New Diagnosis for Energy Flow From Solar Wind to Ionosphere During Substorm: Global MHD Simulation

Abstract

To investigate the energy involved by a substorm, we performed global magnetohydrodynamics simulation for different solar wind conditions. The intensity of the auroral electrojet increases with the southward component of interplanetary magnetic field (IMF) and the solar wind velocity, which is consistent with observations. To evaluate the energy that enters the magnetosphere, we first defined a solar wind effective cross-sectional area in which all the Poynting fluxes entering the magnetosphere pass through. We found that the solar wind magnetic energy is insufficient to provide the magnetic energy entering the magnetosphere (intake magnetic energy) for southward IMF. About 33–88% of the intake magnetic energy is converted from the solar wind kinetic energy. About 2–7% of the solar wind kinetic energy (passing through the effective area) is converted to the magnetic energy that enters the magnetosphere. Significant contribution from the solar wind kinetic energy makes the energy coupling function complicated. The effective area also depends on the solar wind parameters, also making it complicated. An interesting point is that the rates of energy stored and released in the lobe also depend on the solar wind parameters. The ionospheric Joule heating rate is well correlated with the intake magnetic energy at onset, and during the substorm expansion. The correlation coefficients between them are better than that of the ε parameter. These results imply that both the directly driven process (manifested by intake magnetic energy) and loading-unloading process (manifested by stored/released energy in the lobe) are largely regulated by the solar wind condition.