Possible evidence for large, transient electric fields in the magnetotail from oppositely directed anisotropies of energetic protons and electrons

Abstract

Observations of energetic (Ep ≳ 0.29 MeV, Ee ≳ 0.22 MeV) proton and electron bursts in the magnetotail obtained with The Johns Hopkins University Applied Physics Laboratory (JHU/APL) detectors on the earth‐orbiting (∼ 40 Re × 22 Re) IMP‐8 satellite during the magnetically disturbed day of November 26, 1973 are presented. Two periods are selected (1100 to 1130 UT, and 1230 to 1300 UT) during which intensification of the auroral electrojet was seen and large fluxes of both electrons and protons appeared at the location of IMP‐8 (XSM ≃ −26 Re; YSM ≃ 17.8 Re; ZSM ; 1.7 Re). In both cases electrons and protons exhibit large anisotropies (∼ 30:1 and 5:1 for protons and electrons, respectively), with the electrons propagating tailward while the protons are propagating earthward. These oppositely directed anisotropies last as long as one minute, and are not necessarily aligned with the local magnetic field. While these enhanced fluxes are observed, there exist significant depressions (ΔB ≈ 15 γ) in the local magnetic field accompanied with the appearance of both positive and negative components of BX and BZ, i.e., a signature of the presence of the neutral sheet. We interpret these observations as suggesting the presence of an electric field in association with the acceleration region in the magnetotail. The associated spectral and species measurements are presented and discussed in the context of proposed acceleration models.