Solar Plasmas

"Plasma", consisting of a sea of positively-charged ions and negatively-charged electrons, is sometimes called the fourth state of matter, and more than 99% of the matter in the universe is in the plasma state.

We research plasma processes in the Sun and beyond, developing mathematical models of the complex interactions between plasmas and magnetic fields.

Some key research questions are:

• How is plasma in the solar corona heated to temperatures of millions of degrees?
• How does the process of magnetic reconnection work in the solar corona and elsewhere?
• What is the origin of high-energy charged particles in solar flares?

We are also involved in modelling the interaction of plasma with magnetic fields in magnetically-confined fusion devices such as spherical tokamaks.

Our Research

The solar corona in X-rays
The solar corona in X-rays. Credit: Hinode/XRT.

A central theme of our research is modelling the nature and effects of magnetic reconnection in the solar atmosphere. Magnetic reconnection is a process of fundamental importance in astrophysics, with relevance ranging from galactic magnetic fields, to stellar and solar activity, to planetary magnetospheres, as well as in magnetically-confined fusion plasmas. Reconnection restructures magnetic fields and provides an efficient means for converting magnetic energy into thermal energy and particle kinetic energy – thus, it plays a central role in solar flares and coronal heating.
Our knowledge and understanding of the solar atmosphere has been revolutionised in recent decades by a wealth of observations, especially from space observatories such as (currently) Solar Dynamic Observatory, RHESSI, IRIS and Hinode. However, new observations have increasingly revealed the complex and dynamic nature of solar magnetic activity, and significant advances in theory and modelling are required in order to understand the physical processes. A particular challenge for theory is the vast range of spatial scales – from global scales of the order of Mm, which can be well treated using a fluid approach, down to plasma scales, such as the ion gyro-radius, of the order of m or less, which require kinetic models. Our research thus utilises both magnetohydrodynamic (fluid) models and kinetic plasma approaches, and we are currently developing new techniques to bridge the gap between fluid and kinetic scales. Our modelling involves both large-scale numerical simulations and analytical calculations.

Field lines and emission in a simulated nanoflare
Field lines and emission in a simulated nanoflare (Pinto et al, A&A 2016)

Solar flares are dramatic and complex events which are of interest both in their own right and because of their significant effects on the Earth's space environment by shaping "space weather". A solar flare involves the release of up to 1025 Joules of stored magnetic energy, in timescales of minutes - hours, with signatures across the electromagnetic spectrum. There is now a considerable body of evidence demonstrating that magnetic reconnection is the fundamental process of energy release in solar flares. However, many mysteries remain unsolved. One challenge is to explain the origin of the large numbers of high-energy charged particles which are observed, and we are developing models to tackle this question.

Explaining how solar coronal plasma maintained at temperatures of millions of degrees Kelvin, while the surface temperature is only a few thousand degrees, is a major unsolved problem in astrophysics. A promising candidate for effective dissipation of stored magnetic energy is magnetic reconnection, the same process which is responsible for the energy release in large-scale solar flares. It has thus been proposed that coronal heating is due to the combined effect of many small flares, known as nanoflares. We are currently involved in modelling heating by nanoflares, focusing on energy dissipation in twisted magnetic fields.

Over recent years, we have pioneered models of heating in twisted magnetic flux ropes triggered by the ideal kink instability, with application both to understanding confined flares and to coronal heating. Recent work has focused on predicting the observational signatures of heating in twisted fields, considering both thermal (EUV) and non-thermal emission (Hard X-rays, microwaves).
Our research is supported by grants from STFC. We have active collaborations with groups in the UK and internationally.

 

Research Activities

  • Energy release, heating and avalanches in kink-unstable twisted coronal loops

    We have used 3D magnetohydrodynamic simulations to show that the onset of ideal kink instability in a twisted magnetic field leads to dissipation of the stored magnetic energy through magnetic
    reconnection, occurring at multiple sites throughout the loop volume. Recently, it has been shown that a single unstable flux rope can trigger an avalanche of heating events in neighbouring stable flux ropes. This process is currently being further investigated, developing also a new model based on relaxation theory, allowing prediction of heating in complex systems of loops.

  • Observational signatures of twisted fields and magnetic reconnection

    The coronal magnetic field cannot be directly measured, and it is thus important to predict observable signatures of heating in twisted magnetic fields. Using magnetohydrodynamic simulations coupled to a test-particle code, we are able to simulate the evolution of both thermal plasma and energetic particles in a flaring coronal loop, and predict observable quantities such as EUV emission, Hard X ray emission and microwaves - both spatial and temporal variations.

  • Acceleration and transport of charged particles in reconnecting magnetic fields

    New approaches are required to explain the acceleration and transport of high-energy electrons and ions in solar flares. We are currently using a test particle approach, coupled to time-evolving magnetohydrodynamic simulations, to explore the acceleration of particles in merging magnetic islands within a reconnecting current sheet. Particle acceleration at 2D magnetic nulls points – which are likely to be common in the solar corona – is also under investigation. We are also pioneering a new “Reduced Kinetics” approach to modelling particle acceleration, which bridges the gap between fluid and kinetic models.

  • Measurement of inhomogeneous photospheric magnetic fields

    There is a strong evidence that fine intense magnetic fluxtubes with sizes of 10-100 km carry most of the magnetic flux through the solar surface, the photosphere. Knowing the characteristics of these features  is very important for understanding the dynamics of the lower atmosphere of the Sun  and the generation of coronal magnetic fields.

    A large sunspot (left) and two photospheric iron spectral lines showing strong Zeeman splitting (right). Credit: Hinode/SOT/SP

    A large sunspot (left) and two photospheric iron spectral lines showing strong Zeeman splitting (right). Credit: Hinode/SOT/SP


    We investigate solar magnetic fields,  focusing on the small-scale magnetic fields at the photosphere in active regions. Using spectrapolarimetric data from Hinode space telescope and some ground-based instruments, we study tthe Zeeman effect in several solar magneto-sensitive spectral lines. Based on simple multi-component magnetic field models, we fit the observed spectral line profiles and evaluate the real field strengths and sizes of these small-scale magnetic elements.

  • Modelling magnetic reconnection in spherical tokamaks

    Plasma filaments in the MAST spherical tokamak. Credit: MAST team, CCFE

    We have developed two-fluid simulations and associated relaxation models of magnetic reconnection when two flux ropes merge during the merging-compression formation of spherical tokamaks.
    We are also interested in the role of reconnection in plasma filaments in the edge region of tokamaks.

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