Dynamic variations in intensity and energy spectra of electrons in the inner radiation belt

Abstract

The energy spectra and intensities of electrons >130 kev have been measured since October 1971 with two 256-channel electron spectrometers on board the low-altitude (∼800-km) polar-orbiting earth-oriented satellite 1971 089A. In the inner radiation belt the energy spectra of the trapped electrons can often be characterized as exponential in shape, but during magnetic disturbances the trapped fluxes at the lower edge of the inner belt (L ≲ 1.3) may be quite variable and may extend to lower L values, and the energy spectra may display pronounced structure, such as the presence of one or more peaks. Several features of the data are in agreement with a set of observations made at a time when the Starfish electrons were dominant and are consistent with the origin of the peaks being a redistribution of the existing population. Peaks in the energy spectra are also frequently observed at the outer edge of the inner belt (L ≈ 1.4–1.8), but several features of these peaks are in distinct contrast to those observed on very low L shells. The energies of the peaks seen in the outer parts of the belt usually decrease very rapidly with increasing L value, whereas the energies of those seen at the lower edge of the belt are generally not strongly dependent upon L. At L ≈ 1.4 to 1.8 the most pronounced peaks occur more often at low hmin values; in contrast, structure in the spectra seen at the inner edge of the belt is usually more pronounced at higher values of hmin. These and other differing characteristics suggest that the peaks in the outer portions of the belt are associated with precipitation processes and do not result from the same type of ‘redistribution in L’ mechanism proposed for the nearly monoenergetic spectra sometimes seen at the lower edge of the inner belt. The observations suggest that peaks may originate from a cyclotron resonance interaction of electrons with whistlers of a nearly constant frequency over the L range of concern.