Properties of AE indices derived from real‐time global simulation and their implications for solar wind‐magnetosphere coupling

Abstract

Real‐time magnetohydrodynamic (MHD) simulation of the solar wind‐magnetosphere‐ionosphere (S‐M‐I) coupling system was used to calculate auroral electrojet (AE) indices. This simulation reproduces the magnetic field configurations in the magnetosphere, magnetospheric convection, and field‐aligned currents (FACs) using the upstream boundary conditions with the interplanetary magnetic field (IMF), solar wind speed, temperature, and proton number density measured by the ACE spacecraft. The electrical potential at 3 RE (Earth radii) from the center of the Earth is mapped on the ionosphere. The ionospheric currents are deduced from Ohm's law to match the divergence of Pedersen and Hall currents from FACs. The AE indices are obtained from the magnetic field perturbation caused by the simulated ionospheric currents. We compared the simulated AE indices for 247 d with the AE indices deduced from the magnetic variations at up to 12 stations located around the auroral latitude. The results show that the simulated AE reproduces the observed AE indices well. Of the 247 d, 64% had cross‐correlation coefficients of more than 0.5. We also found that the simulated AE indices do not correlate well with the observed AE indices when the standard deviations of variations in the observed AE indices are less than 100 nT. When variations in the AE indices are small, some of the short‐period perturbations of the electromagnetic energy flowing from the solar wind into the magnetosphere is absorbed or filtered in the real S‐M‐I coupling system by some mechanism that is not included in our MHD simulation and that the resulting fluctuation in the AE indices is damped compared with the simulation.