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 and narrow-band emission features, even during moderately quiet solar conditions. These non-thermal 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 seconds 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.

arXiv comment15 pages, 14 figures, Accepted for publication in ApJ

References in corpus (14)

• The NumPy array: a structure for efficient numerical computation
• scikit-image: Image processing in Python
• The Murchison Widefield Array: the Square Kilometre Array Precursor at low radio frequencies
• The Murchison Widefield Array: Design Overview
• A Review of Solar Type III Radio Bursts
• Science with the Murchison Widefield Array
• All-sky Galactic radiation at 45 MHz and spectral index between 45 and 408 MHz
• A decade of solar Type III radio bursts observed by the Nancay Radioheliograph 1998-2008
• First Spectroscopic Imaging Observations of the Sun at Low Radio Frequencies with the Murchison Widefield Array Prototype
• LOFAR tied-array imaging and spectroscopy of solar S bursts
• Estimating solar flux density at low radio frequencies using a sky brightness model
• Spectral Structures and Their Generation Mechanisms for Solar Radio Type-I Bursts
• A New Look at Type III Bursts and their Use as Coronal Diagnostics
• Noise storm continua: power estimates for electron acceleration

Cited by in corpus (18)

• Solar Physics with the Square Kilometre Array
• Type III Solar Radio Burst Source Region Splitting Due to a Quasi-Separatrix Layer
• 4D Data Cubes from Radio-Interferometric Spectroscopic Snapshot Imaging
• The Low-Frequency Solar Corona in Circular Polarization
• Science with the Murchison Widefield Array: Phase I Results and Phase II Opportunities
• First radio evidence for impulsive heating contribution to the quiet solar corona
• Unsupervised generation of high dynamic range solar images: A novel algorithm for self-calibration of interferometry data
• A weak coronal heating event associated with periodic particle acceleration episodes
• Quantifying weak non-thermal solar radio emission at low radio frequencies
• Radio Stars: from kHz to THz
• Propagation Effects in Quiet Sun Observations at Meter Wavelengths
• Transient brightenings in the quiet Sun detected by ALMA at 3 mm
• Discovery of correlated evolution in solar noise storm source parameters: Insights on magnetic field dynamics during a microflare
• Energetics of small electron acceleration episodes in the solar corona from radio noise storm observations
• Detection of weak ubiquitous impulsive nonthermal emissions from the solar corona
• Robust absolute solar flux density calibration for the Murchison Widefield Array
• Applying wavelet analysis to the X-ray light curves of active galactic nuclei and quasi-periodic eruptions
• New Insights into Type-I Solar Noise Storms from High Angular Resolution Spectroscopic Imaging with the Upgraded Giant Metrewave Radio Telescope