Effects of ambipolar diffusion on waves in the solar chromosphere

Abstract

Context. The chromosphere is a partially ionized layer of the solar atmosphere that mediates the transition between the photosphere where the gas motion is determined by the gas pressure and the corona dominated by the magnetic field. Aims. We study the effect of partial ionization for 2D wave propagation in a gravitationally stratified, magnetized atmosphere characterized by properties that are similar to those of the solar chromosphere. Methods. We adopted an oblique uniform magnetic field in the plane of propagation with a strength that is suitable for a quiet sun region. The theoretical model we used is a single fluid magnetohydrodynamic approximation, where ion-neutral interaction is modeled by the ambipolar diffusion term. Magnetic energy can be converted into internal energy through the dissipation of the electric current produced by the drift between ions and neutrals. We used numerical simulations in which we continuously drove fast waves at the bottom of the atmosphere. The collisional coupling between ions and neutrals decreases with the decrease in the density and the ambipolar effect thus becomes important. Results. Fast waves excited at the base of the atmosphere reach the equipartition layer and are reflected or transmitted as slow waves. While the waves propagate through the atmosphere and the density drops, the waves steepen into shocks. Conclusions. The main effect of ambipolar diffusion is damping of the waves. We find that for the parameters chosen in this work, the ambipolar diffusion affects the fast wave before it is reflected, with damping being more pronounced for waves which are launched in a direction perpendicular to the magnetic field. Slow waves are less affected by ambipolar effects. The damping increases for shorter periods and greater magnetic field strengths. Small scales produced by the nonlinear effects and the superposition of different types of waves created at the equipartition height are efficiently damped by ambipolar diffusion.