Polarisation of microwave emission from reconnecting twisted coronal loops

TL;DR

This study investigates how magnetic twist in coronal loops affects microwave emission polarisation, aiming to improve detection of magnetic twist during solar flares through synthetic modelling of gyro-synchrotron emission.

Contribution

It demonstrates that magnetic twist influences microwave polarisation patterns, providing a potential observational signature for detecting magnetic twist in solar flare loops.

Findings

Polarisation gradients are produced in twisted loops during flares.

Visibility of polarisation patterns depends on loop orientation and viewing angle.

Patterns are short-lived due to rapid magnetic reconfiguration.

Abstract

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. 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. We use time-dependent magnetohydrodynamic and test-particle models developed using LARE3D and GCA codes to investigate twisted coronal loops relaxing following the kink-instability. Synthetic GSMW emission maps (I and V Stokes components) are calculated using GX simulator. 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 due to fast magnetic reconfiguration after the instability. Their visibility also depends on the orientation and position of the loop on solar disk. Typically, it would be difficult to see these characteristic polarisation pattern in a twisted loop seen from the top (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).