Simultaneous observations of electrons (E> 45 keV) at 2000-kilometer altitude and at 100,000 kilometers in the magnetotail

Abstract

Geiger counters on the University of Iowa's Injun 4 satellite and on Vela satellites 2A, 2B, 3A, and 3B have provided simultaneous measurements of energetic electron fluxes (Ee > 45 kev) at ∼2000-km altitude over the auroral and polar regions of the earth and at ∼17 earth radii (RE) in th plasma sheet of the magnetotail. The data discussed here were obtained in March, April, September, and October 1965 periods when Injun 4 crossed the northern auroral zone between 1600 and 2400 magnetic local time (MLT) and between 0400 and 1200 MLT. The Injun measurements confirm a recent report that the peak flux of electrons in the outer belt in the late morning hours often is substantially increased shortly after the onset of a magnetic bay in the midnight sector of the auroral zone. They show, in addition, that the early evening sector of the outer belt does not respond promptly in this way to a bay. This difference in response supports the view that the electrons that enhance the morning sector of the outer belt are injected by some process acting in the midnight sector of the magnetosphere. The Vela measurements confirm an earlier report that an essentially isotropic flux of energetic electrons appears in the plasma sheet, generally after the peak of a bay. Furthermore, the concurrent measurements by Vela and Injun 4 show that the flux of electrons in the plasma sheet after a bay is usually much less intense than the peak flux attained in the outer zone after the same bay. Both the delayed appearance of the electrons in the magnetotail and their relatively lower intensity imply that the energization process acts not in the very distant tail, but closer to the earth than 17 RE. It is suggested that electron energization (along with a general heating of the ambient plasma) takes place within the auroral and/or outer-zone tubes of force during a bay and that these tubes do not extend as far as 17 RE into the tail during the energization process. Movement of the hot plasma to the distant tail signals the start of recovery of a bay and results in the bursts of energetic electrons seen in the tail. Thus, the distant tail is not a source of energetic (>45-kev) outer-belt electrons, but an additional sink for the process that generates them. It is argued, on the basis of these and other observations, that some process other than merging of magnetic field lines is primarily responsible for generation of the energetic electrons.