#### Title

Solar flares: The onset of magnetic reconnection and the structure of radiative slow-mode shocks

#### Date of Award

Spring 1992

#### Project Type

Dissertation

#### Program or Major

Physics

#### Degree Name

Doctor of Philosophy

#### First Advisor

Terry G Forbes

#### Abstract

This is a theoretical study of two magnetohydrodynamic (MHD) processes associated with magnetic reconnection in large solar flares and other related eruptive phenomena.

The first process determines the effect of magnetic reconnection in the corona upon the loss of equilibrium triggered by the slow evolution of the field lines mapping to the region in the photosphere where the flare occurs. The quasi-static MHD equations are solved for a magnetic field configuration which satisfies line-tied boundary conditions during the eruption of the flare. Such boundary conditions occur because the inertial mass of the photosphere plasma is much greater than that of the corona. The effect of magnetic reconnection is investigated by assuming three distinct characteristic time scales, namely the convective time scale $\tau\sb{p}$ (days) for motion in the photosphere, the reconnection time scale $\tau\sb{r}$ (hours) for reconnection in the corona, and the Alfven time scale $\tau\sb{\rm A}$ (minutes) for wave propagation in the corona. Magnetic energy can be gradually stored in the corona in a time period of $\tau\sb{p}$ as the system evolves through a series of equilibria until it reaches a point where no nearby equilibria are available.

The second process is the magnetic energy conversion occurring in slow-mode MHD shocks in a plasma where both radiation and thermal conduction are important. Standing slow shocks actually convert magnetic energy into heat and kinetic energy. In a radiative and conducting plasma a slow shock dissociates into an extended foreshock, an isothermal subshock, and a downstream radiative cooling region. The research reported here shows that for typical flare conditions, about 2/3 of the magnetic energy conversion in the slow shocks occurs in the subshock while the remaining 1/3 occurs in the foreshock. It is also shown that no stable, steady-state solutions exist for radiative slow shocks in a coronal like environment unless the temperature in the radiative region downstream of the subshock falls below 10$\sp5$ K. (Abstract shortened with permission of author.).

#### Recommended Citation

Xu, Peng, "Solar flares: The onset of magnetic reconnection and the structure of radiative slow-mode shocks" (1992). *Doctoral Dissertations*. 1693.

https://scholars.unh.edu/dissertation/1693