The two‐dimensional evolution of thermospheric ∑O/N2 response to weak geomagnetic activity during solar‐minimum observed by GOLD

We conduct observational and modeling studies of thermospheric composition responses to weak geomagnetic activity (nongeomagnetic storms). We found that the thermospheric O and N2 column density ratio (∑O/N2) 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/N2 depletions were pushed westward by zonal winds. The ∑O/N2 evolution during weak geomagnetic activity suggests that the formation mechanism of the ∑O/N2 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.