Global thermospheric neutral density and wind response to the severe 2003 geomagnetic storms from CHAMP accelerometer data



[1] Measurements of atmospheric density near 410 km from the STAR accelerometer on the CHAMP satellite are used to illustrate the spatial-temporal dependence of the thermospheric response to the severe solar storms occurring during 29 October to 1 November 2003. This interval includes periods of elevated magnetic activity with KP values of 5–9, as well as undisturbed intervals that serve to define quiet time baseline densities. Measurements are available from −87° to +87° latitude during both day and night at local times near 1300 and 0100 hours, respectively. During times of maximum geomagnetic activity for this study, density measurements exhibit enhancements of 200–300%. Northern Hemisphere daytime responses are much larger than in the Southern Hemisphere; the origins of this effect are unknown. Nighttime density disturbances more readily propagate to equatorial latitudes, possibly facilitated by the predominant equatorward flow in both hemispheres due to the diurnal tides driven by in situ EUV heating. The CHAMP density measurements are compared with density predictions from the NRL-MSISe00 empirical density model and demonstrate some model shortcomings. Measurements of cross-track accelerations provide the opportunity to estimate zonal winds from the equator to about ±60° latitude, transitioning to a measure of purely meridional winds at the turning point of the orbit near ±87° latitude. A periodic variation in cross-track winds with an apparent period of 24 hours appears at high latitudes and exhibits similar amplitudes and temporal-latitudinal structures to the empirical HWM-93 wind model when projected into the cross-track direction. This periodicity is due to the displacement of geomagnetic and geographic coordinates. At low latitudes, CHAMP and HWM-93 both yield westward winds of order 100 ms−1 during midday under quiet magnetic conditions; however, during severely disturbed periods the HWM-93 winds generally show a greater westward intensification (to 250 ms−1) than the CHAMP measurements. At night, CHAMP winds are near zero under quiet conditions whereas HWM-93 indicates eastward winds of order 50–100 ms−1. Under disturbed conditions the CHAMP winds shift to westward values of order 200 to 250 ms−1, while HMW-93 values do not exceed about 50 ms−1 in the westward direction. The physical origins of the observed effects are difficult to isolate, and unequivocal interpretation will require sophisticated numerical modeling taking into account self-consistent interactions between the neutral winds, drifts, and ionization densities.