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 Alfven 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 F-10.7 index is a dominant controlling factor of the mass density. For the equatorial mass density rho(*)(eq) that is normalized to the value at L = 11, we obtain an empirical formula log rho(*)(eq)= -0.136 + 1.78 x 10(-3) = 10(-3) F-10.7, where the units of rho(*)(eq) and F-10.7 are amu cm(-3) and solar flux units (sfu; 1 sfu =10(-22) W m(-2) Hz(-1)), respectively. This formula indicates that rho(*)(eq) changes by a factor of 1.8, if F-10.7 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 F-10.7, the mass density at L approximate to 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 F-10.7 dependence of the O+ outflow rate than the H+ outflow rate, and entry of solar wind H+ ions to the outer magnetosphere.