A type II solar radio burst without a coronal mass ejection association

TL;DR

This paper documents a rare Type II solar radio burst on 2023-11-02 that lacks a corresponding white-light CME. Using multi-instrument observations (LOFAR, e-CALLISTO, ORFEES, NRH, GOES-16, SDO/AIA, STEREO-A EUVI), the authors characterize a split-band, fundamental-harmonic Type II burst coincident with an M1.6 flare and an EUV disturbance but no coronagraphic CME. The authors derive shock parameters from the radio spectrum via plasma-density relations and compare them with EUV-imaged disturbance speeds, finding consistency around ~500 km s$^{-1}$ and an Alfvén speed minimum near ~350 km s$^{-1}$ at ~1.5 $R_\odot$. They argue the EUV ejecta near the disk center likely represents the near-Sun CME manifestation driving the shock, which remained unseen in white light due to projection, highlighting that disk-center eruptions can drive inner-corona shocks without detectable CMEs and calling for enhanced low-corona observations to refine models of Type II generation.

Abstract

Type II solar radio bursts are commonly associated with shocks generated by coronal mass ejections (CMEs), where plasma waves are excited by magnetohydrodynamic (MHD) processes and converted into radio waves at the local plasma frequency or its harmonics. However, there are instances where type II bursts occur in the absence of whitelight CMEs. We analysed one such metric type II radio burst observed on November 2, 2023, characterized by split band features and fundamental-harmonic lanes. Notably, no CME was detected with space-based coronagraphs during this event. However, an intense M1.6 class flare was observed just before the type II burst and an extreme ultraviolet (EUV) disturbance was observed expanding into surrounding regions. The absence of any whitelight CME seen in any coronagraph field of view even though the EUV shock had a moderate speed of $\approx500~km/s$, which was close to the shock speed derived from radio observations, %indicates that the shock in the inner corona was most-likely produced by the very intense solar flare and the type II was associated with the EUV disturbance seen in the lower corona. These observations indicate that the shock in the inner corona was most-likely driven by the EUV ejecta seen in the lower corona, but the ejecta did not survive as a CME in the coronagraph field of view.

Figures (4)

• Figure 1: Dynamic spectra of the type II solar radio burst on 02 Nov 2023. The lower band is with the LOFAR data (20-80 MHz), the middle band is with e-CALLISTO (Birr), and the upper band is with ORFEES observations. The fundamental and harmonic bands are marked with dots in the spectra. We did the radio imaging for the harmonic part of the type II burst (see Fig. \ref{['fig1:figure2']}).
• Figure 2: (a): The X-ray flare as seen with the GOES-16 satellite. This was a M1.6 class flare with a very short duration of around 4 minutes and 40 seconds, as shown in the figure. (b): Zoomed-in dynamic spectrum of the type II solar radio burst. The vertical dashed line shows the time of the image made in panel (c). The two horizontal lines show the frequency of the radio images in panel (c). (c): The radio contours (at $70\%$ level) at show the location of the type II radio bursts at 170 MHz ('grey' color) and 150.9 MHz ('black' color) with the data obtained from the NRH instrument.
• Figure 3: Left panel: Snapshots at different time instances from the two coronagraphs, SOHO/LASCO-C2 and STEREO-B/SECCHI-COR2 images. These show that no white-light CME was observed in the coronagraph FOV for the observed type II radio burst. Right Panel: Snapshots of different channels of SDO/AIA wavelengths images at different times. The arrow shows the movement of a faint feature in the AIA FOV.
• Figure 4: The electron density obtained from radio observations using equation 2 (left panel). The ejecta speed plot showing the speed of the disturbance in the EUV field of view and the shock speed derived from the radio observations (right panel).