Up to now, observations had been unable to show conclusively a one-to-one correspondence between perpendicular ion acceleration and a particular type of plasma wave within the O + source region below 2000 km. In this paper we demonstrate that intense (100-300 mV/m) lower hybrid waves are responsible for transvemely accelerating H + and O + ions to characteristic energies of up to 6 eV. This wave-particle interaction takes place in thin fdamentary density cavities oriented along geomagnetic field lines. The measurements we discuss were conducted in the nightside auroral zone at altitudes between 500 km and 1100 km. Our results are consistent with theories of lower hybrid wave condensation and collapse. the ionosphere below 2000 km, where the O + density is sufficient to account for the large observed upward ion fluxes. ally bound and therefore unable to escape to higher altitudes. Since a significant parallel electric field is not likely to exist in the ionosphere (its high conductivity would short the field out), the ions have to be "raised" by a mechanism other than a single-step parallel energization process. Recent particle measurements conducted in the topside auroral ionosphere have exhibited numerous examples of ion conics. This name refers to the shape the observed ion distribution functions have in velocity space: strongly peaked in pitch angle, indicating some form of heating transverse to the ambient magnetic field. This transverse heating allows the ions to surge upward via the grad B force. The parallel energy of the upgoing ions will adiabatically increase at the expense of their perpendicular energy, gradually transforming the original transversely heated distribution into a more field-aligned one, i.e., a conic (please see Figure 1). As will be discussed in the body of this work, the mechanism responsible for this behavior in the topside ionosphere is a wave-particle interaction in the form of lower hybrid waves, which accelerate the ions perpendicular motion. The results of this process are known as transversely accelerated ions (TAI). Many investigators suggest that the source driving the ion-energizing waves is the free energy associated with energetic auroral electrons [Chang and Coppi, 1981; Retterer et al., 1986; Crew and Chang, 1987]. The first observations of TAI were reported by Sharp et al. [1977]. Since then, various other measurements have been carried out, but only a small number of them at the lower altitudes in the source region where the O + transverse heating must originate [Whalen et al., 1978; Klurnpar, 1979; Yau et al., 1983; Yau et al., 1986; Kintner et al., 1986; LaBelle et al., 1986b; Kintner et al., 1989; Kintner et al., 1991; Garbe et al., 1992; Arnoldy et al., 1992; Kintner et al., 1992]. At these heights, the energy of the upward traveling ions is rarely detected above 500 eV and their conical distribution is "shallow" (pitch angles of 90 ø to 140ø). Once in the auto-16,935 16,936 VAGO ET AL.: ION HEATING BY LOCALIZED LOWER HYBRID WAVES Conic Ions Heated Ions Thermal Ions...
The widespread existence of naturally excited broadband electrostatic wave activity up to 15 kHz, with predominantly parallel polarization, have been detected in the topside auroral ionosphere by the wave experiment onboard the Freja S/C. Amplitudes of a few mV/m electric field fluctuations and a few % density fluctuations are typically observed, but stronger cases are quite common. These emissions are most often observed within the auroral energization region according to the plasma instrument data, and coincide frequently with Alfvén wave activity from a few Hz to tens of Hz. This suggests that the electrostatic emission is linked to the nonlinear evolution of kinetic Alfvén waves.
High time resolution ion mass spectrometer distribution function measurements and wave data from a sounding rocket flight over an aurora have revealed the fine structure of the transverse ion acceleration mechanism in the upper ionosphere. The transversely accelerated ion (TAI) events can occur in a volume with a cross‐field dimension as small as several tens of meters and thus appear as 50–100 ms ion bursts due to the rocket payload motion. Bulk heating to a characteristic energy of several eV and tail heating in the direction perpendicular to B of a few percent of ambient ions to a characteristic energy the order of 10 eV occur for both hydrogen and oxygen ions. The TAI at 90° pitch angle occur in localized regions of intense lower hybrid waves and in regions of density depletion. On close examination of the correlation between the wave bursts and the TAI it is believed that the waves produce the ion acceleration. The TAI occur during periods of field‐aligned auroral electron bursts. Finally, near 1000 km altitude they occur about once every second. If the event presented here is considered average, the flux of TAI oxygen ions above 7 eV could account for the ion conic fluxes measured by the ISIS spacecraft.
414 in the third paragraph in the right-hand column, the second complete sentence should read "Since about 75 % of the few hundred ion bursts measured aboard TOPAZ 3 are associated with wave bursts, the acceleration occurs along an extended length of field line and lasts for several seconds or more."
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