Electron density in the plasmasphere: Whistler data on solar cycle, annual, and diurnal variations

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Abstract
Whistler data are used to present a statistical view of equatorial plasmaspheric electron density neq and associated tube electron content NT (defined as the number of electrons in a geomagnetic flux tube of 1cm² cross‐sectional area at 1000km altitude and extending to the magnetic equator). The data were acquired between 1959 and 1973 at Byrd (L ≃ 7), Eights (L ≃ 4), and Siple (L ≃ 4), Antarctica, which are within 1 hour of the same geomagnetic meridian, and from Stanford, California (L ≃ 2). The plasmaspheric neq profile beyond L ≃ 3 is dominated by variations associated with magnetic disturbances and subsequent recovery. In the aftermath of disturbances the plasmasphere tends to be divided into an inner ‘saturated’ region, which is in equilibrium with the underlying ionosphere in a diurnal average sense, and an outer ‘unsaturated’ region, which is still filling with plasma from below. In the outer plasmasphere byond ˜3.5 RE, diurnal variations appear as relatively small effects superimposed on larger storm‐associated variations. Large numbers of whistler traces (as many as 3000 in some cases) were scaled for each of several months. These data sets form the basis for approximations to neq profiles to form log10 (neq) = aL + b. These profiles are offered for reference use in estimating plasmasphere density levels. The previously reported annual and solar cycle variations are further documented by new evidence that these effects diminish with increasing L beyond L ≃ 3.