Magnetospheric modes and solar wind energy coupling efficiency

Abstract

Using observations and two different global MHD simulations, we demonstrate that the solar wind speed controls the magnetospheric response such that the higher the speed, the more dynamic and irregular is the magnetospheric response. For similar level of driving solar wind electric field, the magnetospheric modes can be organized in terms of speed: Low speed produces steady convection events, intermediate speeds result in periodic sawtooth oscillations, and high speeds drive large geomagnetic storms. We show that the control parameter of energy transfer and coupling is the electric field along the large‐scale X line. We demonstrate using global MHD simulations that for slowly varying interplanetary magnetic field (IMF), the reconnection line is tilted approximately by an angle θ/2, where θ is the IMF clock angle. Then, for clock angles away from northward, the magnetospheric energy entry and response scale with the electric field along the reconnection line (EPAR), rather than the traditionally used EY. If we define the energy coupling efficiency as response/EPAR, we can show it to be independent of the IMF clock angle and only weakly dependent on the solar wind dynamic pressure. These results demonstrate the ability of the localized reconnection line to control the energy input through the entire magnetopause.