Growing drift-Alfvén modes in collisional solar plasma

Abstract

Context. Solar plasmas are structured and stratified both vertically and horizontally. The presence of density gradients and magnetic fields results in an additional wave which can be electrostatic (the drift wave) and electromagnetic (the drift-Alfven wave). Aims. The stability is discussed of the drift-Alfven wave which is driven by the equilibrium density gradient, in both unbounded and bounded, collisional solar plasmas, including the effects of both hot ions and a finite ion Larmor radius. The density gradient in combination with the electron collisions with heavier plasma species is the essential source of the instability of the electrostatic drift mode which is coupled to the dispersive Alfven mode. Methods. An analytical linear normal mode analysis is used for the description of the waves in spatially unlimited plasma. In the application to the magnetic structures the complex eigen-modes and the corresponding complex discrete eigen-frequencies in cylindric, radially inhomogeneous, collisional and bounded plasma are derived and discussed. Results. A detailed derivation of the hot ion (the finite ion Larmor radius) contribution is performed within the two fluid model. In the analysis of modes in an unbounded plasma the exchange of identity between the electrostatic and electromagnetic modes is demonstrated. Due to this, the frequency of the electromagnetic part of the mode becomes very different compared to the case without the density gradient. In the case of a bounded plasma the dispersion properties of modes involve a discrete poloidal mode number, and eigen-functions in terms of Bessel functions with discrete zeros at the boundary. The results are applied to coronal and chromospheric plasmas.