A new approach to the maser emission in the solar corona

Department of Mathematics and Information Sciences, Faculty of Engineering and Environment, Northumbria University, NE1 8ST, Newcastle Upon Tyne, UK
e-mail: stephane.regnier@northumbria.ac.uk

Received: 17 November 2014
Accepted: 26 June 2015

Abstract

Aims. The electron plasma frequency ω_pe and electron gyrofrequency Ω_e are two parameters that allow us to describe the properties of a plasma and to constrain the physical phenomena at play, for instance, whether a maser instability develops. In this paper, we aim to show that the maser instability can exist in the solar corona.

Methods. We perform an in-depth analysis of the ω_pe/Ω_e ratio for simple theoretical and complex solar magnetic field configurations. Using the combination of force-free models for the magnetic field and hydrostatic models for the plasma properties, we determine the ratio of the plasma frequency to the gyrofrequency for electrons. For the sake of comparison, we compute the ratio for bipolar magnetic fields containing a twisted flux bundle, and for four different observed active regions. We also study how ω_pe/Ω_e is affected by the potential and non-linear force-free field models.

Results. We demonstrate that the ratio of the plasma frequency to the gyrofrequency for electrons can be estimated by this novel method combining magnetic field extrapolation techniques and hydrodynamic models. Even if statistically not significant, values of ω_pe/Ω_e≤ 1 are present in all examples, and are located in the low corona near to photosphere below one pressure scale-height and/or in the vicinity of twisted flux bundles. The values of ω_pe/Ω_e are lower for non-linear force-free fields than potential fields, thus increasing the possibility of maser instability in the corona.

Conclusions. From this new approach for estimating ω_pe/Ω_e, we conclude that the electron maser instability can exist in the solar corona above active regions. The importance of the maser instability in coronal active regions depends on the complexity and topology of the magnetic field configurations.