Solar jet-induced perturbation propagating through coronal loops and in-loop electron beam transport indicated by type II and type N radio bursts

TL;DR

This work investigates a CME-free, high-frequency metric type II radio burst associated with a C3.1 flare and a coronal jet. By integrating CBSm radio spectra with NRH imaging and complementary EUV/HXR observations, the authors show that a jet-induced perturbation propagates through nearby closed loops at ≈$880$ km s$^{-1}$ and steepens into a fast-mode shock with speed ≈$800$ km s$^{-1}$, capable of accelerating electrons to produce type II emission. The study also analyzes a type N burst, identifying three electron-beam branches with speeds ≈$0.24c$ and energies around ≈$15$ keV, and locates type II sources with respect to the jet-disturbed loops. The findings imply that metric type II bursts in weak flares can arise from loop-embedded shocks driven by jet perturbations rather than CMEs, emphasizing the role of local coronal structure and magnetic topology in shock formation and electron acceleration.

Abstract

Solar type II radio bursts are commonly attributed to coronal shocks driven by coronal mass ejections (CMEs). However, some metric type II bursts have occasionally been reported to occur in the absence of a CME and to be associated with weak solar activities. This study aims to identify the driver of the coronal shock in this kind of type II event. We investigate a high-frequency metric type II burst with clear band splitting, observed simultaneously by the Chashan Broadband Solar radio spectrograph and the Nançay Radioheliograph. It is associated with a C3.1-class flare and a small-scale jet, but without a detectable CME in the coronagraphs. The type II burst is preceded by multiple type III bursts, one of which exhibits characteristics of a type N burst. The type II burst source is associated with the jet-induced perturbation front propagating through nearby closed loops at a speed of $\sim$880 km s$^{-1}$, rather than the much slower jet front. This suggests that the disturbance initiated by the jet can convert to a shock wave within low Alfvénic coronal loops, providing the necessary conditions for electron acceleration and subsequent radio emission. Our findings offer new insights into the formation mechanism of high-frequency type II bursts associated with weak flares and jets.

Figures (7)

• Figure 1: Overview of the event on 2023 May 8. (a) GOES 1$-$8 Å soft X-ray light curve (black) of the C3.1 class flare and the ASO$-$S/HXI count rate from combined total flux detectors (D92+D93+D94) in the 25$-$50 keV energy channel (orange) between 08:22$-$08:28 UT. (b) Radio dynamic spectrum observed by CSO/CBSm (110$-$300 MHz), Learmonth (80$-$110 MHz), and GERMANY-DLR (50$-$80 MHz), displaying various type III and type III-like (J, N) bursts followed by a type II burst. Both fundamental (F) and harmonic (H) emission lanes can be identified for the type N burst and type II burst.
• Figure 2: (a1)$-$(a4), (b1)$-$(b4), and (c1)$-$(c4) EUV images in the AIA 131 Å, 171 Å and 304 Å wavelengths, showing the circular-ribbon flare and jet eruption. White and black contours in panel (c1) represent the positive and negative magnetic fields of the HMI LOS magnetogram scaled at $\pm$120 G, respectively. In panel (c3), green and blue curves mark the locations of filaments F1 and F2, respectively, as observed in panel (c1). [0.8em] (An animation of this figure is available in the online article.)
• Figure 3: (a)-(c) GONG H$\alpha$ images before, during, and after the jet eruption. White arrows in panel (a) indicate the two mini-filaments. (d) Evolution of the positive (red) and negative (blue) magnetic flux between 05:20-09:10 UT. The gray shaded region indicates the period of the flare. (e) HMI vector magnetic field map at 08:24:00 UT. Red and blue arrows represent the transverse field for positive and negative polarities, respectively. The length of the arrows indicates the magnetic field strength and the direction corresponds to the azimuthal orientation.
• Figure 4: HXR light curves and imaging of the flare in the duration of radio bursts. (a) ASO-S/HXI count rates of combined total flux detectors (D92+D93+D94) in three energy channels between 15$-$300 keV. (b) Fermi/GBM HXR flux in five channels between 4$-$300 keV. The red dashed lines indicate the peak of the curves at 08:23:52 UT. (c$-$d) Contours of HXR sources from HXI, in energy ranges of 15$-$25 keV (orange), 20$-$30 keV (blue), and 30$-$50 keV (red), overplotted on the AIA 1600 Å and HMI magnetogram images. The contour levels represent 25$\%$ and 60$\%$ of the maximum. The yellow arrow indicates the positive polarity in the center of the circular-ribbon.
• Figure 5: Radio spectral and imaging observations of the type N burst (harmonic emission). (a) CSO/CBSm radio spectrum, the type N burst is outlined by the black dashed curve. White dashed lines indicate the three NRH imaging frequencies at 150, 173, and 228 MHz. Red symbols mark the six selected data points as shown in panels (c) and (d). (b) Schematic diagram illustrating the generation of a type N burst. Red arrows indicate the propagation directions of the electron beam, while the blue and green curves represent closed and open magnetic field lines, respectively. The ellipses on the closed loop correspond to the radio source locations shown in panels (c) and (d). (c) NRH radio sources superposed on the AIA 193 Å image at three times as marked by red dots in panel (a). The contours represent 70$\%$ of the maximum of each NRH image and are plotted in blue, red, and cyan at three frequencies (150, 173, 228 MHz), respectively. (d) NRH contours corresponding to the data points marked by red diamond, asterisk, and square in panel (a). The white curve represents a schematic of the large-scale closed loop. The yellow arrow points to the position of the jet.