds on refilling outflows.…”

The Influence of Tropospheric Processes on Disturbances in the D and E Ionospheric Layers

“…A quiet geomagnetic environment is a prerequisite for a perfectly clean experiment. Even weak geomagnetic activity can still cause strong and long-term disturbances in the thermosphere [44], suggesting a potential effect on the TEC value. In this case, one should restore the TEC, excluding the influence of geomagnetic activity, and this is a significant problem.…”