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

Lead Research Organisation: University of Manchester
Department Name: Physics and Astronomy

Abstract

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.

Planned Impact

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.

Publications

10 25 50
 
Description We have developed and tested new methodologies for investigating the physics of solar flares, bringing together large scale processes studied using magnetohydrodynamics, and kinetic physics of charged particles on vastly smaller length scales.
Exploitation Route The methodologies may be used by others. We make testable predictions of the observable signatures of energy release in twisted solar loops, which could be used by observers.
Sectors Education

Other

 
Description Reconnection driven waves and oscillations in the flaring solar corona
Amount £459,867 (GBP)
Organisation Science and Technologies Facilities Council (STFC) 
Sector Public
Country United Kingdom
Start 03/2020 
End 03/2023
 
Title Data underpinning: Coronal energy release by MHD avalanches: continuous driving 
Description  
Type Of Material Database/Collection of data 
Year Produced 2018 
Provided To Others? Yes  
URL https://research-portal.st-andrews.ac.uk/en/datasets/data-underpinning-coronal-energy-release-by-mhd...
 
Description Space weather effects of flares 
Organisation Nagoya University
Country Japan 
Sector Academic/University 
PI Contribution MHD modelling and test particle simulations of flaring events
Collaborator Contribution Provision of data for data-driven models of flares
Impact Expect 2 papers Also new models predicting particle impact on solar surface and particle escape into heliosphere
Start Year 2019
 
Description Tokamak filaments 
Organisation EURATOM/CCFE Fusion Association
Country United Kingdom 
Sector Academic/University 
PI Contribution Developing models of reconnection in tokamak filaments based on solar flare models
Collaborator Contribution Collaborating on modelling and providing data on tokamak filaments
Impact Work currently in progress
Start Year 2017
 
Description BBC Breakfast TV interview 
Form Of Engagement Activity A press release, press conference or response to a media enquiry/interview
Part Of Official Scheme? No
Geographic Reach National
Primary Audience Public/other audiences
Results and Impact Interview on total solar eclipse on BBC Breakfast TV
Year(s) Of Engagement Activity 2017
 
Description David Elder public lecture 
Form Of Engagement Activity A talk or presentation
Part Of Official Scheme? No
Geographic Reach National
Primary Audience Public/other audiences
Results and Impact David Elder public lecture at Glasgow science centre. To explain solar flares to members of public including school children and adults. Questions and answers afterwards
Year(s) Of Engagement Activity 2020
 
Description Loreto College Sixth form talk 
Form Of Engagement Activity A talk or presentation
Part Of Official Scheme? No
Geographic Reach Local
Primary Audience Schools
Results and Impact Talk on mathematics in my research area to sixth form mathematics students at local college
Year(s) Of Engagement Activity 2017
 
Description Preshow discussion at dance event 
Form Of Engagement Activity Participation in an activity, workshop or similar
Part Of Official Scheme? No
Geographic Reach Regional
Primary Audience Public/other audiences
Results and Impact Preshow discussion with choreographer Alex Whiteley before "8 minutes" dance event inspired by the sun and solar activity. Panel discussion and audience questions.
Year(s) Of Engagement Activity 2017
 
Description Public outreach event (Oxford) 
Form Of Engagement Activity Participation in an activity, workshop or similar
Part Of Official Scheme? No
Geographic Reach Regional
Primary Audience Public/other audiences
Results and Impact Co-organiser and lead speaker at "Plasma takes 5" public event, Worcester College, Oxford. Attended by over 100 members of the general public. Short presentations on wide range of plasma physics topics, with question and answer.
Year(s) Of Engagement Activity 2017
 
Description Talk at Bluedot Festival 
Form Of Engagement Activity A talk or presentation
Part Of Official Scheme? No
Geographic Reach National
Primary Audience Public/other audiences
Results and Impact Talk on solar flares and their impact on their Earth to about 600 people of all ages at Bluedot Music Festival at Jodrell Bank
Year(s) Of Engagement Activity 2018
 
Description Talk at European Astrofest 2020 
Form Of Engagement Activity A talk or presentation
Part Of Official Scheme? No
Geographic Reach National
Primary Audience Public/other audiences
Results and Impact Talk on solar flares to around 700 people at Astrofest, Kensington, Londo
Very positive feedback received
Year(s) Of Engagement Activity 2020
URL https://europeanastrofest.com/
 
Description Tokamak Energy youtube video 
Form Of Engagement Activity Engagement focused website, blog or social media channel
Part Of Official Scheme? No
Geographic Reach International
Primary Audience Public/other audiences
Results and Impact Interview on synergies between solar plasmas and tokamaks for "Tokamak Energy" (private company developing independent fusion energy programme) outreach video, available on Youtube
Year(s) Of Engagement Activity 2017