Solar wind energy input and magnetospheric substorm activity compared

The energy input from the solar wind into the magnetosphere resulting in storm and substorm activity is estimated in terms of the time integral of Akasofu epsilon parameter epsilon. Reliability of the epsilon measurements is analyzed using two-point measurements in the solar wind. It is shown that for large substorms, the probability of obtaining a different result (more than 15% difference) from two spacecraft is about 20%, whereas for smaller substorms the probability can be as large as 60%. Details of the energy input from the solar wind and its dissipation in the nightside ionosphere during both isolated and storm-time substorms are compared using WIND solar wind data and a local electrojet index from the IMAGE magnetometer chain in the Scandinavian local time sector. We demonstrate that the energy output to the ionosphere is almost independent of the energy input during the growth phase, and that the ionospheric dissipation is best correlated with energy input during the substorm expansion phase. Comparison of isolated and storm-time substorms shows that while the total ionospheric dissipation of storm-time substorms is considerably larger, its relative role in the energy budget is smaller.