Wavelet-based Characterization of Small-scale Solar Emission Features at Low Radio Frequencies

Abstract

Low radio frequency solar observations using the Murchison Widefield Array have recently revealed the presence of numerous weakshort-livednarrowband emission features, even during moderately quiet solar conditions. These nonthermal features occur at rates of many thousands per hour in the 30.72 MHz observing bandwidth, and hencenecessarily require an automated approach for their detection and characterization. Here, we employ continuous wavelet transform using a mother Ricker wavelet for feature detection from the dynamic spectrum. We establish the efficacy of this approach and present the first statistically robust characterization of the properties of these features. In particular, we examine distributions of their peak flux densities, spectral spans, temporal spans, and peak frequencies. We can reliably detect features weaker than 1 SFU, making them, to the best of our knowledge, the weakest bursts reported in literature. The distribution of their peak flux densities follows a power law with an index of −2.23 in the 12-155 SFU range, implying that they can provide an energetically significant contribution to coronal and chromospheric heating. These features typically last for 1-2 s and possess bandwidths of about 4-5 MHz. Their occurrence rate remains fairly flat in the 140-210 MHz frequency range. At the time resolution of the data, they appear as stationary bursts, exhibiting no perceptible frequency drift. These features also appear to ride on a broadband background continuum, hinting at the likelihood of them being weak type-I bursts.

A Weak Coronal Heating Event Associated with Periodic Particle Acceleration Episodes

“…(e.g. Mercier & Trottet 1997;Iwai et al 2014;Mugundhan et al 2016;Suresh et al 2017). A similar situation is likely to prevail here.…”

The Low-Frequency Solar Corona in Circular Polarization

“…A mixture of plasma emission and thermal emission may also help to explain why our higher-frequency Stokes V maps are not well-correlated with the LOS field structure. One of the MWA's main contributions thus far has been to demonstrate the prevalence of very weak nonthermal emissions (Suresh et al, 2017). Sharma, Oberoi, and Arjunwadkar (2018) report that up to 45 % of the total intensity outside of nominal burst periods may be nonthermal during moderately active periods, and preliminary imaging analyses suggest that nonthermal components are present to varying degrees in every environment.…”

Science with the Murchison Widefield Array: Phase I results and Phase II opportunities

“…Detailed non-imaging investigations of weak impulsive nonthermal emissions have been carried out using the MWA Phase I with the aim of providing robust statistics on these emissions. Using an automated continuous wavelet transforms based approach, Suresh et al (2017) found that these impulsive emissions take place at the rate of many thousand per hour (as measured across 30.72 MHz bandwidth). Individual emission features were found to last for 1-2 s and span 4-5 MHz in bandwidth and ∼ 1 − 100 SFU 5 in peak flux densities.…”