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Ann. Geophys., 25, 1603-1615, 2007
www.ann-geophys.net/25/1603/2007/
© European Geosciences Union 2007
CHAMP observation of intense kilometer-scale field-aligned currents, evidence for an ionospheric Alfvén resonator
M. Rother1, K. Schlegel2, and H. Lühr1
1GeoForschungsZentrum Potsdam, Germany
2Max-Planck-Institute for Solar System Research (formerly MPAE Lindau), Katlenburg-Lindau, Germany
Abstract. Bursts of very intense kilometer-scale field-aligned currents (KSFACs) are
observed quite frequently by the CHAMP satellite when passing through the
auroral region. In extreme cases estimated current densities exceed
3 mA/m². Typical scale sizes of these KSFACs are 1 km.
The low-Earth, polar orbiting satellite CHAMP allows one to assess KSFACs down to
scales of a couple of 100 m based on its high-precision magnetic field vector
data sampled at 50 Hz. Using data from 5 years (2001–2005) details of these
currents can be investigated. In our statistical study we find that most of
the KSFAC bursts and the strongest events are encountered in the cusp/cleft
region. Significantly fewer events are found on the nightside. The affected
region is typically 15°–20° wide in latitude. There
seems to be some dependence of the current intensity on the level of magnetic
activity, Kp. On the other hand, no dependence has been found on sunspot
number, the solar flux level, F10.7 or the solar zenith angle. The latitude,
at which KSFAC bursts are encountered, expands equatorward with increasing
Kp. This trend follows well the auroral oval expansion during enhanced
magnetic activity. These KSFACs are generally accompanying large-scale
FAC sheets, and they are predominantly associated with Region 1 currents. We
propose an explanation of the KSFACs in terms of Alfvén waves trapped in a
ionospheric resonator, which is initiated when the convection
electric field or current strength surpasses a critical value. Many
properties of such a resonator are in agreement with our KSFAC results.
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