Broadband microwave burst produced by electron beams

TL;DR

This paper demonstrates that electron beams moving obliquely or along magnetic fields can produce microwave bursts via gyrosynchrotron emission, offering a new diagnostic tool for solar flare analysis through combined spectral, temporal, and polarization data.

Contribution

It introduces a novel forward fitting algorithm using gyrosynchrotron formulas and polarization measurements to diagnose electron beams in solar flares, validated with a real flare event.

Findings

Electron beams can generate microwave bursts via gyrosynchrotron mechanism.

The burst analyzed was produced by an oblique electron beam in a strong magnetic field.

Electrons had a short lifetime of about 0.5 seconds, indicating magnetic mirror reflection.

Abstract

Theoretical and experimental study of fast electron beams attracts a lot of attention in the astrophysics and laboratory. In the case of solar flares the problem of reliable beam detection and diagnostics is of exceptional importance. This paper explores the fact that the electron beams moving oblique to the magnetic field or along the field with some angular scatter around the beam propagation direction can generate microwave continuum bursts via gyrosynchrotron mechanism. The characteristics of the microwave bursts produced by beams differ from those in case of isotropic or loss-cone distributions, which suggests a new tool for quantitative diagnostics of the beams in the solar corona. To demonstrate the potentiality of this tool, we analyze here a radio burst occurred during an impulsive flare 1B/M6.7 on 10 March 2001 (AR 9368, N27W42). Based on detailed analysis of the spectral, temporal, and spatial relationships, we obtained firm evidence that the microwave continuum burst is produced by electron beams. For the first time we developed and applied a new forward fitting algorithm based on exact gyrosynchrotron formulae and employing both the total power and polarization measurements to solve the inverse problem of the beam diagnostics. We found that the burst is generated by a oblique beam in a region of reasonably strong magnetic field ($\sim 200-300$ G) and the burst is observed at a quasi-transverse viewing angle. We found that the life time of the emitting electrons in the radio source is relatively short, $\tau_l \approx 0.5$ s, consistent with a single reflection of the electrons from a magnetic mirror at the foot point with the stronger magnetic field. We discuss the implications of these findings for the electron acceleration in flares and for beam diagnostics.