Explorer VIII satellite measurements in the upper ionosphere

Abstract

This is a more extensive report than earlier publications on upper-ionospheric ion com position (Bourdeau, Donley, Whipple & Bauer 1962) and electron temperature (Serbu, Bourdeau & Donley 1961) and on spacecraft-plasma interaction (Bourdeau, Donley, Serbu & Whipple 1961), all measured by use of the Explorer VIII satellite. Results from an ion retarding potential experiment show that the upper ionospheric composition responds to the neutral gas temperature. Specifically, during the satellite’s active life (November-December 1960), O + ions predominated from the perigee altitude (425 km) up to about 800 km at night and 1500 km at diurnal maximum; the base of the helium ion region was located at 800 km during the sunrise period and at 1500 km in the day-time; the base of the protonosphere possibly was located at 1200 km during the sunrise period and above 1800 km at diurnal maximum. For the latitudes indicated, the electron temperature ( T e ) data which in this report are restricted to magnetically quiet days ( A p < 15) are consonant with current models of the diurnal electron density behaviour and with a hypothesis of solar ultra-violet radiation as the only ionizing agent. For the 6 h period centred at midnight, the average T e observed at altitudes of 425 to 600 km was 900 °K with a standard deviation of 150 °K. There were no anomalously high values and no significant change with magnetic dip (0 to 75° N). For the period 10 to 16 h d.m.t., at magnetic dips between 50 and 70° S and at altitudes between 1000 and 2400 km , the observed average T e was 1600 °K (s.d. 200 °K). The most pronounced feature of the measured diurnal variation is that T e exceeds the neutral gas temperature by a factor of about 2·5 during the sunrise period at 600 to 900 km . Suggestions are made for the explanation of differences between the Explorer VIII T e observations and other measurements. The measured satellite-plasma interaction is consistent with theoretical expectation lending confidence to the above-described geophysical results. The observed ‘average’ potential of the satellite varied from a few tenths of a volt negative at night, to zero when the measured day-time charged particle density was 104 cm -3 and thence to a few tenths of a volt positive for day-time densities of 103 cm -3 . Superimposed on the ‘average’ potential were potential gradients across the satellite skin, an effect produced by the movement of a conducting body through a magnetic field. The measured orientation sensitivity of three types of current flowing between the satellite and the ionosphere is described.