The current systems responsible for SYM and ASY indices variation during geomagnetic storm

The SYM and ASY indices have been long used to characterize the current systems during geomagnetic storms. We study the contribution of different current systems to the variations of the SYM-H, ASY-H and ASY-D indices. For more than 150 storms, the indices were computed using the empirical magnetospheric models: T01, TS05 (and the newest TS07 for a few events). Comparing the model indices with the real ones we test the models performance in the near-earth region. Of these three models, the T01 was found to be best at predicting the SYM-H and ASY-H indices while TS05 was somewhat better for predicting the ASY-D index. The superposed epoch analysis of the contribution of the different current systems to the SYM and ASY indices revealed the following: (1) During the main phase, the main contribution to SYM-H index comes from the tail module of the models. The symmetric ring current module contribution becomes comparable to or stronger during the recovery phase. (2) The ASY-H and ASY-D indices are controlled entirely by current systems which close via field-aligned currents: the partial ring current and region1/region 2 field-aligned currents. In addition, the analysis was conducted separately for strong and moderate storms as well as for CIR and CME driven storms. No meaningful difference was found between the results obtained for these subsets.