Causes of low thermospheric density during the 2007-2009 solar minimum

Satellite drag data indicate that the thermosphere was lower in density, and therefore cooler, during the protracted solar minimum period of 2007-2009 than at any other time in the past 47 years. Measurements indicate that solar EUV irradiance was also lower than during the previous solar minimum. However, secular change due to increasing levels of CO₂ and other greenhouse gases, which cool the upper atmosphere, also plays a role in thermospheric climate, and changes in geomagnetic activity could also contribute to the lower density. Recent work used solar EUV measurements from the Solar EUV Monitor (SEM) on the Solar and Heliospheric Observatory, and the NCAR Thermosphere-Ionosphere-Electrodynamics General Circulation Model, finding good agreement between the density changes from 1996 to 2008 and the changes in solar EUV. Since there is some uncertainty in the long-term calibration of SEM measurements, here we perform model calculations using the MgII core-to-wing ratio as a solar EUV proxy index. We also quantify the contributions of increased CO₂ and decreased geomagnetic activity to the changes. In these simulations, CO₂ and geomagnetic activity play small but significant roles, and the primary cause of the low temperatures and densities remains the unusually low levels of solar EUV irradiance.