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| Titel |
Multi-spacecraft determination of wave characteristics near the proton gyrofrequency in high-altitude cusp |
| VerfasserIn |
D. Sundkvist, A. Vaivads, M. André, J.-E. Wahlund, Y. Hobara, S. Joko, V. V. Krasnoselskikh, Y. V. Bogdanova, S. C. Buchert, N. Cornilleau-Wehrlin, A. Fazakerley, J.-O. Hall, H. Rème, G. Stenberg |
| Medientyp |
Artikel
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| Sprache |
Englisch
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| ISSN |
0992-7689
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| Digitales Dokument |
URL |
| Erschienen |
In: Annales Geophysicae ; 23, no. 3 ; Nr. 23, no. 3 (2005-03-30), S.983-995 |
| Datensatznummer |
250015196
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| Publikation (Nr.) |
 copernicus.org/angeo-23-983-2005.pdf |
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| Zusammenfassung |
| We present a detailed study of waves with frequencies near the proton
gyrofrequency in the high-altitude cusp for northward IMF as observed by
the Cluster spacecraft. Waves in this regime can be important for
energization of ions and electrons and for energy transfer between
different plasma populations.
These waves are present in the entire cusp with the highest amplitudes
being associated with localized regions of downward precipitating ions,
most probably originating from the reconnection site at the magnetopause.
The Poynting flux carried by these waves is downward/upward at frequencies
below/above the proton gyrofrequency, which is consistent with the waves being
generated near the local proton gyrofrequency in an extended region along
the flux tube. We suggest that the waves can be generated by the precipitating
ions that show shell-like distributions.
There is no clear polarization of the perpendicular wave components with
respect to the
background magnetic field, while the waves are polarized in a
parallel-perpendicular plane. The coherence length
is of the order of one
ion-gyroradius in the direction perpendicular to the ambient magnetic
field and a few times larger or more in the parallel direction. The
perpendicular
phase velocity was found to be of the order of 100km/s, an order of
magnitude lower than the local Alfvén speed.
The perpendicular wavelength is of the order of a few proton gyroradius or less.
Based on our multi-spacecraft observations
we conclude that the waves cannot be ion-whistlers,
while we suggest that the waves can
belong to the
kinetic Alfvén
branch
below the proton gyrofrequency fcp
and
be described as
non-potential ion-cyclotron waves
(electromagnetic ion-Bernstein waves)
above.
Linear wave growth calculations using kinetic code show considerable wave
growth of non-potential ion cyclotron waves at wavelengths agreeing with
observations.
Inhomogeneities in the plasma on
the order of the ion-gyroradius suggests that inhomogeneous (drift) or nonlinear
effects or both of these should
be taken into account. |
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