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Abstract The time profile of solar flare radio emission is often modeled as an injection of energetic particles onto a closed magnetic loop, where they may be trapped by the pinching of field lines and remain for a time before decaying through loss of energy to the background or escaping to the solar surface through the bottom of the loop. These injection, trapping, and precipitation models for energetic particle transport have often been used to explain the characteristics of spatially integrated microwave emissions in solar flares. With the high-cadence imaging spectroscopy capabilities of modern radio instruments, these ideas can be probed with new depth. Radio imaging allows for the selection of particular regions of flares to spatially and temporally isolate individual injections and determine individual decay parameters that could be confused in spatially integrated spectra. Simultaneous spectroscopy allows the fitting of light curves versus frequency for insight into the evolution of the particle energy spectrum and a deeper physical understanding of the decay process. Using currently available time resolution and data quality, injections and decays can be fit simultaneously to the order of 1 s. These considerations motivate the creation of the Pulsed-Injection-Precipitation Decomposition Fitter (PIP_Decomp), which implements an automated method for fitting a series of light curves with injection functions convolved with exponential decays to produce spectrally resolved fit parameters. Herein, PIP_Decomp is introduced and tested by applying it to model flares. Then, PIP_Decomp is used to investigate two relatively simple flares observed by the Expanded Owens Valley Solar Array. 

Abstract Noise in images of strong celestial sources at radio wavelengths using Fourier synthesis arrays can be dominated by the source itself, so-called self-noise. We outlined the theory of self-noise for strong sources in a companion paper. Here we consider the case of noise in maps of radio emission from the Sun which, as we show, is always dominated by self noise. We consider several classes of science use cases for current and planned arrays designed to observe the Sun in order to understand limitations imposed by self-noise. We focus on instruments operating at decimeter and centimeter wavelengths but the results are applicable to other wavelength regimes. 

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 ∼3–7R_⊙, which do not have any obvious counterparts in other wave bands, like white-light and extreme ultraviolet. 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 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. 

Abstract In this Letter, taking advantage of microwave data from the Expanded Owens Valley Solar Array and extreme-ultraviolet (EUV) data from the Atmospheric Imaging Assembly, we present the first microwave imaging spectroscopy diagnosis for the slow-rise precursor of a major coronal mass ejection (CME) on 2022 March 30. The EUV images reveal that the CME progenitor, appearing as a hot channel above the polarity inversion line, experiences a slow rise and heating before the eruption. The microwave emissions are found to mainly distribute along the hot channel, with high-frequency sources located at the ends of the hot channel and along precursor bright loops underneath the hot channel. The microwave spectroscopic analysis suggests that microwave emissions in the precursor phase are dominated by thermal emission, largely different from the main phase when a significant nonthermal component is present. These results support the scenario that the precursor reconnection, seeming to be moderate compared with the flare reconnection during the main phase, drives the buildup, heating, and slow rise of CME progenitors toward the explosive eruption. 

Abstract The Sun is a powerful source of radio emissions, so much so that, unlike most celestial sources, this emission can dominate the system noise of radio telescopes. We outline the theory of noise in maps formed by Fourier synthesis techniques at radio wavelengths, with a focus on self-noise: that is, noise due to the source itself. As a means of developing intuition we consider noise for the case of a single dish, a two-element interferometer, and an$$n$$ $n$-element array for simple limiting cases. We then turn to the question of the distribution of noise on a map of an arbitrary source observed at radio wavelengths by an$$n$$ $n$-element interferometric array. We consider the implications of self-noise for observations of the Sun in a companion paper. 

Abstract Routine measurements of the magnetic field of coronal mass ejections (CMEs) have been a key challenge in solar physics. Making such measurements is important both from a space weather perspective and for understanding the detailed evolution of the CME. In spite of significant efforts and multiple proposed methods, achieving this goal has not been possible to date. Here we report the first possible detection of gyroresonance emission from a CME. Assuming that the emission is happening at the third harmonic, we estimate that the magnetic field strength ranges from 7.9 to 5.6 G between 4.9 and 7.5R_⊙. We also demonstrate that this high magnetic field is not the average magnetic field inside the CME, but most probably is related to small magnetic islands, which are also being observed more frequently with the availability of high-resolution and high-quality white-light images. 

Abstract A subclass of early impulsive solar flares, cold flares, was proposed to represent a clean case, where the release of the free magnetic energy (almost) entirely goes to the acceleration of the nonthermal electrons, while the observed thermal response is entirely driven by the nonthermal energy deposition to the ambient plasma. This paper studies one more example of a cold flare, which was observed by a unique combination of instruments. In particular, this is the first cold flare observed with the Expanded Owens Valley Solar Array and, thus, for which the dynamical measurement of the coronal magnetic field and other parameters at the flare site is possible. With these new data, we quantified the coronal magnetic field at the flare site but did not find statistically significant variations of the magnetic field within the measurement uncertainties. We estimated that the uncertainty in the corresponding magnetic energy exceeds the thermal and nonthermal energies by an order of magnitude; thus, there should be sufficient free energy to drive the flare. We discovered a very prominent soft-hard-soft spectral evolution of the microwave-producing nonthermal electrons. We computed energy partitions and concluded that the nonthermal energy deposition is likely sufficient to drive the flare thermal response similarly to other cold flares. 

