The Storm-Substorm Relationship during Different Solar Wind Conditions

Abstract:

Solar-terrestrial coupling is the study of processes which transfer solar wind energy to the magnetosphere creating geomagnetic activity. This coupling depends on properties of the solar wind and particularly on the discrete structures present in the solar wind. These include CMEs, CIRs, and HSS (high speed streams) which have very different characteristics causing different modes of response of the magnetosphere such as magnetic storm, substorms, steady magnetospheric convection (SMC) and others. Among these different modes of response, magnetic storms and substorms are two of the primary ones because they occur frequently and because they can cause considerable problems in technological systems.

The differences in solar wind driving conditions during CME, CIR, and HSS provide a good opportunity to examine how the properties of substorms change with solar wind structures and the activity they create. Since the probability of observing these structures is a function of solar cycle phase, the storm-substorm relationship changes with solar cycle phase. It is possible that the nature of solar wind coupling changes with the type of storm, storm intensity, and/or the phase of the storms.

The original hypothesis was that a magnetic storm is simply a superposition of substorms. However, some evidences have been found that storms begin to develop before substorms occur. Is substorm occurrence independent of storm development? This would suggest that substorms can be seen in any phase of a storm and even a storm without substorms. There are some recent studies suggest this is possible. However, most of the studies did not take into consideration different solar wind driving conditions. In this study, we will investigate several aspects of the relation of substorms to storms such as: Does the frequency and intensity of substorms change with phase of the solar cycle? Are there differences in these properties between cycles? Are the characteristics of substorms different if they occur within a storm or outside of a storm? Are CME storm time substorms different from CIR storm substorms? Do substorm properties change with phase of the storm?