Interferometric imaging with LOFAR remote baselines of the fine structures of a solar type-IIIb radio burst^⋆

¹ CAS Key Laboratory of Geospace Environment, School of Earth and Space Sciences, University of Science and Technology of China, Hefei, Anhui 230026, PR China
e-mail: pjer1316@mail.ustc.edu.cn
² CAS Center for the Excellence in Comparative Planetology, USTC, Hefei, Anhui 230026, PR China
³ Anhui Mengcheng Geophysics National Observation and Research Station, USTC, Mengcheng, Anhui 233500, PR China
⁴ ASTRON, The Netherlands Institute for Radio Astronomy, Oude Hoogeveensedijk 4, 7991 PD Dwingeloo, The Netherlands
⁵ Department of Physics, University of Helsinki, PO Box 64 00014 Helsinki, Finland
⁶ Space Radio-Diagnostics Research Centre, University of Warmia and Mazury in Olsztyn, Olsztyn, Poland
⁷ RAL Space, United Kingdom Research and Innovation (UKRI) – Science and Technology Facilities Council (STFC) – Rutherford Appleton Laboratory (RAL), Harwell Campus, Oxfordshire OX11 0QX, UK
⁸ Solar-Terrestrial Centre of ExcellenceSIDC, Royal Observatory of Belgium, 1180 Brussels, Belgium
⁹ Leibniz-Institut für Astrophysik Potsdam, An der Sternwarte 16, 14482 Potsdam, Germany

Received: 14 February 2020
Accepted: 16 May 2020

Abstract

Context. Solar radio bursts originate mainly from high energy electrons accelerated in solar eruptions like solar flares, jets, and coronal mass ejections. A sub-category of solar radio bursts with short time duration may be used as a proxy to understand wave generation and propagation within the corona.

Aims. Complete case studies of the source size, position, and kinematics of short term bursts are very rare due to instrumental limitations. A comprehensive multi-frequency spectroscopic and imaging study was carried out of a clear example of a solar type IIIb-III pair.

Methods. In this work, the source of the radio burst was imaged with the interferometric mode, using the remote baselines of the LOw Frequency ARray (LOFAR). A detailed analysis of the fine structures in the spectrum and of the radio source motion with imaging was conducted.

Results. The study shows how the fundamental and harmonic components have a significantly different source motion. The apparent source of the fundamental emission at 26.56 MHz displaces away from the solar disk center at about four times the speed of light, while the apparent source of the harmonic emission at the same frequency shows a speed of < 0.02 c. The source size of the harmonic emission observed in this case is smaller than that in previous studies, indicating the importance of the use of remote baselines.