Abstract Measuring plasma parameters in the upper solar corona and inner heliosphere is challenging because of the region’s weakly emissive nature and inaccessibility for most in situ observations. Radio imaging of broadened and distorted background astronomical radio sources during solar conjunction can provide unique constraints for the coronal material along the line of sight. In this study, we present radio spectral imaging observations of the Crab Nebula (Tau A) from 2024 June 9 to June 22 when it was near the Sun with a projected heliocentric distance of 5–27 solar radii, using the Owens Valley Radio Observatory’s Long Wavelength Array at multiple frequencies in the 30–80 MHz range. The imaging data reveal frequency-dependent broadening and distortion effects caused by anisotropic wave propagation through the turbulent solar corona at different distances. We analyze the brightness, size, and anisotropy of the broadened images. Our results provide detailed observations showing that the eccentricity of the unresolved source increases as the line of sight approaches the Sun, suggesting a higher anisotropic ratio of the plasma turbulence closer to the Sun. In addition, the major axis of the elongated source is consistently oriented in the direction perpendicular to the radial direction, suggesting that the turbulence-induced scattering effect is more pronounced in the direction transverse to the coronal magnetic field. Lastly, when the source undergoes large-scale refraction as the line of sight passes through a streamer, the apparent source exhibits substructures at lower frequencies. This study demonstrates that observations of celestial radio sources with lines of sight near the Sun provide a promising method for measuring turbulence parameters in the inner heliosphere. 

Abstract A major challenge in understanding the initiation and evolution of coronal mass ejections (CMEs) is measuring the magnetic field of the magnetic flux ropes (MFRs) that drive CMEs. Recent developments in radio imaging spectroscopy have paved the way for diagnosing the CMEs’ magnetic field using gyrosynchrotron radiation. We present magnetic field measurements of a CME associated with an X5-class flare by combining radio imaging spectroscopy data in microwaves (1–18 GHz) and meter waves (20–88 MHz), obtained by the Owens Valley Radio Observatory’s Expanded Owens Valley Solar Array (EOVSA) and Long Wavelength Array (OVRO-LWA), respectively. EOVSA observations reveal that the microwave source, observed in the low corona during the initiation phase of the eruption, outlines the bottom of the rising MFR-hosting CME bubble seen in extreme ultraviolet and expands as the bubble evolves. As the MFR erupts into the middle corona and appears as a white-light CME, its meter-wave counterpart, observed by OVRO-LWA, displays a similar morphology. For the first time, using gyrosynchrotron spectral diagnostics, we obtain magnetic field measurements of the erupting MFR in both the low and middle corona, corresponding to coronal heights of 0.02 and 1.83R_⊙. The magnetic field strength is found to be around 300 G at 0.02R_⊙during the CME initiation and about 0.6 G near the leading edge of the CME when it propagates to 1.83R_⊙. These results provide critical new insights into the magnetic structure of the CME and its evolution during the early stages of its eruption. 

When in situ solar energetic electron (SEE) events are closely associated with nonthermal flares, the escaping electron population is frequently observed to be much smaller than the nonthermal-radiation-emitting population near the solar surface. If a single accelerated population drives both signatures, the physical mechanism causing this severe deficit of upward-propagating electrons remains poorly understood. Focusing on one of the 2022 November 10–12 SEE events associated with recurrent solar jets and interplanetary type III radio bursts, we present a new, combined microwave–X-ray analysis using the Expanded Owens Valley Solar Array and the Spectrometer/Telescope for Imaging X-rays on board Solar Orbiter. For the first time for such an event, this synergy enables spatially resolved diagnostics over a broad energy spectrum of the near-Sun energetic electrons, complemented by in situ measurements made by spacecraft at multiple heliocentric longitudes and distances. Consistent with earlier results based on in situ and X-ray data, our results show that only 0.1%–1% of energetic electrons escape into interplanetary space. Crucially, the new microwave spectral imaging analysis suggests that energetic electrons are strongly concentrated in a compact region just above a miniflare arcade at the base of the jet spire and that their number density decreases by at least 2 orders of magnitude in the direction of the jet spire away from this region. This steep gradient, revealed by the microwave diagnostics, points to efficient local acceleration and trapping in the region analogous to the above-the-loop-top “magnetic bottle” region in major eruptive flares, allowing only a small fraction of electrons to access open magnetic field lines and enter interplanetary space.