Introduction

Ion heating and acceleration perpendicular to the magnetic field is a common feature in the auroral region. These ions acquire energies of up to several keV and then move up the field line, adiabatically exchanging perpendicular for parallel energy to produce ion conics [Sharp et al., 1977]. A wide variety of waves, including electromagnetic ion cyclotron (EMIC) waves [e.g., Chang et al., 1986], have been suggested as transverse acceleration mechanisms. Preferential acceleration of O⁺ by auroral EMIC waves has been reported [Erlandson et al., 1994], and it has recently been shown that for low concentrations of helium (n_He⁺ / n_e < 0.01), EMIC waves can heat He⁺ to produce the double-conic He⁺ distributions seen near the equator in the dayside outer magnetosphere [Horne and Thorne, 1997]. The existence of a mechanism which preferentially heats He⁺ at auroral latitudes was inferred from DE-1 data by Collin et al. [1988], who found that number fluxes for a large fraction of upflowing He⁺ events greatly exceeded the fluxes expected from a cold ionosphere.

EMIC waves have also been linked to several other physical processes in the aurora. The finite E_|| of oblique EMIC waves can accelerate electrons to energies of several keV, producing modulations in the downgoing electron flux and thereby creating flickering aurora [Temerin et al., 1986]; more recently, EMIC waves have been observed together with modulated electron fluxes [Lund et al., 1995; McFadden et al., 1998]. These waves have also been linked to the recently discovered short-duration solitary electric field structures, which may carry a significant fraction of the parallel potential drop [Ergun et al., 1998].

The generation mechanism for these waves is unclear. The leading candidate is an instability driven by the auroral electron beam [Temerin and Lysak, 1984; Oscarsson et al., 1997]. However, unless the electron distribution used in the linear instability calculation is carefully chosen, the growth rates obtained are too low to account for the observed waves, and observations of heavy ion cyclotron waves cannot be explained by this mechanism (see Lund and LaBelle [1997] for discussion).

In this letter we report observations of EMIC waves in association with resonantly accelerated He⁺ in the aurora. The acceleration mechanism that we infer from the data is analogous to the cyclotron resonant acceleration mechanism which has been proposed in impulsive solar flares [Temerin and Roth, 1992].

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