Energetic protons of magnetospheric origin in the plasma sheet associated with substorms

Abstract

Energetic particle detectors on three of the Vela satellites provide three-dimensional measurements of fluxes of energetic electrons and protons and have revealed that after some substorms the expanding plasma sheet is populated with protons (0.5 MeV < Ep < 0.9 MeV) at flux levels above the intensity threshold of the detector, ∼20 protons/cm² s sr MeV, which corresponds to one count per sampling interval. The proton fluxes have the following characteristics: (1) they exceed this flux level in association with only a small percentage of substorms; (2) they are present only in the plasma sheet (i.e., not in the tail lobes); (3) the proton flux rises above the threshold of the detectors near the time of the appearance of the intense flux of energetic electrons (Ee ≳ 30 keV) that is always observed in the recovering plasma sheet; (4) the proton flux usually decays to a level below the detector threshold after 10–20 min unlike the electron flux, which typically remains intense (i.e., at levels of ≈105/cm² s sr) for an hour or more; (5) the proton spectrum is very soft, γ (the index of a power law spectrum) being 5–10; (6) the protons appear predominantly in the evening sector of the plasma sheet; (7) they almost always display a strong unidirectional anisotropy having a peak flux perpendicular to B and directed approximately parallel to the magnetospheric equatorial plane and into the 0°–90° range of solar magnetospheric longitude; and (8) proton fluxes above the instrument threshold are not observed outside the magnetosphere (i.e., in the magnetosheath or in the solar wind) in evident correlation with even the most intense bursts seen in the magnetotail. We conclude that the protons are energized within the magnetosphere in the substorm process and that the flux anisotropy is the consequence of a gradient in the proton density directed generally earthward and equatorward. The peak proton flux during any individual proton event tends to be directed perpendicular to the radius vector from the earth to the Vela satellite, suggesting that the magnetic field in the recovering plasma sheet lies more nearly in magnetic meridian planes than it is shown to do in published models based on magnetic field measurements.