Thermal instabilities in the solar corona

Abstract

In this thesis, several problems relating to thermal instabilities in the solar corona are examined. Chapter 1 gives a brief description of the Sun and corresponding events with particular attention focused on prominences, their formation and eruption. Various problems concerning thermal instabilities are then tackled in the later Chapters. In Chapter 2, the basic MHD equations are introduced and a physical description of the thermal instability mechanism given. The MHD equations are linearised in a uniform, infinite medium and the basic instability criteria obtained. Chapter 3 investigates the normal mode spectrum for the linearised MHD equations for a cylindrical equilibrium. This spectrum is examined for zero perpendicular thermal conduction, with both zero and non-zero scalar resistivity. Particular attention is paid to the continuous branches of this spectrum, or continuous spectra. For zero resistivity there are three types of continuous spectra present, namely the Alfven, slow and thermal continua. It is shown that when dissipation due to resistivity is included, the slow and Alfven continua are removed and the thermal continuum is shifted to a different position (where the shift is independent of the exact value of resistivity). The 'old' location of the thermal continuum is covered by a dense set of nearly singular discrete modes called a quasi-continuum, for equilibria with the thermal time scale much smaller than the Alfven time scale. This quasi-continuum is investigated numerically and the eigenfunctions are shown to have rapid spatial oscillating behaviour. These oscillations are confined to the most unstable part of the equilibrium based on the Field criterion and may be the cause of fine structure in prominences. In Chapter 4, the normal mode spectrum for the linearised MHD equations is examined for a plasma in a cylindrical equilibrium. The equations describing these normal modes are solved numerically using a finite element code. In the ideal case the Hain-Lust equation is expanded and a WKB solution obtained for large axial wave numbers. This is compared to the numerical solutions. In the non-ideal case, the ballooning equations describing localised modes are manipulated in an arcade geometry and a dispersion relation derived. It is illustrated that as the axial wave number k is increased, the fundamental thermal and Alfven modes can coalesce to form overstable magnetothermal modes. The ratio between the magnetic and thermal terms is varied and the existence of the magnetothermal modes examined. The corresponding growth rates are predicted by a WKB solution to the ballooning equations. The interaction of thermal and magnetic instabilities and the existence of these magnetothermal modes may be significant in the eruption of prominences into solar flares. Chapter 5 extends the work presented in Chapter 4 to include the effects of line-tying in a coronal arcade. The ballooning equations which were introduced in Chapter 4 are manipulated to give a dispersion relation. This relation is a quadratic in the square of the azimuthal wave number m if parallel thermal conduction is neglected and a cubic in m2 if parallel conduction is included. Rigid wall boundary conditions are applied to this dispersion relation. This dispersion relation is then solved numerically subject to these boundary conditions and the solutions plotted. Unfortunately the expression for the thermal continuum in line-tied arcades is required since the thermal continuum must play an important role in the proceedings. This calculation is left for future work. From the results obtained, it can be seen that the thermal instability can play a major part in prominence formation and destruction. The thermal instability may help create the prominence. Resistivity and perpendicular thermal conduction can cause of the observed fine scale structure. Finally, a neighbouring thermal instability may trigger a magnetic instability that causes the prominence to erupt.