Introduction

In the years since Sharp et al. [1977] first deduced the existence of a mechanism that accelerates ions transversely to the magnetic field at auroral latitudes, ion conics have come to be recognized as a ubiquitous feature of the aurora. Since that time many studies have attempted to determine the mechanisms which create this heating. Suggested wave modes include heating by lower hybrid cavitons [Kintner et al., 1986], electromagnetic ion cyclotron (EMIC) waves [e.g., Erlandson et al., 1994], and a variety of broadband extremely low frequency (BBELF) modes including solitary kinetic Alfv\'en waves [Knudsen and Wahlund, 1998], ion acoustic waves [Wahlund et al., 1998], inhomogeneous energy density driven instability [Amatucci et al., 1998; Koepke et al., 1999], and electrostatic solitary waves [Ergun et al., 1998]. Recently, the following consistent picture has emerged from rocket and satellite observations at altitudes below 4200 km [Lynch et al., 1996; Knudsen et al., 1998; André et al., 1998; Lund et al., 1997]: most of the transverse ion heating is due to one or more of the BBELF modes, but EMIC waves also contribute, particularly on the nightside. While lower hybrid waves may be important at sounding rocket altitudes, they are less important above 2000 km.

One important constraint on models of ion outflow is the species dependence of the transverse acceleration. For BBELF waves, the surprising answer is that the degree of energization is independent of ion mass [Knudsen et al., 1994; Norqvist et al., 1996; Lund et al., 1997]. By contrast, when EMIC waves accompany an ion conic, He⁺ is most effectively heated [Lund et al., 1998], and O⁺ is heated more effectively than H⁺ [Erlandson et al., 1994]. Such a preferential heating is needed to explain the observed He⁺ fluxes in a sizeable fraction of the upflowing ion events examined by Collin et al. [1988]. The X-type He⁺ distributions observed in the dayside outer magnetosphere have been interpreted as signatures of preferential heating by EMIC waves [Anderson and Fuselier, 1994], and a similar mechanism is believed to account for anomalously high ³He abundances in impulsive solar flares [Temerin and Roth, 1992; Roth and Temerin, 1997].

Because more than one wave mode can accelerate ions transversely to the geomagnetic field, a comparison of how effectively such acceleration mechanisms operate under similar conditions can shed light on the relative importance of these mechanisms. Previous studies of O⁺ outflow [André et al., 1998; Norqvist et al., 1998] have shown that BBELF waves are responsible for most of the number flux; however, these studies do not investigate energy fluxes or outflow of lighter ions. Because EMIC waves can selectively enhance the energies and number fluxes of certain species [Temerin and Roth, 1992; Lund et al., 1998], we must ask whether EMIC waves can dominate the outflow of He⁺ in particular or whether BBELF ion conics remain most important in accounting for the energy fluxes and number fluxes of outflowing ions. This paper presents a direct comparison of transverse heating by BBELF and EMIC waves under similar conditions. The next section shows an auroral pass in which transverse heating by both BBELF and EMIC waves occurs. Using data from several such passes, we then compare the relative importance of the two mechanisms in terms of energies, energy fluxes, number fluxes, and densities.


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Title Page | Abstract | Introduction | Data | Direct Comparison of Mechanisms | Conclusion | Acknowledgements | References