Microwave response to kink oscillations of a plasma slab

TL;DR

This paper investigates how kink oscillations in a plasma slab influence microwave emissions, revealing non-linear responses and frequency doubling at anti-nodes, which impacts the interpretation of solar flare observations.

Contribution

It demonstrates the non-linear microwave response to kink waves in a plasma slab and identifies frequency doubling at anti-nodes, advancing understanding of flare-related microwave pulsations.

Findings

Microwave response is non-linear to kink oscillations.

At anti-nodes, microwave oscillation frequency doubles.

Transformation of light curves occurs from node to anti-node.

Abstract

The modulation of the intensity of microwave emission from a plasma slab caused by a standing linear kink fast magnetoacoustic wave is considered. The slab is stretched along a straight magnetic field, and can represent, for example, a current sheet in a flaring active region in corona of the Sun, or a streamer or pseudostreamer stalk. The plasma density is non-uniform in the perpendicular direction and described by a symmetric Epstein profile. The plasma parameter $\beta$ is taken to be zero, which is a good approximation for solar coronal active regions. The microwave emission is caused by mildly relativistic electrons which occupy a layer within the oscillating slab and radiate via the gyrosynchrotron (GS) mechanism. Light curves of the microwave emission were simulated in the optically thin part of the GS spectrum, and their typical Fourier spectra were analysed. It is shown that the microwave response to a linear kink magnetohydrodynamic wave is non-linear. It is found that, while the microwave light curves at the node oscillate with the same frequency as the frequency of the perturbing kink mode, the frequency of the microwave oscillations at the anti-node is two times higher than the kink oscillation frequency. Gradual transformation the one type of the light curves to another occurs when sliding from the node to the anti-node. This result does not depend on the width of the GS-emitting layer inside the oscillating slab. This finding should be considered in the interpretation of microwave quasi-periodic pulsations in solar and stellar flares.