Polarisation of microwave emission from reconnecting twisted coronal loops

¹ Jodrell Bank Centre for Astrophysics, University of Manchester, Manchester, M13 9PL, UK
e-mail: mykola.gordovskyy@manchester.ac.uk
² Astronomy & Astrophysics Group, University of Glasgow, Glasgow G15 8QQ, UK

Received: 18 July 2016
Accepted: 27 March 2017

Abstract

Context. Magnetic reconnection and particle acceleration due to the kink instability in twisted coronal loops can be a viable scenario for confined solar flares. Detailed investigation of this phenomenon requires reliable methods for observational detection of magnetic twist in solar flares, which may not be possible solely through extreme UV and soft X-ray thermal emission. Polarisation of microwave emission in flaring loops can be used as one of the detection criteria.

Aims. The aim of this study is to investigate the effect of magnetic twist in flaring coronal loops on the polarisation of gyro-synchrotron microwave (GSMW) emission, and determine whether it could provide a means for magnetic twist detection.

Methods. We consider time-dependent magnetohydrodynamic and test-particle models developed using the LARE3D and GCA codes to investigate twisted coronal loops that relax after kink instability. Synthetic GSMW emission maps (I and V Stokes components) are calculated using GX simulator.

Results. It is found that flaring twisted coronal loops produce GSMW radiation with a gradient of circular polarisation across the loop. However, these patterns may be visible only for a relatively short period of time owing to fast magnetic reconfiguration after the instability. Their visibility also depends on the orientation and position of the loop on the solar disk. Typically, it would be difficult to see these characteristic polarisation patterns in a twisted loop seen from the top (i.e. close to the centre of the solar disk), but easier in a twisted loop seen from the side (i.e. observed very close to the limb).