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| Titel |
Concerning the generation of geomagnetic giant pulsations by drift-bounce resonance ring current instabilities |
| VerfasserIn |
K.-H. Glassmeier, S. Buchert, U. Motschmann, A. Korth, A. Pedersen |
| 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 ; 17, no. 3 ; Nr. 17, no. 3, S.338-350 |
| Datensatznummer |
250013690
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| Publikation (Nr.) |
 copernicus.org/angeo-17-338-1999.pdf |
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| Zusammenfassung |
Giant pulsations are nearly monochromatic
ULF-pulsations of the Earth's magnetic field with periods of about 100 s and
amplitudes of up to 40 nT. For one such event ground-magnetic observations as
well as simultaneous GEOS-2 magnetic and electric field data and proton flux
measurements made in the geostationary orbit have been analysed. The
observations of the electromagnetic field indicate the excitation of an odd-mode
type fundamental field line oscillation. A clear correlation between variations
of the proton flux in the energy range 30-90 keV with the giant pulsation event
observed at the ground is found. Furthermore, the proton phase space density
exhibits a bump-on-the-tail signature at about 60 keV. Assuming a drift-bounce
resonance instability as a possible generation mechanism, the azimuthal wave
number of the pulsation wave field may be determined using a generalized
resonance condition. The value determined in this way,
m = - 21 ± 4, is in accord with the value m = - 27 ± 6 determined
from ground-magnetic measurements. A more detailed examination of the observed
ring current plasma distribution function f shows that odd-mode type
eigenoscillations are expected for the case ∂f / ∂W > 0,
much as observed. This result is different from previous theoretical studies as
we not only consider local gradients of the distribution function in real space,
but also in velocity space. It is therefore concluded that the observed giant
pulsation is the result of a drift-bounce resonance instability of the ring
current plasma coupling to an odd-mode fundamental standing wave. The generation
of the bump-on-the-tail distribution causing ∂f / ∂W > 0
can be explained due to velocity dispersion of protons injected into the ring
current. Both this velocity dispersion and the necessary substorm activity
causing the injection of protons into the nightside magnetosphere are observed.
Key words. Magnetospheric physics (energetic particles
, trapped; MHD waves and instabilities) · Space plasma physics (wave-particle
interactions). |
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