The Alfven Mach Number Control of the Solar Wind-Magnetosphere Coupling Efficiency and the Saturation of the Geomagnetic Indices

Abstract:

We have investigated the effect of key solar wind driving parameters on the solar wind-magnetosphere coupling efficiency and saturation of the cross polar cap potential (CPCP) during sheath and magnetic cloud driven storms. The particular focus of the study was on the coupling efficiency dependence with Alfven Mach number (M_A).

Since we are studying the instantaneous coupling efficiency instead of the average efficiency over the whole solar wind structure, we needed to take into account the communication time between the solar wind and the magnetosphere. We present the results of the time delay analysis between geomagnetic indices (PCN, AE and SYM-H) and the interplanetary electric field y-component (E_Y, GSM coordinate system) and Newell and Borovsky functions.

The study shows that the M_A has a clear effect to the saturation of the PCN index, which can be used as a proxy of the polar cap potential. The higher the M_A the higher the limit E_Y value after which the saturation starts to occur. Thus, the coupling efficiency increases as a function of M_A. Also, the AE index saturates during high solar wind driving but the saturation is not M_A depended. However, the results also suggest that the M_A it is not the primary cause for the PCN saturation.