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| Titel |
Compressive fluctuations in the solar wind and their polytropic index |
| VerfasserIn |
B. Bavassano, R. Bruno, H. Rosenbauer |
| 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 ; 14, no. 5 ; Nr. 14, no. 5, S.510-517 |
| Datensatznummer |
250012289
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| Publikation (Nr.) |
 copernicus.org/angeo-14-510-1996.pdf |
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| Zusammenfassung |
| Magnetohydrodynamic compressive fluctuations
of the interplanetary plasma in the region from 0.3 to 1 AU have been
characterized in terms of their polytropic index. Following Chandrasekhar's
approach to polytropic fluids, this index has been determined through a fit of
the observed variations of density and temperature. At least three different
classes of fluctuations have been identified: (1) variations at constant thermal
pressure, in low-speed solar wind and without a significant dependence on
distance, (2) adiabatic variations, mainly close to 1 AU and without a relevant
dependence on wind speed, and (3) variations at nearly constant density, in fast
wind close to 0.3 AU. Variations at constant thermal pressure are probably a
subset of the ensemble of total-pressure balanced structures, corresponding to
cases in which the magnetic field magnitude does not vary appreciably throughout
the structure. In this case the pressure equilibrium has to be assured by its
thermal component only. The variations may be related to small flow-tubes with
approximately the same magnetic-field intensity, convected by the wind in
conditions of pressure equilibrium. This feature is mainly observed in
low-velocity solar wind, in agreement with the magnetic topology (small open
flow-tubes emerging through an ensemble of closed structures) expected for the
source region of slow wind. Variations of adiabatic type may be related to
magnetosonic waves excited by pressure imbalances between contiguous flow-tubes.
Such imbalances are probably built up by interactions between wind flows with
different speeds in the spiral geometry induced by the solar rotation. This may
account for the fact that they are mainly found at a large distance from the
sun. Temperature variations at almost constant density are mostly found in fast
flows close to the sun. These are the solar wind regions with the best examples
of incompressible behaviour. They are characterized by very stable values for
particle density and magnetic intensity, and by fluctuations of Alfvénic type.
It is likely that temperature fluctuations in these regions are a remnant of
thermal features in the low solar atmosphere. In conclusion, the polytropic
index appears to be a useful tool to understand the nature of the compressive
turbulence in the interplanetary plasma, as far as the frozen-in magnetic field
does not play a crucial role. |
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