Statistical survey of pitch angle distributions in core (0‐50 eV) ions from Dynamics Explorer, 1: Outflow in the auroral zone, polar cap, and cusp

Abstract

Core (0‐50 eV) ion pitch angle measurements from the retarding ion mass spectrometer on Dynamics Explorer 1 are examined with respect to magnetic disturbance, invariant latitude, magnetic local time, and altitude for ions H+, He+, O+, M/Z=2 (D+ or He++), and O++. Included are outflow events in the auroral zone, polar cap, and cusp, separated into altitude regions below and above 3 RE. In addition to the customary division into beam, conic, and upwelling distributions, the high‐latitude observations fall into three categories corresponding to ion bulk speeds that are (1) less than, (2) comparable to, or (3) faster than that of the spacecraft. This separation, along with the altitude partition, serves to identify conditions under which ionospheric source ions are gravitationally bound and when they are more energetic and able to escape to the outer magnetosphere. Features of the cleft ion fountain inferred from single event studies are clearly identifiable in the statistical results. In addition, it is found that the dayside pre‐noon cleft is a consistent source of escape velocity low‐energy ions regardless of species or activity level and the dayside afternoon cleft, or auroral zone, becomes an additional source for increased activity. The auroral oval as a whole appears to be a steady source of escape velocity H+, a steady source of escape velocity He+ ions for the dusk sector, and a source of escape velocity heavy ions for dusk local times primarily during increased activity. The polar cap above the auroral zone is a consistent source of low‐energy ions, although only the lighter mass particles appear to have sufficient velocity, on average, to escape to higher altitudes. The observations support two concepts for outflow: (1) The cleft ion fountain consists of ionospheric plasma of 1‐20 eV energy streaming upward into the magnetosphere where high‐latitude convection electric fields cause poleward dispersion. (2) The auroral ion fountain involves field‐aligned beams which flow out along auroral latitude field lines; and, in addition, for late afternoon local times, they experience additional acceleration such that the ion energy distribution tends to exceed the detection range of the instrument (>50‐60 eV).