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| Titel |
Nonlinear evolution of the parametric instability: numerical predictions versus observations in the heliosphere |
| VerfasserIn |
F. Malara, L. Primavera, P. Veltri |
| Medientyp |
Artikel
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| Sprache |
Englisch
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| ISSN |
1023-5809
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| Digitales Dokument |
URL |
| Erschienen |
In: Nonlinear Processes in Geophysics ; 8, no. 3 ; Nr. 8, no. 3, S.159-166 |
| Datensatznummer |
250005517
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| Publikation (Nr.) |
 copernicus.org/npg-8-159-2001.pdf |
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| Zusammenfassung |
| Low-frequency turbulence in the solar
wind is characterized by a high degree of Alfvénicity close to the Sun.
Cross-helicity, which is a measure of Alfvénic correlation, tends to
decrease with increasing distance from the Sun at high latitudes as well
as in slow-speed streams at low latitudes. In the latter case, large scale
inhomogeneities (velocity shears, the heliospheric current sheet) are
present, which are sources of decorrelation; yet at high latitudes, the
wind is much more homogeneous, and a possible evolution mechanism is
represented by the parametric instability. The parametric decay of an
circularly polarized broadband Alfvén wave is then investigated, as a
source of decorrelation. The time evolution is followed by numerically
integrating the full set of nonlinear MHD equations, up to instability
saturation. We find that, for <beta> ~ 1, the final cross-helicity
is ~ 0.5, corresponding to a partial depletion of the initial correlation.
Compressive fluctuations at a moderate level are also present. Most of the
spectrum is dominated by forward propagating Alfvénic fluctuations, while
backscattered fluctuations dominate large scales. With increasing time,
the spectra of Elsässer variables tend to approach each other. Some
results concerning quantities measured in the high-latitude wind are
reviewed, and a qualitative agreement with the results of the numerical
model is found. |
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