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

Core(0-50 eV) ion pitch angle measurements from the retarding ion mass spectrometer on Dynamics Explorer 1 are examined with respect to magn etic disturbance, invariant latitude, magnetic local time, and altitud e for ions H+, He+, O+, M/Z=2 (D+ or He++), and O++. Included are outf low events in the auroral zone, polar cap, and cusp, separated into al titude regions below and above 3 R(E) In addition to the customary div ision into beam, conic, and upwelling distributions, the high-latitude observations fall into three categories corresponding to ion bulk spe eds 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 ar e gravitationally bound and when they are more energetic and able to e scape to the outer magnetosphere. Features of the cleft ion fountain i nferred from single event studies are clearly identifiable in the stat istical results. In addition, it is found that the dayside pre-noon cl eft is a consistent source of escape velocity low-energy ions regardle ss of species or activity level and the dayside afternoon cleft, or au roral zone, becomes an additional source for increased activity. The a uroral oval as a whole appears to be a steady source of escape velocit y 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 prima rily during increased activity. The polar cap above the auroral zone i s a consistent source of low-energy ions, although only the lighter ma ss particles appear to have sufficient velocity, on average, to escape to higher altitudes. The observations support two concepts for outflo w: (1) The cleft ion fountain consists of ionospheric plasma of 1-20 e V energy streaming upward into the magnetosphere where high-latitude c onvection electric fields cause poleward dispersion. (2) The auroral i on 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 dis tribution tends to exceed the detection range of the instrument (>50-6 0 eV).