Acceleration of electrons near the Earth's bow shock and beyond

Abstract

Groups of electrons of energies >30 kev were observed in the earth's magnetosheath, at the earth's bow shock, and in a region extending out to several earth radii beyond the shock. These measurements were made with a gold-silicon surface barrier detector on board the Imp 1 satellite, in the period of Nov. 27, 1963, to Feb. 13, 1964. A comparison of these results with observations made with a Geiger-Müller counter (University of California) on the same satellite shows that both instruments were recording the same electron structures. Since the response characteristics of the two detector systems for counting electrons are different, it is possible to estimate the spectrums of the electrons in these groups or spikes by comparing the counting rates of the respective detectors for each spike. For an assumed inverse power integral spectrum, F = FoE−γ, we determine the value of γ and find that it varies from 2.5 to 4.5 over a wide range of intensities and locations in space. At times of enhanced solar wind velocity, having a 27-day recurrence, we also observe electron spikes out to ∼200,000 km, which is near satellite apogee. To investigate the origin of the electrons in spikes found in the magnetosheath, at the bow shock, and beyond the shock to distances of several earth radii, the temporal variation and energy spectrums of the electrons in the outer radiation belt, but near the magnetospheric boundary, also were analyzed. It was found that the hardening in the energy spectrum of the trapped electrons in the radiation belt follows closely the compression of the boundary of the magnetosphere controlled by the strength of the solar wind. However, this change in spectrum was not observed for the electrons in the spikes. For this reason it is argued that the spikes are not groups of electrons escaping from the outer radiation belt, but rather are locally accelerated electrons. The Fermi acceleration mechanism proposed by Jokipii and Davis is discussed.