Dark and elliptical, coronal cavities yield important clues to the magnetic structures that cradle prominences, and to the forces that ultimately lead to their eruption. We review observational analyses of cavity morphology, thermal properties (density and temperature), line-of-sight and plane-of-sky flows, substructure including hot cores and central voids, linear polarization signatures, and observational precursors and predictors of eruption. We discuss a magnetohydrodynamic interpretation of these observations which argues that the cavity is a magnetic flux rope, and pose a set of open questions for further study.

Coronal cavities: observations and implications for the magnetic environment of prominences

In some flares the thermal component appears much earlier than the nonthermal component in X-ray range. Using sensitive microwave observations we revisit this finding made by Battaglia et al. (2009) based on thorough analysis of RHESSI data. We have found that nonthermal microwave emission produced by accelerated electrons with energy of at least several hundred keV, appears as early as the thermal soft X-ray emission indicative that the electron acceleration takes place at the very early flare phase. The non-detection of the hard X-rays at that early stage of the flares is, thus, an artifact of a limited RHESSI sensitivity. In all considered events, the microwave emission intensity increases at the early flare phase. We found that either thermal or nonthermal gyrosynchrotron emission can dominate the low-frequency (optically thick) part of the microwave spectrum below the spectral peak occurring at 3-10 GHz. In contrast, the high-frequency optically thin part of the spectrum is always formed by the nonthermal, accelerated electron component, whose power-law energy spectrum can extend up to a few MeV at this early flare stage. This means that even though the total number of accelerated electrons is small at this stage, their nonthermal spectrum is fully developed. This implies that an acceleration process of available seed particles is fully operational. While, creation of this seed population (the process commonly called ‘injection’ of the particles from the thermal pool into acceleration process) has a rather low efficiency at this stage, although, the plasma heating efficiency is high. This imbalance between the heating and acceleration (in favor of the heating) is difficult to reconcile within most of available flare energization models. Being reminiscent of the trade off between the Joule heating and runaway electron acceleration, it puts additional constraints on the electron injection into the acceleration process. As a byproduct of this study, we demonstrate that for those cases when the optically thick part of the radio spectrum is dominated by the thermal contribution, the microwave spectral data yields reliable estimates of the magnetic field and source area at the early flare phase. Subject headings: Sun: filaments — Sun: flares — Sun: radio radiation — Sun: UV radiation

Thermal to Nonthermal Energy Partition at the Early Rise Phase of Solar Flares

The far-side solar eruptive event SOL2014-09-01 produced hard electromagnetic and radio emissions that were observed with detectors at near-Earth vantage points. Especially challenging was a long-duration > 100 MeV $\gamma$
 -ray burst that was probably produced by accelerated protons exceeding 300 MeV. This observation raised the question how high-energy protons could reach the Earth-facing solar surface. Some preceding studies discussed a scenario in which protons accelerated by a shock driven by a coronal mass ejection high in the corona return to the solar surface. We continue with the analysis of this challenging event, involving radio images from the Nancay Radioheliograph and hard X-ray data from the High Energy Neutron Detector (HEND) of the Gamma-Ray Spectrometer onboard the Mars Odyssey space observatory located near Mars. HEND recorded unocculted flare emission. The results indicate that the emissions observed from the Earth’s direction were generated by flare-accelerated electrons and protons trapped in static long coronal loops. They can be reaccelerated in these loops by a shock wave that was excited by the eruption, being initially not driven by a coronal mass ejection. The results highlight ways to address the remaining questions.

Radio, Hard X-Ray, and Gamma-Ray Emissions Associated with a Far-Side Solar Event

In some flares the thermal component appears much earlier than the nonthermal component in X-ray range. Using sensitive microwave observations we revisit this finding made by Battaglia et al. (2009) based on RHESSI data analysis. We have found that nonthermal microwave emission produced by accelerated electrons with energy of at least several hundred keV, appears as early as the thermal soft X-ray emission indicative that the electron acceleration takes place at the very early flare phase. The non-detection of the hard X-rays at that early stage of the flares is, thus, an artifact of a limited RHESSI sensitivity. In all considered events, the microwave emission intensity increases at the early flare phase. We found that either thermal or nonthermal gyrosynchrotron emission can dominate the low-frequency part of the microwave spectrum below the spectral peak occurring at 3-10 GHz. In contrast, the high-frequency optically thin part of the spectrum is always formed by the nonthermal, accelerated electron component, whose power-law energy spectrum can extend up to a few MeV at this early flare stage. This means that even though the total number of accelerated electrons is small at this stage, their nonthermal spectrum is fully developed. This implies that an acceleration process of available seed particles is fully operational. While, creation of this seed population (the process commonly called `injection' of the particles from the thermal pool into acceleration) has a rather low efficiency at this stage, although, the plasma heating efficiency is high. This imbalance between the heating and acceleration (in favor of the heating) is difficult to reconcile within most of available flare energization models. Being reminiscent of the tradeoff between the Joule heating and runaway electron acceleration, it puts additional constraints on the electron injection into the acceleration process.

The SOL2014-09-01 far-side solar eruptive event produced hard electromagnetic and radio emissions observed with detectors at near-Earth vantage points Especially challenging was a long-duration >100 MeV $\gamma$-ray burst probably produced by accelerated protons exceeding 300 MeV This observation raised a question of how high-energy protons could reach the Earth-facing solar surface Some preceding studies discussed a scenario in which protons accelerated by a CME-driven shock high in the corona return to the solar surface We continue with the analysis of this challenging event, involving radio images from the Nan\c{c}ay Radioheliograph and hard X-ray data from the High Energy Neutron Detector (HEND) of the Gamma-Ray Spectrometer onboard the Mars Odyssey space observatory located near Mars HEND recorded unocculted flare emission The results indicate that the emissions observed from the Earth's direction were generated by flare-accelerated electrons and protons trapped in static long coronal loops Their reacceleration is possible in these loops by a shock wave, which was excited by the eruption, being initially not CME-driven The results highlight the ways to address remaining questions

Multi-instrument view on solar eruptive events observed with the Siberian Radioheliograph: From detection of small jets up to development of a shock wave and CME

An upgrade of the Siberian Solar Radio Telescope (SSRT) [Smolkov et al., 1986; Grechnev et al., 2003] to a multiwave radio heliograph has been started. The radio heliograph being created will be designed mainly to measure coronal magnetic fields, to determine the locations of solar-flare energy release, and to investigate coronal mass ejections. These tasks define the parameters of next-generation radio heliographs. A high spatial resolution, a high image acquisition rate, and a high sensitivity are required simultaneously. All these parameters should be realized in the widest possible frequency range—from fractions to tens of GHz). The expected parameters of the future SSRT-based radio heliograph are listed below: spatial resolution 12″–24″, temporal resolution 0.02–1.0 s, frequency range 4–8 GHz, sensitivity up to 100 K, left-hand and right-hand circular polarizations, data rate 0.5–20 Mb s−1 (normal and flare modes). In this paper, we describe the broadband antennas, analog optical data transmission lines, and correlator used in the 10-antenna radio heliograph prototype.

A.V. Gubin

Papers

Multi-instrument view on solar eruptive events observed with the Siberian Radioheliograph: From detection of small jets up to development of a shock wave and CME

Ten-antenna prototype of a radio heliograph based on the Siberian Solar Radio Telescope