Enigmatic centi-SFU and mSFU nonthermal radio transients detected in the middle corona

TL;DR

This paper reports the first detections of faint, transient, extended nonthermal radio emissions in the middle corona at heliocentric distances of roughly $3$–$7\,R_\odot$ using OVRO-LWA. The authors develop an automated, threshold-based identification pipeline, validate detections against potential ionospheric artifacts, and present three events (April 12, 14, 22, 2024) with brightness temperatures around $10^4$–$10^5$ K and lifetimes of a few minutes, often without white-light counterparts in LASCO/C2. Through density estimates and multi-mechanism modeling, they argue that plasma emission and nonthermal gyrosynchrotron emission from accelerated electrons are plausible, while gyroresonance is disfavored by velocity and density requirements. The findings indicate the middle corona hosts a population of nonthermal electrons detectable with high-dynamic-range, low-frequency radio imaging, offering new insights into coronal dynamics and particle acceleration outside traditional active regions and CMEs.

Abstract

Decades of solar coronal observations have provided substantial evidence for accelerated particles in the corona. In most cases, the location of particle acceleration can be roughly identified by combining high spatial and temporal resolution data from multiple instruments across a broad frequency range. In almost all cases, these nonthermal particles are associated with quiescent active regions, flares, and coronal mass ejections (CMEs). Only recently, some evidence of the existence of nonthermal electrons at locations outside these well-accepted regions has been found. Here, we report for the first time multiple cases of transient nonthermal emissions, in the heliocentric range of $\sim 3-7R_\odot$, which do not have any obvious counterparts in other wavebands, like white-light and extreme ultra-violet. These detections were made possible by the regular availability of high dynamic range low-frequency radio images from the Owens Valley Radio Observatory's Long Wavelength Array. While earlier detections of nonthermal emissions at these high heliocentric distances often had comparable extensions in the plane-of-sky, they were primarily been associated with radio CMEs, unlike the cases reported here. Thus, these results add on to the evidence that the middle corona is extremely dynamic and contains a population of nonthermal electrons, which is only becoming visible with high dynamic range low-frequency radio images.

Figures (14)

• Figure 1: Shows the output of various steps involved in detecting a potential candidate. Data corresponds to 70 MHz at 16:57:37 UT on April 12, 2024. Panel a shows the original image. Contours of this image is also indicated to better show the extended structure towards the south east. Subsequent steps lead to panels b, c and d. In Panel a, the lowest contour level is at 1.25% of the peak and then increases in steps of 2. The dashed circle shows the size of the Sun at 70 MHz. The solid circle shows the optical disc of the Sun.
• Figure 2: Left panel: Contours at 43 MHz and 17:16 on April 12, 2024, are overlaid on the nearest LASCO C2 difference image. Difference has been taken with respect to a 2-hour prior image. The lowest contour is at 0.04 MK and then increases in multiples of 2. Middle panel: Multiple frequency radio contours are overlaid on the same LASCO C2 difference image. The outer contour level at each frequency is at 0.04 MK. Black dashed circle in the left and middle panels shows the radio sun at 43 MHz. The solid circles show the optical disc of the Sun. Right panel: Shows the spectrum extracted from the region marked in the left panel. The spectrum has been extracted from the original image before any smoothing was done. Circles have been used to denote detections. The black line shows a spectrum following $\nu^{-2}$ dependence, where $\nu$ is the observation frequency. All radio contours, except the ones shown in the left panel using blue color, have been obtained by smoothing the images using a Gaussian function with standard deviation of $7.5\hbox{$^\prime$}$.
• Figure 3: Left panel: Difference 43 MHz radio image between 17:16:00 and 17:03:28 UT. The red contour shows the 0.03 MK. The black dashed line indicates the extent of the transient emission at 43 MHz. The solid and dashed circles show the solar limb in the optical and 43 MHz images, respectively. Right panel: Shows the total flux of the transient. The triangles indicate the $5\sigma$ upper limits.
• Figure 4: Multi-frequency contours at different times overlaid on the nearest available LASCO C2 difference image. Difference has been taken with respect to LASCO C2 image at 14:18 UT. The lowest contour at each frequency and time is at 0.05 MK.
• Figure 5: Top left panel: Radio contours at 34 MHz and 16:48 on April 14, 2024 are overlaid on the nearest LASCO C2 difference image. Difference has been taken with respect to a LASCO C2 image 1 hour prior to this time. The lowest contour is at 0.08 MK and then increases in multiples of 2. Negative contour at -0.08 MK (corresponding to lowest positive contour) is drawn using a cyan dashed line. The fact that no dashed line is visible, indicates the absence of any negative peak whose absolute value exceeds or is equal to 0.08 MK. Top middle panel: Multiple frequency radio contours are overlaid on the same LASCO C2 difference image. The outer contour level at each frequency is at 0.08 MK. The black dashed line in left and middle panel shows the radio sun at 34 MHz. The solid circles show the optical disc of the Sun. Top right panel: Brightness temperature spectrum of 2 regions shown. The marker color used to show the spectrum of each region is same as that used to indicate the location of the corresponding region in the top left panel. Properties of the markers and colors are the same as those in Figure \ref{['fig:20240412_event']}. Triangles have been used to denote the non-detections and represent the 5$\sigma$ upper limit. The red and magenta lines show the spectra following $\nu^{-2}$ dependence, where $\nu$ is the observation frequency. Bottom panels: Difference 34 MHz images for 16:45, 16:48 and 16:51 UT are shown in the bottom left, bottom middle and bottom right panels respectively. Difference has been taken with respect to the image at 16:32. The colorbar is highly saturated and non-linear and is the same for the 3 images.