Energization of Polar-Cusp Electrons at the Noon Meridian

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Observations gained with an electrostatic analyzer on board the low-altitude satellite Ariel 4 demonstrate that the directional, differential spectra of polar-cusp electron intensities are regulated by the sign of the elevation angle of the interplanetary magnetic field, theta(IMF). The altitude of the nearly circular, polar orbit was about 550 km. In the energy range 200 E 700 eV, spectra of polar-cusp electron intensities were not observed to respond to changes in the sign of theta(IMF) . At greater energies, spectra were found to be significantly harder when theta(IMF) 0 deg, with enhancements of a factor of about 10 typical for 2-keV electron intensities. Furthermore, these enhanced intensities appear to be localized within about a 1-hour sector of magnetic local time centered on the noon meridian. Enhanced intensities were detected during each sampling of electron intensities at local times MLT = 11.5 - 12.6 hours when theta(IMF) 0 deg, but were observed only occasionally at lesser and greater local times. The physical processes responsible for the observed enhancements in electron intensities are of an unknown nature. Trapped magnetospheric electrons of sufficient intensities, which are generally present just equatorward of the polar cusp, are ruled out as a possible source.