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Ann. Geophys., 24, 667-677, 2006
www.ann-geophys.net/24/667/2006/
© European Geosciences Union 2006
Relationship between low-frequency electric-field fluctuations and ion conics around the cusp/cleft region
W. Miyake1, A. Matsuoka2, and T. Mukai2
1National Institute of Information and Communications Technology, 4-2-1, Nukui-kita, Koganei, Tokyo 184-8795, Japan
2Institute of Space and Astronautical Science, 3-1-1, Yoshinodai, Sagamihara, 229-8510, Japan
Abstract. We investigated the relationship between low-frequency (0.2-4.0 Hz)
electric-field fluctuations (LEFs) and ion conics around the dayside
cusp/cleft region in the altitude range from 5000 to 10000km from
observations made by the Akebono satellite. Ion conics were generally
associated with intense LEFs. We found a significant correlation between the
power spectral density of LEFs at any frequency and the energy of
simultaneously observed ion conics. Ion conics with a conic angle near
90 deg and those more aligned with magnetic field lines both had an
equivalent correlation with the local intensity of the LEFs. The LEFs associated
with near-perpendicular ion conics were, however, generally more intense than
those associated with folded conics. The difference was clearer for
low-energy conics. These results are in agreement with a scenario of
height-integrated heating of ions and energization of ions by electromagnetic
energy supplied by local LEFs. Ions generally stay in the energization region
during their upward motion along the field line, so that more folded ion
conics with weak energization reach the same energy level as
near-perpendicular conics with strong energization, due to the difference in
integration time. The limit on residence time in the intense heating region
causes the clearer difference for low-energy conics. We set up a simple model
to examine the relationship between the energization rate and the evolution
of ion conics along the field lines, and obtained good agreement with the
observation results.
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