Ion composition and temperatures at 1000 km as deduced from simultaneous observations of a VLF plasma resonance and topside sounding data from the Alouette 1 satellite

Abstract

Evidence is presented to support the hypothesis that the low frequency cutoff of a VLF noise band observed by the Alouette 1 satellite is due to a plasma resonance (the lower hybrid resonance) which defines a cutoff frequency for propagation transverse to the earth's field. If this hypothesis is accepted, the observations of this cutoff frequency by the satellite's VLF receiver, and simultaneous measurements of the electron plasma frequency by the topside sounder, can be used to determine an effective mean mass for the ions in the ambient plasma. The values, together with the scale height measured from the electron density profiles, set limits on the ion composition and temperature. The main support for the hybrid resonance hypothesis lies in the plausibility of the temperature and ion mass information that is derived using this interpretation and the agreement between this information and the results of other workers. In general, is found to be larger during the day than at night and to increase with latitude. In polar regions ranges from about 7 at midnight to about 13 at noon; at midnight the composition of the polar ionosphere at 1000 km is more than 60% O+, whereas at noon it is about 95% O+. At middle latitudes, L = 2 to 3, ranges from 1.8 to as high as 7. For low latitudes, L < 2, the lowest observed values of of 1.3 require that the H+ concentration be greater than 68%. An effective ionospheric temperature of about 1200°K is determined for middle latitudes, increasing to high values for high latitudes. A marked diurnal variation of temperature is found at high latitudes.