  In this study, we investigate thermospheric neutral mass density heating
associated with 168 CME-driven geomagnetic storms in the period of May 2001 to
September 2011. We use neutral density measured by two low-Earth orbit
satellites: CHAMP and GRACE. For each storm, we superpose geomagnetic and
density data for the time when the IMF B$_\mathrm{z}$ component turns sharply
southward chosen as the zero epoch time. This indicates the storm main phase
onset. We find that the average SYM-H index reaches the minimum of $-$42 nT
near 12 hours after storm main phase onset. The Joule heating is enhanced by
approximately 200\% in comparison to quiet values. In respect to thermosphere
density, on average, high latitude regions (auroral zones and polar caps) of
both hemispheres are highly heated in the first 1.5 hour of the storm. The
equatorial response is presumably associated with direct equator-ward
propagation of TADs (traveling atmospheric disturbances). A slight north-south
asymmetry in thermosphere heating is found and is most likely due to a positive
B$_\mathrm{y}$ component in the first hours of the storm main phase.

Space Physics