Harmonic Electron Cyclotron Maser Emission along the Coronal Loop

TL;DR

This study investigates harmonic electron cyclotron maser emission along coronal loops during solar flares, revealing how different loop regions contribute to the emission and emphasizing the role of specific electron distribution features.

Contribution

It extends previous models by analyzing emission properties along the entire coronal loop, demonstrating the spatial variation and dominance of harmonic X-mode and Z-mode emissions.

Findings

Strong X2 emission from the loop top and upper sections

Persistent Z-mode excitation across all loop sections

No significant fundamental X mode emission observed

Abstract

Efficient radiation at second and/or higher harmonics of Wce has been suggested to circumvent the escaping difficulty of the electron cyclotron maser emission mechanism when it is applied to solar radio bursts, such as spikes. In our earlier study, we developed a three-step numerical scheme to connect the dynamics of energetic electrons within a large-scale coronal loop structure with the microscale kinetic instability energized by the obtained nonthermal velocity distribution and found that direct and efficient harmonic X-mode (X2 for short) emission can be achieved due to the strip-like features of the distribution. That study only considered the radiation from the loop top at a specific time. Here we present the emission properties along the loop at different locations and timings. We found that, in accordance with our earlier results, few to several strip-like features can appear in all cases, and the first two strips play the major role in exciting X2 and Z (i.e., the slow extraordinary mode) that propagate quasi-perpendicularly. For the four sections along the loop, significant excitation of X2 is observed from the upper two sections, and the strongest emission is from the top section. In addition, significant excitation of Z is observed for all loop sections, while there is no significant emission of the fundamental X mode. The study provides new insight into coherent maser emission along the coronal loop structure during solar flares.