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
Received: 17 November 2014
Accepted: 26 June 2015
Aims. The electron plasma frequency ωpe and electron gyrofrequency Ωe 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 ωpe/Ωe 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 ωpe/Ωe 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 ωpe/Ωe≤ 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 ωpe/Ωe 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 ωpe/Ωe, 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.
Key words: Sun: magnetic fields / Sun: radio radiation / magnetohydrodynamics (MHD) / masers
© ESO, 2015