EDP Sciences
Free access
Volume 375, Number 1, August III 2001
Page(s) 264 - 274
Section The Sun
DOI http://dx.doi.org/10.1051/0004-6361:20010869

A&A 375, 264-274 (2001)
DOI: 10.1051/0004-6361:20010869

Nonlinear fast magnetosonic waves in solar coronal holes

K. Murawski1, R. Oliver2 and J. L. Ballester2

1  Department of Environmental Physics, Technical University of Lublin, ul. Nadbystrzycka 40, 20-618 Lublin, Poland
2  Departament de Física, Universitat de les Illes Balears, 07071 Palma de Mallorca, Spain
    e-mail: ramon.oliver@uib.es, dfsjlb0@uib.es

(Received 2 March 2001 / Accepted 14 June 2001 )

A coronal hole is modeled as a slab of cold plasma threaded by a vertical, uniform magnetic field. A periodic driver acting at the coronal base is assumed to drive the velocity component normal to the equilibrium magnetic field. Previous works indicate that, in the linear regime, only fast mode perturbations propagate, since Alfvén waves are excluded from the model and the slow wave is absent in the cold plasma limit. However, in this work, it is shown that nonlinear terms in the magnetohydrodynamic (MHD) equations give rise to excitation of the velocity component parallel to the equilibrium $\vec {B}$, with a lower amplitude than the normal component. Another consequence of nonlinearities is the generation of higher-frequency Fourier modes, which can be detected by Fourier analyzing the velocity variations above the photosphere. The nature of the nonlinear interactions in the MHD equations determines the frequency of those modes. These interactions are quadratic in the case of the parallel component, while they are cubic in the case of the normal component. Therefore, nonlinearly excited frequencies $2\omega_{\rm d}$, $4\omega_{\rm d}$, $6\omega_{\rm d}$, $\ldots$ are present in the parallel velocity, whereas frequencies $3\omega_{\rm d}$, $5\omega_{\rm d}$, $7\omega_{\rm d}$, $\ldots$ are present in the normal velocity, with $\omega_{\rm d}$ the driving frequency.

Key words: Sun: corona -- Sun: magnetic fields -- magnetohydrodynamics

Offprint request: K. Marawski kamur@akropolis.pol.lublin.pl

© ESO 2001