Impact of Solar Irradiance on Storm-Time Thermospheric Mass Density Response

As a key parameter in the externally forced ionosphere–thermosphere (IT) system, mass density response to storms is affected by the prevailing solar irradiance. To better understand this relationship, we examine mass density responses to the same storm but with different background solar irradiance levels. We simulate an ensemble of cases for the 2015 St. Patrick's Day storm by TIE-GCM with realistic geomagnetic forcing conditions but prescribed higher and lower F_10.7 values. We find that the impact of F_10.7 on the mass density response depends on both altitude and storm phase. Higher F_10.7 produces larger mass density responses during the main phase, but smaller responses around the recovery phase, and vice versa. This behavior is found to be related to NO cooling modulation of the thermosphere, which starts before the recovery phase. The results indicate that interpretation and prediction of storm-time mass density require accurate knowledge of pre-storm solar irradiance.