Direct Comparison of Mechanisms

The ratios of the He⁺ energy to the O⁺ energy fall in the range 0.64–1.39 for the BBELF events and 1.10–4.62 for the EMIC events, with the latter ratio larger on each pass. Similarly, the ratios of the He⁺ energy to the H⁺ energy are 0.45–1.36 for the BBELF events and 2.10–6.75 for the EMIC events. For the EMIC events we find E_He > E_O > E_H, consistent with our earlier report [Lund et al., 1998]; the mass ordering of energies in the BBELF events, although monotonic, is not consistent from one event to the next.

Figure 2 shows the relative importance of BBELF and EMIC waves in ion heating for the events listed in Table 1. We see that in all five cases, the ratio of energy fluxes between the EMIC event and the BBELF event is larger for He⁺ than for H⁺ or O⁺, as would be expected for preferential acceleration of He⁺ by EMIC waves. This pattern, however, does not hold for number fluxes; in two of the five cases the overall number flux is at least as high in the EMIC event as in the BBELF event while the He⁺ number flux actually drops. This result is contrary to the prediction of He⁺ flux enhancements for the Temerin-Roth acceleration mechanism, although the range of parameters appropriate in the auroral zone differ greatly from the range of parameters considered by Temerin and Roth [1992], which are appropriate for impulsive solar flares. The EMIC events are also not associated with enhancements in the relative or absolute He⁺ density. This point is illustrated in Figure 3 for the pass shown in Figure 1; the He⁺ concentration, shown by the squares, is enhanced only poleward of the EMIC event. The discrepancy between these results and the predictions of Temerin and Roth [1992] could arise because the acceleration by EMIC waves is occurring at a higher altitude than the source region of the plasma, and some transverse ion heating, possibly by lower hybrid waves, may occur at these lower altitudes [Lund et al., 1998].

We also see from Figure 2 that for all five passes studied, the BBELF event has the higher overall energy flux, and that the ratio of overall energy fluxes is smaller than the ratio of overall number fluxes for the EMIC events compared with the BBELF events. This result also holds for H⁺ and O⁺ separately. The BBELF emissions therefore are more effective than the EMIC emissions at heating H⁺ and O⁺. This result is expected since BBELF emissions by definition can be cyclotron resonant with all three major species at a given altitude and can maintain this resonance over a sizeable altitude range even if the spectral shape is independent of altitude. In fact, a previous study of Freja data [Norqvist et al., 1996] showed that as little as 3% of the energy at the O⁺ gyrofrequency in BBELF emissions can account for the observed O⁺ energization. EMIC emissions, being narrow-banded, can only be resonant in a narrow altitude range. The fundamental resonance with He⁺ seems more efficient than the harmonic resonance with~O⁺; indeed, three of the five passes have a higher He⁺ energy flux in the EMIC event than the BBELF event. Since their frequency is below the local proton gyrofrequency, EMIC waves cannot be cyclotron resonant with H⁺ at or anywhere below the observation point, so minimal effects on H⁺ are observed.

There are indications that EMIC waves may play a larger role in transverse ion acceleration during geomagnetically active periods than during quiet periods. We have found that the occurrence probability of EMIC ion conics increases with Kp [Lund et al., 1999] in a manner consistent with the Kp dependence reported for EMIC waves alone [Saito et al., 1987]. In addition, of the five passes examined here, all but the two most magnetically quiet had higher total number fluxes during the EMIC events than during the BBELF events. However, a study by Norqvist et al. [1998], which examined only O⁺ outflow, showed that number fluxes in BBELF events are also well-correlated with Kp. A more detailed study would be needed to determine which effect is more important.

Note also that because we have restricted this study to passes in which both EMIC and BBELF emissions are associated with ion conics, we have artificially confined this study to the premidnight sector, where EMIC emissions are most prevalent [Saito et al., 1987; Erlandson and Zanetti, 1998]. BBELF emissions, by contrast, are found at all local times [André et al., 1998; Lund et al., 1997]. While EMIC emissions can be locally important for He⁺ outflow in the aurora, a fair comparison of the two mechanisms would show an even more lopsided dominance of BBELF emissions in transverse ion acceleration in the aurora.

Next: Conclusion
Previous: Data

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