Astronomy and Astrophysics – Astrophysics
Scientific paper
Dec 2003
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2003a%26a...412..529m&link_type=abstract
Astronomy and Astrophysics, v.412, p.529-539 (2003)
Astronomy and Astrophysics
Astrophysics
4
Magnetohydrodynamics (Mhd), Waves, Sun: Corona
Scientific paper
The propagation and the dissipation of small-amplitude Alfvén waves in an equilibrium configuration characterized by three-dimensional inhomogeneities is investigated. Disturbances are supposed to have a typical wavelength smaller than the scale of nonuniformity of the background structure, which allows us to use a WKB expansion technique. The approach we used is similar to that employed by Petkaki et al. (\cite{petetal98}), who studied the case of an incompressible plasma. In the present paper a compressible cold plasma is considered, which is more suitable than an incompressible plasma to describe the situation of the solar Corona, where beta << 1. Small wavelengths allow to decouple Alfvén from magnetosonic fluctuations at the linear level. Considering small Alfvénic wavepackets, the evolution equations for the position, wavevector, and energy are derived. These equations are similar to those obtained by Petkaki et al. (\cite{petetal98}), and they have the same form only if the background density rho (0) is supposed to be constant. Then, the results found by Petkaki et al. (\cite{petetal98}) for an incompressible plasma are valid also in a compressible cold plasma, provided that rho (0)=const.: in particular, in the presence of regions of chaotic fieldlines the wavevector of Alfvénic perturbations exponentially grows and the dissipation time of the wave is td~ log S, S being the Reynolds number. Thus, a fast dissipation is attained even with large values of S, as in the Corona. The equations derived in the present paper are more general than those of Petkaki et al. (\cite{petetal98}), since they can be used also with a nonuniform background density. The present model is discussed with reference to the problem of coronal heating.
de Franceschis M. F.
Malara Francesco
Veltri Pierluigi
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