A comparison of the effects of CIR- and CME-induced geomagnetic activity on thermospheric densities and spacecraft orbits: Case studies

So far studies of the effect of geomagnetic storms on thermospheric density and satellite orbits have been mainly focused on severe storm events caused by Coronal Mass Ejections (CMEs). The effect of long-duration, less intensive geomagnetic activity that is related to Corotating Interaction Regions (CIRs) has not been fully explored. In this paper, thermospheric densities observed by the CHAMP satellite and its orbit parameters are used to compare the responses of satellite orbital altitudes to geomagnetic activity caused by CMEs and CIRs. Three cases are investigated in this paper. Each case had one or two CME storm(s) and one CIR storm that occurred successively. In these cases three out of four CME-storms were stronger than their corresponding CIR-storms, but the durations of these CME-storms were much shorter. Thus, the satellite orbit decay rates during CME-storms are usually larger than those during CIR-storms. However, CIR-storms often had long durations that perturbed satellite orbits for longer periods of time. As a result, the total thermospheric density changes and satellite orbit decays for the entire periods of CIR-storms were much greater than those for the CME-storms since these parameters were related to the total energy deposited into the thermosphere/ionosphere, which depended on both the strengths and the durations of the storms. This study indicates that more attention should be paid to CIR storms during the declining phase and during solar minimum, when they occur frequently and periodically. Whereas fewer CME storms occurring under these conditions. We also found that changes in thermospheric densities and CHAMP orbit decay rates correlated well with variations of auroral hemispheric power, but lagging by about 3-6 h.