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Ann. Geophys., 20, 1943-1958, 2002
www.ann-geophys.net/20/1943/2002/
© European Geosciences Union 2002
A statistical study of ion energization at 1700 km in the auroral region
M. Hamrin1, P. Norqvist1, T. Hellström2, M. André3, and A. I. Eriksson3
1Theoretical Physics, Umeå University, SE-901 87 Umeå, Sweden
2Department of Computing Science, Umeå University, SE-901 87 Umeå, Sweden
3Swedish Institute of Space Physics, Uppsala Division, Box 537, SE-751 21 Uppsala, Sweden
Correspondence to: M. Hamrin (hamrin@space.umu.se)
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)
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