Comprehensive study of solar type II radio bursts and the properties of the associated shock waves

TL;DR

This study addresses where in the solar corona CME-driven shocks accelerate electrons to generate metric type II radio bursts and what the local shock properties are at those sites. It combines metric radio imaging from multiple observatories with MAS-based MHD coronal modeling and 3D shock reconstructions (via PyThea) across ten Solar Cycle 25 events to locate radio sources and infer local plasma conditions, including $M_{ m A}$, $M_{ m FMS}$, and $\theta_{ m BN}$. The main findings show type II sources reside in low-Alfvén-speed streamer regions, with fast, supercritical shocks ($M_{ m A}$ in $3.6$–$6.4$, $M_{ m FMS}$ in $2.3$–$4.5$) and predominantly oblique geometries ($\theta_{ m BN} < 80^{\circ}$); electron-beam energies from herringbones reach up to hundreds of keV in some cases. These results support the CME–streamer interaction scenario for type II generation and demonstrate the value of integrating radio imaging with MHD modeling to obtain location-specific shock parameters relevant to particle acceleration and space weather forecasting.

Abstract

Type II radio bursts are solar radio emissions generated by electrons accelerated by coronal shocks. These bursts are typically found close to expanding coronal mass ejections (CMEs), making them valuable for studying the properties and dynamics of CME-driven shocks in the solar corona. Here, we aim to determine the regions in the solar corona where shock waves accelerate electrons and determine their characteristic properties. To do this, we combine radio observations of type II solar radio bursts with magneto-hydrodynamic (MHD) simulations of the solar corona. We analyse ten type II radio bursts from Solar Cycle 25 exhibiting emissions. The novelty of this study lies in using radio imaging data for all type II bursts to examine the positions of the radio sources. The radio source positions, combined with a geometrical fitting of the CME shock and the MHD simulations, are used to determine essential shock parameters at the acceleration region, such as the Alfvén Mach number $(M_{\rm A}$ and $θ_{\rm BN}$. The shock parameters are then combined with the properties of the radio emission and the associated eruption in a comprehensive study. We found that for all events, the type II bursts are located near or inside coronal streamers. The estimated shock speeds are high, resulting in the formation of super-critical shocks $(3.6~\leq~M_{\rm A}~\leq~6.4$) at the type II locations. In most events, type II bursts are located at oblique shocks rather than near-perpendicular geometries, suggesting that the shock structure is more complex at local scales than the simple spherical shock models usually applied to CME shocks. Our results suggest that CME-streamer interaction regions are necessary for the generation of type II bursts, as they provide ideal plasma conditions for the formation of super-critical shocks and the subsequent acceleration of electrons.

Figures (11)

• Figure 1: Composite dynamic spectrum and radio source locations from 29 May 2024. Panel a: Low-frequency dynamic spectrum (315–40 MHz) combines NDA, e-CALLISTO Landschlacht, and ORFEES data. Horizontal dashed lines mark the NRH imaging frequencies used for panel b at the time indicated by the vertical dashed line. Panel b: Type II radio source contours are shown on an AIA 211$\AA$ running-difference image of the associated CME eruption. Contours mark different frequencies—150 MHz (red), 173 MHz (blue), 228 MHz (green)—matching dashed horizontal lines in panel a, and are drawn at 40% and 80% of maximum intensity. Panel c: De-projected radio source positions from panel b are plotted on the photospheric magnetogram, along with closed field lines (green) near the sources (viewed from Earth, x-axis toward Earth). The reconstructed CME shock at 14:26 UT appears as a magenta mesh.
• Figure 2: Herringbone burst. Panel a: Zoomed-in dynamic spectrum showing the herringbone structures, at 173 MHz (blue) and 150 MHz (red). The dashed horizontal lines represent the two imaging frequencies used. Panel b: AIA running difference image overlaid with NRH contours at 40% and 80% of the maximum value.
• Figure 3: Date: 2021/06/09
• Figure 5: Date: 2022/03/28
• Figure 6: Date: 2022/04/22