Solar Cycle Variation of Plasma Mass Density in the Outer Magnetosphere: Magnetoseismic Analysis of Toroidal Standing Alfven Waves Detected by Geotail

Abstract

We study the variation of plasma mass density in the outer magnetosphere over a solar cycle using mass density estimated from the frequency of fundamental toroidal standing Alfvén waves observed by the Geotail spacecraft. We identify wave events using ion bulk velocity data covering 1995–2006 and use events in the 0400–0800 magnetic local time sector for statistical analysis. We find that the F10.7 index is a dominant controlling factor of the mass density. For the equatorial mass density  [inline image]  that is normalized to the value at L = 11, we obtain an empirical formula  [inline image] , where the units of  [inline image]  and F10.7 are amu cm−3 and solar flux units (sfu; 1 sfu =10−22 W m−2 Hz−1), respectively. This formula indicates that  [inline image]  changes by a factor of 1.8, if F10.7 changes from 70 sfu (solar minimum) to 210 sfu (solar maximum). A formula derived in a similar manner using GOES magnetometer data indicates that, for the same range of F10.7, the mass density at L ∼ 7 varies by a factor of 4.1 We attribute the smaller factor at L = 11 to the lower O+/H+ number density ratio at higher L, the stronger F10.7 dependence of the O+ outflow rate than the H+ outflow rate, and entry of solar wind H+ ions to the outer magnetosphere.