A statistical study of ion energization at 1700 km in the auroral region

Abstract

Abstract. We present a comprehensive overview of several potentially relevant causes for the oxygen energization in the auroral region. Data from the Freja satellite near 1700 km altitude are used for an unconditional statistical investigation. The data are obtained in the Northern Hemisphere during 21 months in the declining phase of the solar cycle. The importance of various wave types for the ion energization is statistically studied. We also investigate the correlation of ion heating with precipitating protons, accelerated auroral electrons, suprathermal electron bursts, the electron density variations, K p index and solar illumination of the nearest conjugate ionosphere. We find that sufficiently strong broadband ELF waves, electromagnetic ion cyclotron waves, and waves around the lower hybrid frequency are foremost associated with the ion heating. However, magnetosonic waves, with a sharp, lower frequency cutoff just below the proton gyrofrequency, are not found to contribute to the ion heating. In the absence of the first three wave emissions, transversely energized ions are rare. These wave types are approximately equally efficient in heating the ions, but we find that the main source for the heating is broadband ELF waves, since they are most common in the auroral region. We have also observed that the conditions for ion heating are more favourable for smaller ratios of the spectral densities S E /S B of the broadband ELF waves at the oxygen gyrofrequency.

Multi-spacecraft determination of wave characteristics near the proton gyrofrequency in high-altitude cusp

“…Gurnett and Frank, 1977;Kintner et al, 2000;Wahlund et al, 2003) and are often associated with local ion energization (e.g. André et al, 1998;Hamrin et al, 2002;Bouhram et al, 2003aBouhram et al, , 2003b. Several studies with Interball-1 (see Savin et al, 2004, and references therein) focus on the boundary between cusp and magnetosheath.…”

On the altitude dependence of transversely heated O<sup>+</sup> distributions in the cusp/cleft

“…In the cusp/cleft and at the poleward edge of the auroral oval, recent statistical studies using FAST (Lund et al, 2000) and Freja (Hamrin et al, 2002) satellite data reported a strong link between ion conic events and enhancements of the broad-band extremely low frequency (BBELF) turbulence. Its spectral shape may be modeled by a power law profile versus frequency (Kintner, 1976), as parametrized by a spectral index α.…”

A global comparison of O+ upward flows at 850 km and outflow rates at 6000 km during nonstorm times

“…We use the OP global empirical model of broadband aurora [ Newell et al , 2009], statistical patterns of O + outflow from Chaston et al [2007], correlations between heated O + and resonant waves of Hamrin et al [2002a, 2002b] and averages of broadband electric fields of Kasahara et al [2001] as proxies for transverse wave heating in the altitude range between the DMSP and Polar data sets from 850 km to ∼6000 km. Newell et al [2009] used their empirical model of broadband aurora at 850 km [ Newell et al , 2009, Figure 4a] to argue that these broadly peaked down flowing electron distributions are associated with Dispersive Alfven Waves (DAWs) operating at higher altitudes.…”

“…'s results, it is reasonable to assume that the locations of enhanced ion outflow and the “% Alfvén Wave Driven” shown apply reasonably well to the lower activity levels reported here, though with a reduction in the percent driven by Alfvén waves in the midnight sector (C. Chaston, private communication, 2011). At altitudes below FAST, Hamrin et al [2002a, 2002b] have used observations on the Freja spacecraft (∼1700 km) to show that the existence of O + outflows and waves, which transversely heat O + , are highly correlated. Hamrin et al similarly found a greater occurrence rate of heated O + and resonant waves near midnight.…”