JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 97, NO. All, PAGES 16,855-16,863,NOVEMBER 1, 1992 UpflowingIonospheric Ionsin theAuroralRegion G. Lu, • P. H. REIFF, 2 T. E. MOORE, a AND R. A. HœœLIS • Observations of upfiowing ionospheric ions areobtained nearly simultaneously by DE 1 andDE 2 over the nightside auroral regions. At low altitudes, the mean value of the net upward ionnumber flux is of theorder of 10 .9 cm '2 s 4. The ionosphere ispredominantly O*, and the flux ofions with energy greater than 5 eVisa very small fraction (less than1%) of thetotal ion flux. At highaltitudes, theupflowing ions areaccelerated by a parallel electric field andheated (with characteristic energies of hundreds of electron volts). Comparing upflowing fluxes at high andlow altitudes yields an estimated height of the bottom of the auroral acceleration region of 1400-1700 km for the region of peak potential drop. Thislow-altitude acceleration could either be from a parallel electric field orfrom perpendicular acceleration. The fluxes atthe edges of the arc are mostly H* thus implying a higher-altitude base of theacceleration region at theedges where thepotential drop is lower. INrFRODUCTION In order to overcome the Earth's gravitational field, heavy oxygen atoms musthave about10 electron volts of energy, whereas lightatoms and molecules, such ashydrogen, need much less energy to escape the ionosphere. Thus, the Earth is surrounded with a hydrogen gas"geecorona". The escape of light ions from the topside high-latitude ionosphere leaves O* as the dominantion to altitudes of up to a couple of thousand kilometers, dependingon ,the ion, electron, and neutral temperatures [Banks and Holzer, 1969]. Theupflowing flux for H* was measured to beof the order ell08 cm '2 s 'l [Hoffman and Dodson, 1980], consistent with the theoretical prediction [Banks and Holzer,1969]. Theescaping H* flux is matched by a similar O* flux that supplies the required H* by charge exchange. Upflowing O*fluxes ofmagnitudes larger than 108 cm '2 s 'l have beenobserved in the topside ionosphere throughout the high- latitude region[Lockwood and Titheridge, 1981; Lockwood, 1982, 1984]. Heelis et al. [1984] have studied two cases of exceptionally large flux (>10 løcm '2s 'l) observed atan altitude of 900 km by DE 2, and foundthemto be associated with intense upward field-aligned currents. Unless it is accelerated, the O* outflow is predicted to be much smaller than the upward H* fluxes.In addition, upflowing O+ is likely to be converted to H* via the accidentally resonant charge exchange reaction O++ H -• O + H + astheO+ moves through theneutral hydrogen. However, if the O+ flow time scalebecomes shorter than the charge exchange period,thensignificant portions of the O + plasma can escape from the ionosphere. It was argued [Moore, 1980, 1984] that. in •High Altitude Observatory, National Center for Atmospheric Research, Boulder, Colorado. 2Department of Space Physics andAstronomy, Rice University, Houston, Texas. 3Space Physics Laboratory, NASAMarshall Space Flight Center, Huntsville, Alabama. 4Centerfor Space Physics,Universityof Texas at Dallas, Richardson,Texas. Copyright 1992 by theAmerican Geophysical Union. Paper number 9ZIA01435. 0148-0227/92/92JA-01435505.00 order for appreciable O + outflow to occur, O + acceleration must occur near or below the crossover altitude zHO where neutral O and H have equal number densities. If anacceleration mechanism is notpresent below thisaltitude, then the upward O+ flux should be substantially less than that of H+. Observations from Earth-orbiting satellites have shownthat theenergetic upfiowing ions (O + and H+) areseen both asfield- aligned narrow pitch angle"ion beams" and as confined pitch angle "ionconics" [Shelley et al., 1976; Sharp et al., 1977, 1979; Ghielmetti et al., 1978; Klumpar, 1979; Cellin et at., 1981; Gorney et al., 1981; Yau et al., 1984]. The pitch angle distributions of the ionospheric ions indicatethe existence of processes capable of providing acceleration in two independent ways: (1) acceleration primarily along the magnetic field direction, associated with the signatures of parallelelectrostatic fields, and (2) acceleration or heatingof the cold ionospheric ions perpendicular to themagnetic field, by such mechanisms as ion cyclotronwaves, after which the magneticmirror force converts part of the ions' absorbed perpendicular energyinto parallel energy for upward motion. A realistic scenario has been proposed [Ungstrup et al., 1979; Heelis et al., 1984] that the ionospheric ions are first transversely accelerated or heated by ion cyclotron waves,and thenthe magnetic mirror forceraises the ionsto the regionof largeparallel electric field wherethey are further accelerated. Besides parallel electrostatic fields and ion cyclotron waves, there are at least two other mechanisms involved in the ion acceleration or heating processes. These are ion species pressure gradient forcesand arebipolar electric fields [Moore, 1984]. To distinguishfrom the large scale auroral parallel potential drop of a few kilovolts, the arebipolar electricfields areionospherically generated andaretypically very weak (a few volts). In hydrostatic equilibrium, the pressure gradient forceis balanced by the gravitational forcefor eachspecies.However, the much lighter electrons would then escape, leavingthe ions behind. Therefore an ambipolar electric field develops to equalize thescale heights of electrons and ions. At low altitudes, 4- an O -dominated plasmasetsup a vertical arebipolar electric field under whose influence H + ions experience upward acceleration in excess of their gravitational acceleration. In regions of parallelcurrents, the ambipolar electric field readjusts sothata steady current canbe maintained. Upflowing ionosphericions are a frequently occurring phenomenon overthe auroral regions [Shelley et al., 1976; Sharp et al., 1977;Ghielmetti et al., 1978; Coilin et al., 1981;Gorneyet al., 1981]. Statistical studies by Yau et al. [1984] and 16,855