Quantifying weak non-thermal solar radio emission at low radio frequencies

Abstract

The recent availability of fine grained high sensitivity data from the new generation low radio frequency instruments such as the Murchison Widefield Array (MWA) have opened up opportunities for using novel techniques for characterizing the nature of solar emission at these frequencies. Here we use this opportunity to look for evidence for the presence of weak non-thermal emissions in the 100-240 MHz band, at levels weaker than have usually been probed. The presence of such features is believed to be a necessary consequence of nanoflare-based coronal and chromospheric heating theories. We separate the calibrated MWA solar dynamic spectra into a slowly varying and an impulsive, and hence non-thermal, component. We demonstrate that Gaussian mixtures modeling can be used to robustly model the latter and we estimate the flux density distribution as well as the prevalence of impulsive non-thermal emission in the frequency-time plane. Evidence for presence of non-thermal emission at levels down to $\sim$0.2 SFU (1 SFU = 10${}^4$ Jy) is reported, making them the weakest reported emissions of this nature. Our work shows the fractional occupancy of the non-thermal impulsive emission to lie in the 17--45\% range during a period of medium solar activity. We also find that the flux density radiated in the impulsive non-thermal emission is very similar in strength to that of the slowly varying component, dominated by the thermal bremsstrahlung. Such significant prevalence and strength of the weak impulsive non-thermal emission has not been appreciated earlier.

arXiv comment10 figures, 2 tables, Submitted to ApJ

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