Investigation of small scale weak solar emission features at low radio frequencies

Abstract

Solar observations at low radio frequencies using the MurchisonWidefield Array have recently revealed the presence of a myriad of weak, short-lived and narrow-band emission features, even during quiet and moderately active solar conditions. In terms of their appearance in the timefrequency plane, these features are quite unlike any of the known classes of radio bursts and their detailed observational characteristics are yet to be established. They occur at rates of a few thousands per hour in the 30:72 MHz observational bandwidth and hence, necessarily require an automated approach for their characterization. Here, we develop a wavelet-based pipeline using a 2D Ricker wavelet for automated feature recognition from the dynamic spectrum. We perform separate non-imaging and imaging studies of these features to investigate distributions of their peak flux densities, energies, morphologies in the dynamic spectrum, source sizes and locations, and search for their associations with known solar active regions. We find the typical radiated energies associated with these features to be about 1015 􀀀1018 ergs, placing them amongst the weakest radio bursts reported in literature. The distribution of their peak flux densities is well-fit by a power law with index -2.23 over the 12􀀀155 SFU range, implying that they can contribute to coronal and chromospheric heating in an energetically significant manner. Images of a small subset of these features reveal the presence of two bright, compact, extended sources. While one of these two sources appears only during the occurrence of some features, the other source is persistently present, even during feature-less regions of the dynamic spectrum. The presence of this persistent source together with its association to a flaring region observed at EUV wavelengths suggest that the observed small-scale features correspond to type-I bursts embedded in a type-I storm. Analogous to type-I bursts, these features appear to ride on a variable, enhanced, broadband continuum, and possess short spectral and temporal spans of about 4􀀀5 MHz and 1􀀀2 seconds respectively. A 2D Gaussian model is found to serve as a powerful tool to track the locations and morphologies of their sources with an accuracy better than the intrinsic resolution of the observing instrument.