Effects of partial ionisation in the solar atmosphere

Abstract

In this thesis techniques are developed for the simulation of partially ionised plasmas in the fluid approximation. These techniques are used to model the evolution of magnetic fields in the partially ionised regions of the solar atmosphere. Single fluid equations for a partially ionised plasma are derived based on the individual equations for each species. A Lagrangian Remap MHD code is then adapted to simulate a plasma of arbitrary degree of ionisation. The effects of the presence of neutrals on the propagation and damping of Alfv´en waves in the solar atmosphere are investigated. Ion-neutral collisions are shown to be an efficient damping mechanism for outwardly propagating Alfv´en waves of frequencies greater than 0.1 Hz, showing that high frequency waves in the outer solar atmosphere cannot originate at the surface of the Sun. Next simulations to show the effects of neutrals on the emergence of magnetic flux from beneath the solar surface into the outer atmosphere are performed. Results from 2D and 3D numerical experiments show that the presence of neutrals increases the amount of magnetic flux that can emerge into the corona. Furthermore, ion-neutral collisions are strong enough to dissipate currents perpendicular to the magnetic field as it emerges. This shows that ion-neutral collisions are a viable mechanism for the formation of force-free (j ∧B = 0) coronal magnetic field from sub-surface field, which is not the case when the plasma is assumed to be fully ionised