Response of the Coupled Magnetosphere Ionosphere Outside the Plasmapause During the 24 August 2005 Major Storm.

Geomagnetic storms are driven by changes in solar wind and are known to cause dramatic changes in the coupled magnetosphere-ionosphere system. Simultaneous observations of the magnetospheric and ionospheric densities is vital to determine the timings of the coupled system's response. Whistler mode (WM) sounding from the IMAGE satellite permitted measurements of field-aligned electron density (Ne) below 3000 km before and during the 24 August 2005 major storm (Kp,max=8.67; Dst,min= -216 nT). WM echoes were observed two days before the storm onset and on the first and third day of storm's recovery phase at L~ 3.2 and MLT~3. The IMAGE satellite passive recordings and Carpenter and Anderson [1992] empirical model was used to obtain the plasmapause (Lpp) location. The field line L~3.2 was inside the plasmasphere before the storm's onset (Lpp~4) and it was outside the plasmapause during the onset, main, and first three days of storm's recovery phase (Lpp≤3). Our measurements show evidence of plasma exchange between magnetosphere and ionosphere during the storm's recovery phase. On the first day of recovery phase, relative to the Ne before storm's onset, below 1000 km Ne increased (2-3 times) and that above 1000 km significantly decreased (~3 to 5 times). On the third day of recovery phase, below 1000 km Ne decreased and was comparable to that before the storm's onset. In the 1000 to 2000 km altitude range, Ne decreased and was lower than (30-50%) that during the first day of recovery. Above 2000 km it increased but was 1.5 to 2.5 times lower than the Ne before storm's onset. The observed variations in Ne during the recovery phase can be interpreted as down-flow of plasma from higher to lower altitudes (first day) and up-flow of plasma from lower to higher altitudes (third day). WM sounding results combined with the CHAMP satellite (~350 km) measurements indicate that: (1) significant amounts of plasma is dumped into the ionosphere (<1000 km) from plasmasphere on the first day of recovery phase; (2) the refilling of the plasmasphere started from the ionosphere on the second day of the recovery phase; and (3) the refilling period at L~3.2 is greater than 3 days. WM sounding results combined by physics-based models (e.g., SAMI3) may provide a unique way to study the role of neural winds and electric fields in modifying ionosphere/plasmasphere.