28 th ICPIG, July 15-20, 2007, Prague, Czech Republic C.7 Electron-drift driven mode in the solar atmosphere D. Petrovi´ c 1,2 , J. Vranjes 1 , and S. Poedts 1 1 Centre for Plasma Astrophysics, KU Leuven, Celestijnenlaan 200B, B-3001 Leuven, Belgium 2 PLASMANT, Department of Chemistry, University of Antwerp, Belgium A dispersion relation describing the coupled, drift-driven, and dispersive Alfv´ en 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) Alfv´ en 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 Alfv´ en 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. 1. Introduction The solar surface and photosphere are covered by a net- work 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 mag- netic field. These drifts can, in turn, excite and amplify plasma perturbations, which is the subject of the present work. The behaviour of electromagnetic waves is discussed for a weakly ionized plasma with a neutral flow, in a mag- netization regime in which an electron drift exists relative to the ions. This drift across the magnetic field is caused by the neutral flow. Using a standard normal mode ap- proach, the linear dynamics of small perturbations prop- agating obliquely to the equilibrium magnetic field lines is investigated. According to the dynamo theory, a conductor moving across magnetic field lines produces an electric field per- pendicular both to this magnetic field and to the veloc- ity vector when the following conditions (the ‘dynamo inequalities’) are satisfied: Ω e /ν e ≫ 1, and 1 ≪ Ω e Ω i /ν e ν i ≪∞. Here, ν e and ν i denote the electron and ion collision frequencies, respectively, and Ω e and Ω i are the electron and ion cyclotron frequencies [1], [2]. The physical meaning of the dynamo theory is that, in the initial magnetic field, the ions are less closely bound to the mag- netic field lines than the electrons. As a result, a flow of neutral atoms can carry the ions along more easily than the electrons in such a system. Because of that, a flow of neu- tral atoms can induce a charge separation in a partially ion- ized inhomogeneous plasma. When this happens, a charge- separation electric field occurs, and this field reaches a lim- iting value in the ambipolar diffusion regime, i.e., when the electrons and the ions diffuse at the same rate. In nature, such perpendicular electric fields exist, for ex- ample, in the Earth’s ionosphere (h≃ 85 − 110 km) and in the solar photosphere. Both systems contain regions with weakly ionized plasma where the dynamo inequal- ities and are satisfied. This means that the neutral flow in the photosphere can indeed produce these perpendicu- lar electric fields, which, in combination with the magnetic fields, causes an  E ×  B−drift. Due to the different mag- netization regime, described by the dynamo inequalities, a relative drift of the electrons to the ions will appear. In the present paper, we investigate and discuss the stability of electromagnetic waves in a weakly ionized, strongly colli- sional plasma with such an electron drift across the mag- netic field. 2. Model and basic equations The dynamics of this system, in the regime mentioned above, is described by the electron continuity equation, the equations of motions for both the electrons and the ions, and the quasi-neutrality equation. Viscous effects are negli- gible here since the effects of collisions with neutrals dom- inate for the considered set of parameters and for the wave numbers we consider in the present study. For the hot ions the effect of the finite Larmor radius is included. Hence, the diamagnetic contribution to the polarization drift and the stress tensor drift are taken into account. We consider the limiting case of when the plasma β is higher than the electron-ion mass ratio, i.e. β>m e /m i , so that the electron inertia is negligible. The approximation also implies that the electron-neutral collision frequency ν en is much higher than the wave frequency ω. In the equilibrium, an analytical expression for the charge separation electric field is obtained in terms of the velocity of the neutral convection, v n : e  E 0 ≈−v n m i ν in  Ω e Ω i ν en ν in 1 1+ ν ei /ν en − Ω 2 i ν 2 in 1 1+ ν en /ν ei  28 ICPIG, July 15-20, 2007, Prague, Czech Republic th Topic number: 07 1920