Solar wind density effect on the night-side geomagnetic activity (AL index)

The role of solar wind density in driving of geomagnetic activity is now again in the focus of attention of magnetospheric scientists. We analyze 20 years of hourly AL and OMNI solar wind data to reveal the basic solar wind flow influence on the magnitude of night-side geomagnetic activity and magnetotail dynamics. In such a data set trigger-related specifics of substorm onsets are averaged out. The more pronounced effects of the solar wind speed and interplanetary magnetic field on AL need to be eliminated before the analysis of the smaller density role. With an iterative method we determine the best formula, accounting for the velocity and magnetic field contribution: E=VBy2/2+Bz2sin4(θ/2)+αV2sin0.5(θ/2). Average AL proved to be not sensitive to density changes, when density was above ∼5cm-3, and average |AL| dropped by ∼30%, when density decreased from 5 to 1cm-3.