Predicting storm-time thermospheric mass density variations at CHAMP and GRACE altitudes

Orbit-averaged mass density measurements derived from the satellites CHAMP and GRACE are used to investigate the storm-time prediction model developed by Liu et al. (2010) at different altitudes. This model uses as input only the solar wind merging electric field. From 2002 to 2005 in total 31 major geomagnetic storms with minimum Dst < -100 nT are selected for a statistical study. The results show that the model can successfully predict the storm-time mass density changes at both CHAMP and GRACE altitudes. The orbit-averaged density of CHAMP and GRACE show very similar distribution in shape, regardless of the orbital local time difference, but the amplitude of GRACE density is about 30% of that of CHAMP density. An optimal delay time of 4.5 h has been found for both CHAMP and GRACE densities. During the four years the scale factor a between merging electric field and mass density for CHAMP altitude remains basically at the same level, a = 0.5, while the a for GRACE density shows a declining trend, and its value is only 30% of that for CHAMP density. The storms driven by corotating interaction regions (CIR) have in general larger a values than the storms driven by coronal mass ejections (CMEs).