Observations of energetic electrons ( E ≳ 200 keV) in the Earth's magnetotail: Plasma sheet and Fireball observations

With the California Institute of Technology electron/isotope spectrometer (EIS) aboard the earth‐orbiting spacecraft Imp 8, intense energetic electron events ( E ≳ 200 keV) have been observed at ∼30 R E in the magnetotail on 13 of 28 magnetotail passes. In one class of events, peak absolute intensities j ( E ≳ 200 keV) = 10³–10 4 el (cm² s sr) −1 are detected; the differential energy spectra for these events are very steep, with power law indices of ∼7. For this class of observations, symmetric electron pitch angle distributions are detected with the presence of little or no unidirectional streaming, a finding consistent with quasi‐trapped motion of the particles. Concurrent observations with other instrumentation on the same spacecraft indicate that the local magnetic fields possess northward components while simultaneous plasma (50 eV ≤ E ≤ 45 keV) data show bulk flow speeds of a few hundred kilometers per second or less, with intense plasma heating ordinarily occurring. A second distinct class of events corresponds to lower average absolute electron intensities ( E ≳ 200 keV), typically harder electron energy spectra, and strong intermittent field‐aligned (and tailward) unidirectional streaming of the energetic electrons. During periods when this class of events is observed in the plasma sheet, strong southward components are found in the local magnetic fields, and large tailward bulk plasma flow has been reported (with tailward jetting in excess of 1000 km s −1 in certain cases). The second class of events is consistent with energetic electron motion on essentially open field lines. These streaming events are found to be associated with apparent localized acceleration regions in the magnetotail.