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.

10 figures, 2 tables, Submitted to ApJ

References in corpus (2)

• All-sky Galactic radiation at 45 MHz and spectral index between 45 and 408 MHz
• LOFAR tied-array imaging of Type III solar radio bursts

Cited by in corpus (7)

• Science with the Murchison Widefield Array: Phase I Results and Phase II Opportunities
• A Statistical Study of Solar Radio Type III Bursts and Space Weather Implication
• Unsupervised generation of high dynamic range solar images: A novel algorithm for self-calibration of interferometry data
• Imaging Spectroscopy of CME-Associated Solar Radio Bursts
• Detection of weak ubiquitous impulsive nonthermal emissions from the solar corona
• Robust absolute solar flux density calibration for the Murchison Widefield Array
• Radio, X-ray and extreme-ultraviolet observations of weak energy releases in the `quiet' Sun