Narrowband Spikes Observed during the 2013 November 7 Flare

TL;DR

This study analyzes narrowband radio spikes during a 2013 solar flare, proposing that they are generated by Bernstein modes in turbulent plasma, and compares models to estimate plasma parameters.

Contribution

It provides evidence supporting Bernstein modes as the origin of narrowband spikes and estimates plasma density and magnetic field during the flare.

Findings

Narrowband spike frequencies fit Bernstein mode dispersion branches.

Power-law spectra of spikes suggest turbulent plasma conditions.

Arguments favoring Bernstein mode generation over upper-hybrid models.

Abstract

Narrowband spikes are observed in solar flares for several decades. However, their exact origin is still discussed. To contribute to understanding of these spikes, we analyze the narrowband spikes observed in the 800-2000 MHz range during the impulsive phase of the November 7, 2013 flare. In the radio spectrum, the spikes started with typical broadband clouds of spikes, and then their distribution in frequencies changed into unique, very narrow bands having non-integer frequency ratios. We successfully fitted frequencies of these narrow spike bands by those, calculating dispersion branches and growth rates of the Bernstein modes. For comparison, we also analyzed the model, where the narrow bands of spikes are generated at the upper-hybrid frequencies. Using both models, we estimated the plasma density and magnetic field in spike sources. Then the models are discussed, and arguments in favor of the model with the Bernstein modes are presented. Analyzing frequency profiles of this spike event by the Fourier method, we found the power-law spectra with the power-law indices varying in the -0.8 -- -2.75 interval. Because at some times this power-law index was close to the Kolmogorov spectral index (-5/3), we propose that the spikes are generated through the Bernstein modes in turbulent plasma reconnection outflows or directly in the turbulent magnetic reconnection of solar flares.