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| Titel |
Density and magnetic field fluctuations observed by ISEE 1-2 in the quiet magnetosheath |
| VerfasserIn |
C. Lacombe, G. Belmont, D. Hubert, C. C. Harvey, A. Mangeney, C. T. Russell, J. T. Gosling, S. A. Fuselier |
| 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 ; 13, no. 4 ; Nr. 13, no. 4, S.343-357 |
| Datensatznummer |
250011815
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| Publikation (Nr.) |
 copernicus.org/angeo-13-343-1995.pdf |
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| Zusammenfassung |
| We analyse the fluctuations of the electron
density and of the magnetic field in the Earth's magnetosheath to identify the
waves observed below the proton gyrofrequency. We consider two quiet
magnetosheath crossings i.e. 2 days characterized by small-amplitude waves, for
which the solar wind dynamic pressure was low. On 2 August 1978 the spacecraft
were in the outer magnetosheath. We compare the properties of the observed
narrow-band waves with those of the unstable linear wave modes calculated for an
homogeneous plasma with Maxwellian electron and bi-Maxwellian (anisotropic)
proton and alpha particle distributions. The Alfvén ion cyclotron (AIC) mode
appears to be dominant in the data, but there are also density fluctuations
nearly in phase with the magnetic fluctuations parallel to the magnetic field.
Such a phase relation can be explained neither by the presence of a proton or
helium AIC mode nor by the presence of a fast mode in a bi-Maxwellian plasma. We
invoke the presence of the helium cut-off mode which is marginally stable in a
bi-Maxwellian plasma with α particles: the observed phase relation
could be due to a hybrid mode (proton AIC+helium cut-off ) generated by a non-Maxwellian
or a non-gyrotropic part of the ion distribution functions in the upstream
magnetosheath. On 2 September 1981 the properties of the fluctuations observed
in the middle of the magnetosheath can be explained by pure AIC waves generated
by protons which have reached a bi-Maxwellian equilibrium. For a given wave
mode, the phase difference between B\Vert and the density is
sensitive to the shape of the ion and electron distribution functions: it can be
a diagnosis tool for natural and simulated plasmas. |
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