Implications of X-ray Observations for Electron Acceleration and Propagation in Solar Flares

Abstract

High-energy X-rays and γ -rays from solar flares were discovered just over fifty years ago. Since that time, the standard for the interpretation of spatially integrated flare X-ray spectra at energies above several tens of keV has been the collisional thick-target model. After the launch of the Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI) in early 2002, X-ray spectra and images have been of sufficient quality to allow a greater focus on the energetic electrons responsible for the X-ray emission, including their origin and their interactions with the flare plasma and magnetic field. The result has been new insights into the flaring process, as well as more quantitative models for both electron acceleration and propagation, and for the flare environment with which the electrons interact. In this article we review our current understanding of electron acceleration, energy loss, and propagation in flares. Implications of these new results for the collisional thick-target model, for general flare models, and for future flare studies are discussed.

Nonequilibrium Processes in the Solar Corona, Transition Region, Flares, and Solar Wind (Invited Review)

“…In Oka et al (2013Oka et al ( , 2015 it was argued that there should always be a thermal core component due to thermalization (in the lower energy range) by Coulomb collisions and/or wave-particle interactions (see also Holman et al, 2011). By varying the index κ, the κ-distribution can, in fact, represent not only cases with negligible non-thermal component (including super-hot cases) but also cases with saturated non-thermal component (with no spectral break between the thermal and non-thermal components).…”

Solar Particle Radiation Storms Forecasting and Analysis

“…From Klein and Dalla (2017) with another closed magnetic structure. A more detailed discussion of hard X-ray source morphology and its interpretation can be found in Holman et al (2011). Radio emission of electron beams, such as type III bursts, is another key observation to identify the electron acceleration.…”

“…The UMASEP scheme considers that a common positive derivative with the particle flux near Earth, with a suitable time delay, indicates a magnetic connection between the Earth and a site of particle acceleration near the Sun. It is well known (Neupert 1968;Dennis and Zarro 1993;Holman et al 2011) that hard X-ray (HXR) or microwave bursts, produced by non-thermal electrons in the solar atmosphere through bremsstrahlung and gyrosynchrotron emission (see Chap. 2), have time profiles that mimic the time derivative of the SXR.…”

HESPERIA Forecasting Tools: Real-Time and Post-Event

“…The UMASEP scheme considers that a common positive derivative with the particle flux near Earth, with a suitable time delay, indicates a magnetic connection between the Earth and a site of particle acceleration near the Sun. It is well known (Neupert 1968;Dennis and Zarro 1993;Holman et al 2011) that hard X-ray (HXR) or microwave bursts, produced by non-thermal electrons in the solar atmosphere through bremsstrahlung and gyrosynchrotron emission (see Chap. 2), have time profiles that mimic the time derivative of the SXR.…”