Energetic ion beam in the Earth’s magnetotail lobe

Occurence of a prominent peak in the ion energy spectrum at energies of 0.1 to 0.7 MeV is observed by the IMP‐8 spacecraft during an energetic particle burst in a plasma “dropout” interval at about 37 R e in the earth’s magnetotail. This unusual characteristic in the spectrum is detected for about 2 minutes. By fitting the observed ion spectra to a drifting Kappa distribution, it is found that the ion population can be described as a hot beam with number densities 4 × 10 −7 to 5 × 10 −4 cm −3 and temperatures 15 to 45 keV, jetting tailward at speeds of 3500 to 7000 km/s. The energy flux density associated with the beam can be as high as 8 × 10 −2 ergs cm −2 sec −1 .