Fluxes of =50-keV protons and =30-keV electrons at ∼35 RE , 1. Velocity anisotropies and plasma flow in the magnetotail

Abstract

The NOAA/APL energetic particle experiment (EPE) on Imp 7 (Explorer 47) is capable of measuring hot plasma flow with a temporal resolution of 20 s if the densities exceed ∼0.1 cm−3. The lowest-energy electron channel (30–90 keV) can detect plasma with kT ∼1 keV, while the lowest-energy proton channel (50–200 keV) can detect plasmas at similar temperatures if they have a bulk velocity of ≳50 km s−1. The bulk velocities can be deduced from the measured proton angular distributions (16 sectors in the ecliptic) by using straightforward expressions derived from phase space transformations of a Maxwellian distribution. Representative examples are given for quiet time flow in the magnetosheath (V ∼500 km s−1, kT ≃ 5 keV) and the magnetotail (V ∼ 70 km s−1, kTp ∼10 keV), with Tp/Te > 3 in both cases. We also present a detailed example of a class of high-intensity events that occur in the plasma sheet in the dusk sector of the magnetosphere. The 1-hour event (0520–0620 UT, October 3, 1973) was associated with a 1500-γ depression in the H component of the geomagnetic field at Barrow, Alaska. Tailward velocities of 1260 km s−1 were deduced during the expansive phase of the substorm, and sunward velocities of ∼950 km s−1; during the recovery. Comparison with higher-energy proton measurements on the same spacecraft over the first year of data (September 1972 to October 1973) reveals that the larger events often have a nonthermal tail, although bulk velocities, densities, and temperatures deduced from the smaller events are consistent with previous measurements of plasma flow by other workers.