Interferometric Imaging, and Beam-Formed Study of a Moving Type-IV Radio Burst with LOFAR

Abstract

Type-IV radio bursts have been studied for over 50 years. However, the specifics of the radio emission mechanisms is still an open question. In order to provide more information about the emission mechanisms, we studied a moving Type-IV radio burst with fine structures (spike group) by using the high-resolution capability of the Low-Frequency Array (LOFAR) on August 25, 2014. We present a comparison of Nançay Radioheliograph (NRH) and the first LOFAR imaging data of the Type-IV radio burst. The degree of circular polarization (DCP) is calculated at frequencies in the range 20 – 180 MHz using LOFAR data, and it was found that the value of DCP gradually increased during the event, with values of 20 – 30%. LOFAR interferometric data were combined with white-light observations in order to track the propagation of this Type-IV burst. The kinematics shows a westward motion of the radio sources, slower than the CME leading edge. The dynamic spectrum of LOFAR shows a large number of fine structures with durations of less than 1 s and high brightness temperatures ($T_{ \mathrm{B}}$ T B ), i.e., $10^{12}$ 10 12 – $10^{13}$ 10 13 K. The gradual increase of DCP supports gyrosynchrotron emission as the most plausible mechanism for the Type IV. However, coherent emissions such as Electron Cyclotron Maser (ECM) instability may be responsible for small-scale fine structures. Countless fine structures altogether were responsible for such high $T_{\mathrm{B}}$ T B .

New Insights from Imaging Spectroscopy of Solar Radio Emission

“…A third major advantage is that full-correlation data can be improved by the application of various calibration techniques, including self-calibration, to correct phase and amplitude errors after the data are taken, whereas in beam forming the calibration is frozen in during processing. In fact, LOFAR also has a full-correlation imaging mode that is now coming into more standard use for these reasons (e.g., Murphy et al 2021, Liu et al 2022.…”

“…Most published solar studies (e.g., Morosan et al 2014) have used the LBA in the tied-array beam-forming mode mentioned in Section 3.1.2, mainly due to the technically simpler imaging procedure, although the advantages of full-correlation mode (Murphy et al 2021) and the utility of the higher-frequency HBA (Magdalenić et al 2020, Liu et al 2022) should make the interferometry mode more common.…”

Initial results from multi-beam observations of pulsars and solar transient with the digital beamformer for the Gauribidanur pulsar system

“…Such observations are important for calibrating the dynamic spectra of the latter and thereby inferring the spectral index, emission mechanism, etc., which in some cases are still debated (see, e.g., Refs. 18–23).…”