Source location and evolution of a multi-lane type II radio burst

TL;DR

The paper investigates how a CME-driven shock in the solar corona produces a type II radio burst with evolving multi-lane morphology. Using LOFAR LBA imaging complemented by ORFEES and NRH data, the authors identify three distinct radio-emission regions along the CME front and track their evolution. They correlate each emission lane with local coronal properties, using MAS MHD model outputs to map density and Alfvén speed along the shock. The findings show that the density stratification and varying $V_A$ along the shock control which parts of the front accelerate electrons to emit at specific lanes, providing strong evidence for spatially separate acceleration regions. This work demonstrates the value of high-resolution spectro-imaging for interpreting complex solar radio bursts and informs models of particle acceleration in the corona.

Abstract

Shocks in the solar corona can accelerate electrons that in turn generate radio emission known as type II radio bursts. The characteristics and morphology of these radio bursts in the dynamic spectrum reflect the evolution of the shock itself, together with the properties of the local corona where it propagates. In this work, we study the evolution of a complex type II radio burst showing a multi-lane structure, to find the locations where the radio emission is produced and relate them to the properties of the local environment. Using radio imaging, we track separately each lane composing the type II burst and relate the position of the emission to the properties of the ambient medium such as density, Alfven speed, and magnetic field. We show that the radio burst morphology in the dynamic spectrum changes with time and it is related to the complexity of the local environment. The initial stage of the radio emission show a single lane in the spectrum, while the latter stages of the radio signature evolve in a multi-lane scenario. The radio imaging reveals how the initial stage of the radio emission separates with time into different locations along the shock front as the density and orientation of the magnetic field change along the shock propagation. At the time where the spectrum shows a multi-lane shape, we found a clear separation of the imaged radio sources. By combining radio imaging with the properties of the local corona, we described the evolution of a type II radio burst and, for the first time, identified three distinct radio emission regions above the CME front. Two regions were located at the flanks, producing earlier radio emission than the central position, in accordance with the complexity of density and Alfven speed values in the regions where radio emission is generated. This unprecedented observation provides new insights into the nature of multi-lane type II bursts.

Figures (5)

• Figure 1: Composite dynamic spectrum showing the evolution of the type II radio burst observed on 2022-May-19. The spectrum combines observations from three instruments: ORFEES (300-800 MHz), LOFAR High Band Antenna (HBA, 110-240 MHz), and LOFAR Low Band Antenna (LBA, 30-88 MHz). The event starts at 12:02 UT with a single emission lane at frequencies around 600 MHz and drifts to lower frequencies. The multi-lane structure becomes particularly evident in the LOFAR-LBA frequency range (30-88 MHz) after 12:08 UT. Note that fundamental (F) and harmonic (H) emissions are superposed at high frequencies, in the LBA range they are distinct. The frequency range is plotted on a logarithmic scale to better display the fine structures across the wide frequency range.
• Figure 2: Composite image showing the CME eruption observed on 2022-May-19. The background shows the SDO/AIA running difference image at 193 $\AA$ revealing the CME front structure. The radio contours (in yellow/red) from the Nançay Radioheliograph (NRH) at 432 MHz show a single radio source location associated with the type II radio burst and the erupting CME front at 12:04 UT. The radio source at high frequencies appears as a single emission region, in contrast to the multiple emission regions observed at lower frequencies with LOFAR.
• Figure 3: Overview of the type II radio burst lanes observed with LOFAR-LBA. Top panel: Dynamic spectrum showing the frequency range 30-88 MHz, with marked regions indicating different emission features. Regions 1-5 correspond to fundamental plasma emission: lanes 1-2 show the initial double source structure, while region 3 marks the appearance of a third distinct emission source. Lanes 4-5 continue to show fundamental emission with varying spatial distributions. Lane 6 shows emission comparable to the quiet Sun level. Regions 7a-c, appearing later in time, represent harmonic emission showing three distinct parallel lanes. Lower panels: LOFAR radio imaging of lanes 1-6, showing the spatial distribution of each emission feature. Lanes 1-3 show similar source locations with double source structures on the eastern and southern sides of the shock front. Lane 4 shows emission predominantly on the eastern side, while lane 5 exhibits complex structures in both eastern and southern regions. The detailed imaging of the three parallel harmonic lanes (7a-c) is presented in Figures 4 and 5.
• Figure 4: MAS MHD model results. Density and three lanes in spectra.
• Figure 5: MAS MHD model results. panel shows Alfvén speed with the PFSS model and LOFAR contours on top of it.