Analysis of the effect of neutral flow on the waves in the solar photosphere

¹ Center for Plasma Astrophysics, K.U.Leuven, Celestijnenlaan 200B, 3001 Leuven, Belgium e-mail: Dragana.Petrovic@wis.kuleuven.be
² Institute of Physics, PO Box 57, 11001 Belgrade, Serbia

Received: 6 June 2006
Accepted: 31 July 2006

Abstract

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.