Polar cap potential saturation, dayside reconnection, and changes to the magnetosphere

Abstract

Global MHD simulations of the Earth's magnetosphere using the Block‐Adaptive‐Tree Solarwind Roe Upwind Scheme code at the Community Coordinated Modeling Center are run with a high‐resolution dayside magnetopause and variable‐resolution near‐Earth regions to study the phenomena of polar cap saturation. In the simulations a resistive spot is maintained on the moving magnetopause to ensure that the correct dayside reconnection rate is obtained. The solar wind parameters are held fixed, and the ionospheric Pedersen conductivity is varied. Strong reduction of the cross–polar cap potential is observed, with modest increases in the cross–polar cap current, and no changes in the local and global reconnection rates. Changes in the magnetosphere associated with the saturation of the polar cap are explored: these include a weakening of the dayside magnetic field strength, an equatorward shift of the cusps, a flattening in Z of the closed field line region of the dayside magnetosphere, and a taillike stretching of the dipole field lines at the terminator. Measurements of the currents and voltages of the polar cap indicate that the solar wind acts like a current‐limited voltage generator. A simple circuit model is analyzed using measurements from the MHD simulations, and physically reasonable values for the circuit elements are obtained. Nine models for polar cap saturation are assessed against the findings of the present study.