A systematic study of effects of solar wind density on auroral electrojets

The subject of effects of solar wind density (N_p) on auroral electrojets has recently received increasing attention. For "unbiased" events in which only N_p varied significantly but other solar wind parameters were relatively constant, we calculated rates of change of the auroral electrojets per unit change of N_p (dAU/dN_p and dAL/dN_p). Here we report that the distribution of dAU/dN_p and dAL/dN_p binned by the Z component of interplanetary magnetic field (IMF) shows a butterfly pattern. The rates are found to be small for northward IMF. As the southward IMF becomes larger, the distribution bifurcates-one branch extending to positive rates and the other to negative rates. Although it has been commonly believed that the enhanced N_p increases the auroral electrojets, the enhanced N_p can also decrease the auroral electrojets with a 49% probability.