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, Kp index and solar illumination of the nearest conjugate ionosphere. We find that sufficiently strong broad-band 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 SE /SB of the broadband ELF waves at the oxygen gyrofrequency.Key words. Ionosphere (auroral ionosphere; wave propogation) Magnetospheric physics (electric fields)
Highlights
About 30 years ago it was discovered that the magnetospheric plasma includes a significant amount of plasma which has to be of ionospheric origin (Shelley et al, 1972)
The wave types we focus on are BB-ELF waves, electromagnetic ion cyclotron (EMIC) waves, MSC waves, and waves with frequencies around the lower hybrid frequency, all of which are often observed in the auroral region of the terrestrial magnetosphere
To include only waves that are sufficiently strong enough to cause significant ion heating, we focus on BB-ELF waves with a spectral density larger than 0.012/Hz
Summary
Many different energizing mechanisms can be identified, some of which act at the same time in some regions, while others act separately. Magnetosonic waves with a sharp, lower frequency cut off just below the proton gyrofrequency (in the following denoted MSC waves; Hamrin et al, 2001) are proposed to be able to heat ions (Le Queau et al, 1993; Rauch et al, 1993) These waves are often too weak to cause intense ion heating (Andreet al., 1998). Lund et al (2000) used data from the FAST satellite to conclude that BB-ELF waves and EMIC waves are the main cause for the transverse energization at 2000 km altitude, while lower hybrid waves are much less important. More than one wave type can energize the ions perpendicularly to the geomagnetic field It is, important to investigate and compare the influences of the various waves on the ion heating in order to be able to elucidate the importance of the different energizing mechanisms.
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