Observations of counterstreaming between plasma and energetic particles in the magnetotail

We present fine time resolution (10–20 s) measurements of the energy and angular distributions of three bursts of energetic particles in the magnetotail at distances of approximately 32 R E . These measurements were made near the onset of a magnetic substorm on October 16, 1973, by the NOAA/APL (50 ≤ E p ≤ 220 keV) and JHU/APL ( E p ≥ 290 keV, E e ≥ 220 kev) experiments on Imp 7 and the JHU/APL (210 ≤ E p ≤ 560 keV) experiment on Imp 6. These energetic particle measurements are also correlated with plasma observations made by the University of Iowa Lepedea on Imp 7. These data reveal the presence of two distinct particle populations: (1) an earthward flowing hot plasma usually dominating the particle intensities up to an energy of ∼50 keV and (2) tailward streaming, nonthermal ions and electrons dominating the energy range 200 ≤ E p ≤ 1 MeV. Nonthermal in this context signifies an energetic particle population bearing no direct relationship with the observed earthward flowing ambient hot magnetotail plasma. A careful examination of the angular distribution of the ions in the transition region (50 ≤ E p ≤ 220 keV) has allowed us for the first time to separate the above two particle regimes. Although these initial results apply to a limited set of these events, inspection of the NOAA/APL energetic particle experiment data shows counterstreaming to be a common feature of magnetotail energetic particle events. We have been able to obtain, again for the first time, the energy spectrum of the nonthermal magnetospheric burst particles for energies ≥50 keV. These observations place a number of requirements on various theories which attempt to explain the dynamical processes in the magnetotail. No published theory to date seems to be in satisfactory agreement with all aspects of our observations.