Letter to the Editor

Imaging spectroscopy of a spectral bump in a type II radio burst^⋆

¹ Department of Physics, University of Helsinki, PO Box 64 00014 Helsinki, Finland
² Center for Solar-Terrestrial Research, New Jersey Institute of Technology, Newark, NJ, USA
e-mail: peijin.zhang@njit.edu
³ Cooperative Programs for the Advancement of Earth System Science, University Corporation for Atmospheric Research, Boulder, CO, USA
⁴ Department of Physics and Astronomy, University of Turku, 20500 Turku, Finland
e-mail: diana.morosan@utu.fi
⁵ ASTRON – the Netherlands Institute for Radio Astronomy, Oude Hoogeveensedijk 4, 7991 PD Dwingeloo, The Netherlands
⁶ Space Radio-Diagnostics Research Centre, University of Warmia and Mazury, R. Prawochenskiego 9, Olsztyn 10-719, Poland
⁷ Leibniz-Institut für Astrophysik Potsdam (AIP), An der Sternwarte 16, 14482 Potsdam, Germany

Received: 27 January 2024
Accepted: 28 March 2024

Abstract

Context. Observations of solar, type II radio bursts provide a unique opportunity to analyze the nonthermal electrons accelerated by coronal shocks and diagnose the plasma density distribution in the corona. However, there are very few high-frequency resolution interferometric observations of type II radio bursts that are capable of tracking these electrons.

Aims. Recently, more spatially resolved high-resolution observations of type II radio bursts have been recorded with the Low-Frequency Array (LOFAR). Using these observations, we aim to track the location of a type II radio burst that experienced a sudden spectral bump.

Methods. We present the first radio imaging observations of a type II burst with a spectral bump. We measured the variation in source location and frequency drift of the burst and deducted the density distribution along its propagation direction.

Results. We have identified a type II burst that experiences a sudden spectral bump in its frequency-time profile. The overall frequency drift rate is 0.06 MHz s⁻¹, and this corresponds to an estimated speed of 295 km s⁻¹. The projected velocity of the radio source obtained from imaging is 380 km s⁻¹ toward the east. At the spectral bump, a deviation in the source locations of the type II split bands is observed. The band separation increases significantly in the north–south direction.

Conclusions. The spectral bump shows an 8 MHz deviation at 60 MHz, which corresponds to a 25% decrease in the plasma density. The estimated crossing distance during the spectrum bump was 29 mm, suggesting that this density variation occurs in a confined area. This indicates that the shock most likely encountered the upper extent of a coronal hole.