Astrophysics of the Sun: heating in the solar atmosphere; solar flares; heating and acceleration of the slow solar wind; energy transport processes; time-dependent ionization states; emission line spectroscopy. Plasma physics: electron and ion kinetics; non-equilibrium processes; non-local phenomena; hybrid fluid-kinetic approaches.
Associate Professor of Physics & Astronomy
2000 MS Physics with Planetary & Space Physics, Aberystwyth University
2004 PhD Solar Astrophysics, Cambridge University
Astrophysics of the Sun:
The focus of my research is on developing an understanding of heating in the solar corona and in the coronae of Sun-like stars. Coronal temperatures can reach several million degrees and yet surface temperatures are only a few thousand degrees. The mechanism by which such tremendous temperatures are achieved and maintained remains poorly understood. Indeed, it is one of the great, unsolved mysteries of astrophysics to this day.
One possible mechanism for coronal heating is the small scale, rapid release of energy due to magnetic reconnection – so called nanoflares. Though a promising candidate, they have proven extremely difficult to detect and there is little observational evidence to verify their existence. Theoretical attempts to understand the consequences of this form of heating and to predict more likely observational signatures have been limited. Modeling studies have neglected certain key aspects of coronal plasma physics; in particular, the time-dependence of the ionisation state and the formation of strongly non-Maxwellian electron distributions.
I have carried out work to address these limitations by developing a new and unique computational model which for the first time accounts for both of these processes. Traditional numerical approaches to the coronal heating problem assume an ionisation balance in equilibrium with the plasma, near-Maxwellian electron distributions and a single temperature for all particle species. The new model dispenses with these assumptions and provides a far more realistic physical treatment of coronal plasma.
The computational model that I have developed is based upon the numerical solution of the hydrodynamic equations for multi-species plasma. These are a coupled system of strongly non-linear partial differential equations comprising hyperbolic, parabolic and ordinary source terms which describe a multitude of physical processes: convection; thermal conduction; wave phenomena; shocks; viscous effects; collisions; heating; and optically-thin radiation. Solving these equations requires the implementation of advanced numerical techniques.
In addition, the equations which describe the evolution of the ionisation state, and are coupled to the hydrodynamic equations via the radiation term, are numerically stiff. I have developed novel methods to solve these equations in order that otherwise intractable problems can be handled within reasonable time-frames and using relatively modest computational resources.
The spatial-scales that are important in solar and stellar atmospheres present further challenges. Scale lengths range from several hundred kilometres to just a few metres. It is usually not feasible to employ high resolution everywhere in a numerical solution and so resolving small scale features (while not over-resolving large scale features) requires the use of adaptive computational grids. I have designed and implemented a number of sophisticated adaptive grid strategies including: low-order reconstruction for discontinuities (e.g. shocks) and associated tracking methods; and high-order reconstruction for smooth regions of the solution.
I am particularly interested in phenomena that arise in high temperature and low density plasma, where the constituent particles can become effectively collisionless. A non-Maxwellian distribution can be induced when electrons stream collisionlessly from regions of hot, tenuous plasma (such as the solar corona) into cool, dense plasma (such as the solar chromosphere) whereupon they modify the local electron distribution, by adding a high-energy population to its tail, and drive it away from thermal equilibrium. Thermal conduction can be enhanced by a significant population of energetic tail electrons and so can the collisional ionisation rate, which in turn alters the ionisation state of the plasma and the properties of its emission spectrum. This is particularly important because everything we know about the Sun has been derived from, or confirmed by, its emission spectrum. The decoupling of thermal conduction and the ionisation state from the local thermal properties of the plasma is a phenomena called non-localisation.
I have developed a kinetic component for my computational fluid-based model to allow the study of non-Maxwellian electron distributions and their importance to the physics of the solar atmosphere.