Ring current electron trajectories associated with VLF emissions

Abstract

Intensity enhancements of the ring current electrons associated with the VLF emissions during the geomagnetic storms and substorms, which have been observed by the equatorially orbiting S³‐A satellite (Explorer 45), are so far limited only to electrons of energies below the order of 10 keV. Furthermore, onsets of the enhancement are detected in the lowest observable energy channel first, then shifting toward higher energies with time, a feature, which is opposite to the well‐known energy dependence expected from the drift motion of electrons in the earth's dipole magnetic field. In order to explain these findings on the ring current electron enhancements, trajectories of electrons injected into the nightside of the magnetosphere from the geomagnetic tail are calculated by modifying Ejiri's calculations which were used for the interpretation of the nose events of the ring current protons observed also by the S³‐A satellite. The results indicate that the electron intensity enhancements are limited to electrons below the order of 10 keV simply because of the location of the observations which are initially confined to the dusk‐midnight sector outside the plasmasphere. If the observations are made in the morning sector, where many VLF emissions such as chorus are observed, the enhancement should not be limited only to electrons below the order of 10 keV but should extend to electrons up to the order of 100 keV. The apparent inverse dispersion, i.e., the appearance of the enhancement at lower energies first, is also due to the location of the observations. Namely, if the observations are made in the morning sector outside the plasmasphere, the normal dispersion with the first enhancement by high‐energy electrons should be seen. Since the adiabatic energy increase of charged particles during cross‐magnetic field inward motion in the dipole field is largest for the 90° pitch angle particles, the energy at the source is lowest for 90° pitch angle electrons for the same energy at the observing location. The smaller enhancement of the small pitch angle electron intensity can therefore be explained by the energy spectra of source electrons in the geomagnetic tail.