Multi-scale modelling of heating and particle acceleration in twisted magnetic fields in solar flares and coronal heating

Solar flares are dramatic and complex events, which give off electromagnetic radiation in almost all wavelength bands across the spectrum, and also directly emit high energy particles into space. They are of great interest in their own right, as examplars of fundamental physical processes which take place across the universe - and because of their significant effects on the Earth's space environment through "space weather". The high-energy particles and electromagnetic radiation from flares can damage satellites as well as power systems on the Earth, and are potentially hazardous to astronauts. It is well-established that the primary energy release mechanism is the process of magnetic reconnection. However, there are major outstanding issues to be resolved: in particular, the origin of the large numbers of high energy (non-thermal) ions and electrons. Whilst much new light has been shed on the properties of these particles by recent observations, especially from the Hard X-ray imaging telescope RHESSI, new observations have also posed new challenges to theory and modelling. The vast range of length scales involved - from the global scales of mega-metres down to fundamental plasma scale lengths of metres - makes modelling a particularly difficult task, and no single model can encompass all features.

Another long-standing mystery is to explain the existence of a hot X-ray corona - whose temperature (millions of degrees) greatly exceeds the surface temperature (a few thousand degrees). One very promising scenario is that coronal heating arises from the combined effect of many very small flare-like events, known as nanoflares. Thus, the fundamental energy release process is magnetic reconnection, as in larger scale solar flares. In order to distinguish between different candidates for coronal heating, it is necessary to predict observable signatures, such as the properties of energetic particles, the temperature distribution, and plasma flows.

Twisted magnetic fields provide a reservoir of free magnetic energy which could be dissipated into heating, and such twisted fields are likely to be very common in the solar corona - both as large-scale structures and on smaller scales. We have previously shown that single twisted flux ropes may rapidly release stored magnetic energy if their twist is sufficiently large for onset of the ideal kink instability - this generates small-scale fragmented currents sheets, with efficient plasma heating and particle acceleration through magnetic reconnection. We have developed a powerful set of tools, coupling test-particles to 3D magnetohydrodynamic simulations, and forward-modelling observable signatures such as soft and hard X-ray emission.

In this project, we will build on this work to develop an interlinked hierarchy of models for energy release in twisted magnetic flux ropes, from more idealised 2D models to complex and more realistic larger-scale models. We will develop and exploit an innovative new modelling approach called "reduced kinetics" which bridges the gap between kinetic and fluid approaches. We will use this, and advanced test-particle codes coupled with magnetohydrodynamic simulations, to study both plasma heating and particle acceleration in forced reconnection, driven by an external disturbance, focussing on the merger of twisted flux ropes with the reconnecting current sheet in both 2D and 3D.We will also investigate thermal and non-thermal plasma in more realistic 3D configurations, including curvature and a realistic atmosphere. As well as single unstable loops, we will explore interactions between loops, especially a recently-discovered "avalanche" process whereby one unstable loop may trigger energy release from many stable neighbours. Observable signatures, including microwave emission, will be predicted, so that different scenarios can be compared and tested against observations.

The main impact is academic, making advances in the fundamental science of the Sun and in understanding universal physical processes (especially particle acceleration and magnetic reconnection, also turbulence). We expect the proposed research to lead to a number of significant publications in leading refereed journals, and to be presented at international and UK conferences, including invited talks.

There is societal impact through the improved understanding of space weather. "Space weather" - which is recognised on the National Risk Register - begins at the Sun. We will develop much better understanding of the generation of energetic particles in solar flares, which are a crucial aspect of space weather.

The Sun is a natural plasma physics laboratory, and our work will have wider impact in plasma physics, especially magnetically-confined fusion plasmas. We have strong collaborative links with CCFE and expect some of the techniques and ideas developed in this project to be subsequently applied to magnetically-confined fusion plasmas (e.g. filaments in MAST spherical tokamak).

A further impact of our work is the training of Postdoctoral staff and students. Previous PDRAs from the group have gone on to successful academic careers (e.g. Jain, Dalla). PhD graduates have gone on to successful academic research careers (e.g. Stanier, Los Alamos National Laboratory) and to leading positions in industry and national laboratories (e.g. Randewich - Director of Science, AWE). The skills in problem solving, computing and modelling, as well as more general transferable skills, prove to have very wide applications. Our research also has benefits in training undergraduate students, with MPhys projects frequently being undertaken on small research topics closely associated with this work.

The public and school children find the Sun immensely interesting - and flares in particular spark huge excitement. Our research has been widely communicated to the public - from small children to retired people - in a variety of fora and media. PKB has appeared on the BBC "Sky ay Night" discussing nanoflares, and twice on CBBC Newsround, as well as discussing fusion on Radio 4's "In our Time". She has lectured to large public audiences at science festivals, and at many events at the JBCA Discovery Centre including the "Lovell Lecture" and "Live at Jodrell" music events (which attracts an audience who might not usually attend science talks!). She regularly presents "Ask an Expert" sessions at JBCA, and speaks at careers events for school children as part of widening partipation programme. Recently, PKB presented an invited talk at a History of the Sun conference (attended by members of the public and historians), and has presented keynote talks at national Astronomy festivals. She regularly gives talks to astronomical societies and Cafe Scientifiques, and to school children of all ages.