Effects of substorms on the stormtime ring current index Dst

Abstract. There has been some discussion in recent times regarding whether or not substorm expansive phase activity plays any role of importance in the formation of the stormtime ring current. I explore this question using the Kp index as a proxy for substorm expansive phase activity and the Dst index as a proxy for symmetric ring current strength. I find that increases in Dst are mildly related to the strength of substorm expansive phase activity during the development of the storm main phase. More surprisingly, I find that the strength of Dst during the storm recovery phase is positively correlated with the strength of substorm expansive phase activity. This result has an important bearing on the question of how much the Dst index reflects activity other than that of the stormtime symmetric ring current strength for which it is supposed to be a proxy.

Key words: Ionosphere (electric fields and currents) - Magnetospheric physics (current systems; storms and substorms)