Astronomy and Astrophysics – Astrophysics
Scientific paper
Jan 2007
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2007a%26a...461..277p&link_type=abstract
Astronomy and Astrophysics, Volume 461, Issue 1, January I 2007, pp.277-284
Astronomy and Astrophysics
Astrophysics
3
Waves, Instabilities, Sun: Photosphere
Scientific paper
Context: The solar surface and photosphere are covered by a network of convective motions of a mainly neutral fluid. Such a neutral motion drags the tiny plasma population along, which results in drifts of the plasma species due to the magnetic field. These drifts can, in turn, excite and amplify plasma perturbations, which is the subject of the present work. Aims: The behaviour of electromagnetic waves is discussed for a weakly ionized plasma with a neutral flow, in a magnetization regime in which an electron drift exists relative to the ions. This drift across the magnetic field is caused by the neutral flow. Methods: Using a standard normal mode approach, the linear dynamics of small perturbations propagating obliquely to the equilibrium magnetic field lines is investigated. In the regime of strong perturbations, in which the convective derivatives in the electron and ion momentum equations are within the same order of magnitude as the time derivatives, a nonlinear analysis is performed by considering spatial scales at which the effects due to collisions can be neglected. Results: A dispersion relation describing the coupled, drift-driven, and dispersive Alfvén modes is obtained for a strongly collisional plasma. The results are applied to the solar photosphere. Without electron drift due to the frequent collisions, the real part of the (kinetic) Alfvén wave frequency practically vanishes; i.e., the KAW is completely damped. It is shown that the KAW is much less damped in the presence of the electron drift. However, the kinetic Alfvén wave cannot be destabilized by this drift. The instability of the drift-driven mode (Farley-Buneman type) is shown to develop when the electron drift exceeds a certain threshold. At spatial scales far exceeding the mean free path of the particles, the non-linear effects result in a self-organization in the form of traveling double vortices.
Petrovic Dan
Poedts Stefaan
Vranjes Jovo
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