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.
