First Detailed MeerKAT Imaging Spectroscopy of a Solar Flare

TL;DR

This study delivers the first detailed MeerKAT imaging spectroscopy of a solar flare, achieving dynamic ranges over 1000 and enabling concurrent analysis of bright coherent bursts, faint incoherent emission, and faint hot plasma unseen by EUV instruments. It identifies three spatially distinct coherent sources with varied spectral behavior and places them within different coronal loops via NLFFF magnetic-field extrapolations, indicating multiple electron populations and acceleration sites. The detection of extended incoherent emission beyond AIA structures demonstrates MeerKAT's sensitivity to low-emission plasma in the lower corona, offering a more complete diagnostic of flare energetics. Collectively, the work validates MeerKAT as a powerful solar radio-diagnostics instrument and outlines clear paths to enhance capabilities for SKA-Mid, including higher cadence, broader bandwidth, and flexible observing modes.

Abstract

Radio observations provide powerful diagnostics of energy release, particle acceleration, and transport processes in solar flares. However, despite recent progress in radio interferometric imaging spectroscopy, current instruments still face limitations in image fidelity and resolution, restricting detailed spectroscopic studies of flaring regions. Here we present high-fidelity imaging spectroscopy of a M1.3 GOES class flare with MeerKAT, a precursor to the future-generation array SKA-Mid. Radio emissions at the observed frequencies typically originate in the low corona, offering valuable insights into magnetic reconnection and primary energy-release sites. The obtained images achieve an unprecedented dynamic range exceeding 10^3, enabling simultaneous analysis of bright coherent bursts and faint incoherent emission from the active region. Multiple spatially distinct coherent sources are identified, implying contributions from different populations of accelerated electrons. The incoherent emission extends beyond AIA structures, highlighting MeerKAT's ability to detect dilute but hot plasma invisible to Extreme Ultraviolet instruments. Combined with co-temporal Hard X-ray images and magnetic field extrapolations, the radio sources are located within distinct magnetic structures, further revealing their association with different populations of accelerated electrons. These results demonstrate MeerKAT imaging spectroscopy as powerful diagnostics of solar flares and pave the way for future solar flare studies with SKA-Mid.

Figures (5)

• Figure 1: Overview of the 2024 December 29 M1.3 GOES-class solar flare. (A) MeerKAT Stokes I cross-power dynamic spectrum with GOES 1--8 Å soft X-ray flux (red) overplotted. (B) MeerKAT Stokes V cross-power spectrum. (C) MeerKAT full-disk integrated dynamic spectrum during the burst with STIX light curves overlaid; bad channels and time intervals are flagged. (D) ORFEES dynamic spectrum. Vertical dashed black lines in (A) and (B) denote the interval shown in (C) and (D), while vertical dashed red lines in (C) and (D) mark the times used for imaging in (E--G) and (H--J). The red horizontal line in (D) indicates the low-frequency limit of the MeerKAT band. (E) MeerKAT full-band radio image in brightness temperature (MK) at the burst peak, a linear color scale for values below 1 MK and a logarithmic scale for values above 1 MK is used to enhance the visualization (F) Alpha-blended overlay of MeerKAT emission (inferno colormap, transparency scaled by brightness) on an AIA 131 Å image (grayscale). Red boxes outline the regions shown in the zoomed-in view in (G). (H--J) Same as (E--G), but for the pre-burst quiet time.
• Figure 2: Left: AIA 131 Å image of the flaring active region at 11:20:30 UT, overlaid with different MeerKAT coherent sources (50, 70, and 90% contours). HXI and STIX contours (10, 30, 50, 70, and 90%) are also shown. Right: spatially resolved vector dynamic spectra of the selected sources (boxes in the left panel), obtained after subtracting contamination from nearby sources using two-dimensional Gaussian fitting. The images used to construct the vector dynamic spectrum are generated across four frequency channels, corresponding to a bandwidth of approximately 0.84 MHz.
• Figure 3: Left panels: (Top) AIA 131 Å image of the flaring site at 11:20:30 UT, overlaid with MeerKAT centroids (colored dots) and 50% flux density contours (colored lines) at selected frequencies from 11:20:27 UT. Colors denote the observing frequencies, and the synthesized beam used for the image is shown in the lower-left corner. Two spatially distinct sources are evident at different frequencies. (Bottom) Same as the top panel, but for 11:20:31 UT. The selected frequencies differ, and source extensions appear at relatively lower frequencies. The extended low-frequency feature may be related to the structures indicated by the magenta box in the ORFEES dynamic spectrum, many of which fall below the MeerKAT L-band range. Uncertainties of the centroid positions were estimated as $\sigma \approx \theta_{\mathrm{beam}} / (\sqrt{8\ln2}\,\mathrm{SNR})$1997PASP..109..166C, and are smaller than the symbol size to be visualized. Right panels: (Top and middle) Temporal evolution of the X- and Y-centroid positions, showing only pixels with $T_{\mathrm{B}} > 30$ MK. Black horizontal lines mark the upper limit of the ORFEES frequency coverage, and vertical red lines indicate the times corresponding to the left-panel plots. (Bottom) ORFEES dynamic spectrum during the same interval, with white horizontal lines marking the lower limit of the MeerKAT L-band coverage.
• Figure 4: Left: Contours (10, 30, 50, 70, and 90%) of the incoherent radio source overlaid on the AIA 131 Å image at 11:20:30 UT. Blue contours correspond to the pre-burst quiet time (11:19:51 UT), and red contours to the burst time (11:20:31 UT). Right: Brightness-temperature spectra of the incoherent source at four times: 11:19:51 (pre-burst), 11:20:29, 11:20:31, and 11:21:05 UT (post-burst). Dashed lines show power-law fits obtained through weighted least-squares regression in logarithmic space, with the fitted spectral indices indicated in the legend. Uncertainties combine image rms (estimated from a quiet region) and PB correction errors (assumed to be 10%). The spectra lie in the optically thick regime, exhibiting higher $T_{\mathrm{B}}$ and flatter indices during the burst.
• Figure 5: Upper left: NLFFF magnetic field extrapolation showing a possible configuration of the observed sources. Black curves indicate the lines of sight from Earth. The $X$ and $Y$ axes correspond to heliographic longitude and latitude, while the $Z$ axis extends radially outward from the solar surface. Upper right: Magnetic field lines projected onto the helioprojective frame, with colored curves representing loops associated with different sources. Coherent radio sources are outlined by 50% contours and marked by centroids in cyan, green, and orange, respectively. HXI sources are shown with 10% contours and centroids in pink. Incoherent sources are outlined by 10%, 50%, and 90% contours in red. Bottom panels: Magnetic field lines and associated sources within the field of view indicated by the black box in the upper-right panel. The left panel shows the HXR source (10%, 50%, and 90% pink contours) together with coherent source 1 (50%, 70%, and 90% cyan contours) and the incoherent sources (50%, 70%, and 90% red contours) during the burst, while the middle and right panels display coherent Sources 2 and 3. Centroids in the middle and right panels are determined from different frequencies at 11:20:31 UT and 11:20:27 UT, respectively, corresponding to the times shown in the upper panel of Figure \ref{['fig: cens']}. In the right panel, only the contours and centroids associated with Source 3 are shown.