On the sources of energization of molecular ions at ionospheric altitudes

Abstract

During geomagnetically active times, the suprathermal mass spectrometer on the Akebono satellite frequently observes upflowing molecular ions (NO +, N2 +, 02 +) in the 2-3 Earth radii geocentric distance regions in the auroral zone. Molecular ions originating at ionospheric altitudes must acquire an energy of the order of 10 eV in order to overcome gravitation and reach altitudes greater than 2 Re. This energy must be acquired in a time short compared with the local dissociative recombination lifetime of the ions; the latter is of the order of minutes in the F region ionosphere (300-500 km altitude). Upflowing molecular ions thus provide a test particle probe into the mechanisms responsible for heavy ion escape from the ionosphere. In this paper we analyze the extensive complement of plasma, field, and wave data obtained on the Akebono satellite in a number of upflowing molecular ion events observed at high altitudes (5000 -10,000 km). We use these data to investigate the source of energization of the molecular ions at ionospheric altitudes. We show that Joule heating and ion resonance heating do not transfer enough energy or do not transfer it fast enough to account for the observed fluxes of upflowing molecular ions. We found that the observed field-aligned currents were too weak to support large-scale field-aligned current instabilities at ionospheric altitudes. The data suggest but in the absence of high-resolution wave measurements in the 300 to 500 km altitude range cannot ascertain the possibility that a significant fraction of escape energy is transferred to molecular ions in localized regiom from intense plasma waves near the lower hybrid frequency. We also compared the energization of molecular iota to that of the geophysiqally important O + ions in the 300 to 500 km altitude range, where the energy transfer to O" is believed to occur via small-scale plasma instabilities, ion resonance, and ion-neutral frictional heating. Direct observation of energy input to the ionosphere from all of these sources in combination with in situ measurements of the density and temperature of neutral and ionized oxygen in the 300 to 500 km range are required to determine the relative importance of these energy sources in providing O + with sufficient energy to escape the ionosphere. Introduction Twenty years after the discovery that significant fluxes of O + escape from the ionosphere [Shelley et al., 1972], there exists little or no quantitative information about the relative importance of the various physical processes responsible for the energization and extraction of O + and other heavy ions Paper number 94JA01738. from the Earth's ionosphere. (For a review of the pioneering work on ionospheric ion acceleration and escape, see Johnson [1983]). It is generally agreed that the high latitude auroral ionosphere interacts with the neutral atmosphere to damp plasma motions driven by the solar wind interaction with the magnetosphere. The energy imparted by the solar wind in these time and spatially variable interactions produ...

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