We review the physical processes of particle acceleration, injection, propagation, trapping, and energy loss in solar flare conditions. An understanding of these basic physical processes is inexorable to interpret the detailed timing and spectral evolution of the radiative signatures caused by nonthermal particles in hard X-rays, gamma-rays, and radio wavelengths. In contrast to other more theoretically oriented reviews on particle acceleration processes, we aim here to capitalize on the numerous observations from recent spacecraft missions, such as from the Compton Gamma Ray Observatory (CGRO), the Yohkoh Hard X-Ray Telescope (HXT) and Soft X-Ray Telescope (SXT), and the Transition Region and Coronal Explorer (TRACE). High-precision energy-dependent time delay measurements from CGRO and spatial imaging with Yohkoh and TRACE provide invaluable observational constraints on the topology of the acceleration region, the reconstruction of magnetic reconnection processes, the resulting electromagnetic fields, and the kinematics of energized (nonthermal) particles.

/pdf/particle-acceleration-and-kinematics-in-solar-flares-a-4iyjh5x0g8.pdf

Particle acceleration and kinematics in solar flares – A Synthesis of Recent Observations and Theoretical Concepts (Invited Review)

The collision and reconnection of magnetic flux tubes in the solar corona has been proposed as a mechanism for solar flares and in some cases as a model for coronal mass ejections. We study this process by simulating the collision of pairs of twisted flux tubes with a massively parallel, collocation, viscoresistive, magnetohydrodynamic code using up to 256 × 256 × 256 Fourier modes. Our aim is to investigate the energy release and possible global topological changes that can occur in flux-tube reconnection. We have performed a number of simulations for different angles between the colliding flux tubes and for either co- or counterhelicity flux tubes. We find the following four classes of interaction: (1) bounce (no appreciable reconnection), (2) merge, (3) slingshot (the most efficient reconnection), and (4) tunnel (a double reconnection). We will describe these four classes of flux-tube reconnection and discuss in what range of parameter space each class occurs and the implications our results have for models of flares and coronal mass ejections.

https://iopscience.iop.org/article/10.1086/320974/pdf

Reconnection of Twisted Flux Tubes as a Function of Contact Angle

We investigate the type III radio bursts and X-ray signatures of accelerated electrons in a well observed solar flare in order to find the spatial properties of the acceleration region. Combining simultaneous RHESSI hard X-ray flare data and radio data from Phoenix-2 and the Nan\c{c}ay radioheliograph, the outward transport of flare accelerated electrons is analyzed. The observations show that the starting frequencies of type III bursts are anti-correlated with the HXR spectral index of solar flare accelerated electrons. We demonstrate both analytically and numerically that the type III burst starting location is dependent upon the accelerated electron spectral index and the spatial acceleration region size, but weakly dependent on the density of energetic electrons for relatively intense electron beams. Using this relationship and the observed anti-correlation, we estimate the height and vertical extent of the acceleration region, giving values of around 50 Mm and 10 Mm respectively. The inferred acceleration height and size suggests that electrons are accelerated well above the soft X-ray loop-top, which could be consistent with the electron acceleration between 40 Mm and 60 Mm above the flaring loop.

/pdf/characteristics-of-the-flare-acceleration-region-derived-2lldav9fkd.pdf

Characteristics of the flare acceleration region derived from simultaneous hard X-ray and radio observations

We investigate the type III radio bursts and X-ray signatures of accelerated electrons in a well-observed solar flare in order to find the spatial properties of the acceleration region. Combining simultaneous RHESSI hard X-ray flare data and radio data from Phoenix-2 and the Nancay radioheliograph, the outward transport of flare accelerated electrons is analysed. The observations show that the starting frequencies of type III bursts are anti-correlated with the HXR spectral index of solar flare accelerated electrons. We demonstrate both analytically and numerically that the type III burst starting location is dependent upon the accelerated electron spectral index and the spatial acceleration region size, but weakly dependent on the density of energetic electrons for relatively intense electron beams. Using this relationship and the observed anti-correlation, we estimate the height and vertical extent of the acceleration region, giving values of around 50 Mm and 10 Mm, respectively. The inferred acceleration height and size suggest that electrons are accelerated well above the soft X-ray loop-top, which could be consistent with the electron acceleration between 40 Mm and 60 Mm above the flaring loop.

/pdf/characteristics-of-the-flare-acceleration-region-derived-3yt8yi3ce8.pdf

Observations relevant to the relative timing of hard X-ray, microwave and lower frequency radio bursts in different phases of flare are reviewed. It is shown that such timing comparisons give important information concerning the electron acceleration/injection process, the magnetic field topology at the acceleration site and the flare development itself. In particular it is shown that acceleration begins before the flash phase of flares and that it keeps going on continuously during the entire duration of a flare. Moreover, despite their wide separation in altitude, hard X-ray, microwave and lower frequency sources appear to arise from a common injection of electrons going on continuously through the different phases of flare. In situ acceleration by shock waves giving rise to type II radio emission is briefly discussed. As an alternative interactions between small and large scale magnetic structures is proposed.

Relative timing of hard X-rays and radio emissions during the different phases of solar flares - Consequences for the electron acceleration

The first measurements of the spatial structure of a group of type III solar radio bursts associated with an impulsive hard X-ray burst are presented. At 169 MHz the radio source has been found to consist of two principal regions separated by about 300,000 km. The two regions together produced a total of four component bursts in good time correlation with spikes in the hard X-ray emission. The observations indicate that electron acceleration/injection occurs over a region which covers a wide range of magnetic field lines.

Correlated observations of a spatially resolved type III solar radio burst group and the associated hard X-ray emission

New Correlated Observations of Impulsive Hard X-ray and Type III Radio Bursts

A correlative study of metric type III/V bursts and hard X-ray events during the preflash and the impulsive phases of solar flares is presented. A total of 55 groups of type III bursts were observed with the Nancay radiospectrograph, and 32 of these were associated with hard X-ray bursts detected with instruments on ISEE 3, SMM, or both. X-ray bursts associated with type III or U bursts, and type V continuum emission lasting less than 10 s are relatively small, with peak fluxes at 30 keV of not more than 1 photon/sq cm per sec per keV. X-ray bursts associated with type III bursts and continuum emission lasting longer than 30 s are considerably more intense, with peak fluxes at 30 keV exceeding 1 photon/sq cm per sec per keV and spectra extending to not less than 100 keV. For these latter events, type III emission was observed at 169 MHz with the Nancay radioheliograph from several locations during the preflash phase. At the time of the rapid increase in the hard X-ray flux, a new source appeared at 169 MHz and proceeded to fluctuate in phase with one of the original sources that brightened at the same time. This type of behavior, observed in 13 flares, is interpreted as being caused by the rapid interaction of two or more magnetic structures giving rise to the impulsive phase of the flare.

Solar flare development during the impulsive phase as deduced from correlated radio and hard X-ray observations

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Correlated observations of a spatially resolved type III solar radio burst group and the associated hard X-ray emission

New Correlated Observations of Impulsive Hard X-ray and Type III Radio Bursts

Solar flare development during the impulsive phase as deduced from correlated radio and hard X-ray observations