Energy Release and Particle Acceleration in Flares: Summary and Future Prospects

Abstract

RHESSI measurements relevant to the fundamental processes of energy release and particle acceleration in flares are summarized. RHESSI's precise measurements of hard X-ray continuum spectra enable model-independent deconvolution to obtain the parent electron spectrum. Taking into account the effects of albedo, these show that the low energy cutoff to the electron power-law spectrum is typically ∼ < tens of keV, confirming that the accelerated electrons contain a large fraction of the energy released in flares. RHESSI has detected a high coronal hard X-ray source that is filled with accelerated electrons whose energy density is comparable to the magnetic-field energy density. This suggests an efficient conversion of energy, previously stored in the magnetic field, into the bulk acceleration of electrons. A new, collisionless (Hall) magnetic reconnection process has been identified through theory and simulations, and directly observed in space and in the laboratory; it should occur in the solar corona as well, with a reconnection rate fast enough for the energy release in flares. The reconnection process could result in the formation of multiple elongated magnetic islands, that then collapse to bulk-accelerate the electrons, rapidly enough to produce the observed hard X-ray emissions. RHESSI's pioneering γ-ray line imaging of energetic ions, revealing footpoints straddling a flare loop arcade, has provided strong evidence that ion acceleration is also related to magnetic reconnection. Flare particle acceleration is shown to have a close relationship to impulsive Solar Energetic Particle (SEP) events observed in the interplanetary medium, and also to both fast coronal mass ejections and gradual SEP events. New instrumentation to provide the high sensitivity and wide dynamic range hard X-ray and γray measurements, plus energetic neutral atom (ENA) imaging of SEPs above ∼2 R ⊙ , will enable the next great leap forward in understanding particle acceleration and energy release is large solar eruptions -solar flares and associated fast coronal mass ejections (CMEs).

On the probability of occurrence of extreme space weather events

“…The initiation of coronal mass ejections, for example, may occur through flux cancelation, “break out,” the kink instability, flux emergence, or some other mechanism [ Forbes , 2000], each of which could produce CMEs with different distributions of speeds. Similarly, energetic particles are thought to be produced either through an impulsive mechanism or a gradual one [e.g., Lin , 2011].…”

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

“…Various new features of solar flare X-rays have been discovered by RHESSI and these have already been reviewed elsewhere (e.g. Benz, 2008;Krucker and Lin, 2008;Fletcher et al, 2011;Hannah et al, 2011;Kontar et al, 2011;Lin, 2011;Vilmer, MacKinnon, and Hurford, 2011;White et al, 2011;Zharkova et al, 2011).…”

Type III bursts produced by power law injected electrons in Maxwellian background coronal plasmas

“…Further discussion regarding prediction of 2 f p emission rate is deferred to section 5. [31] In situ observations of energetic electrons and Xray observations during flares suggest that both the power law index p and the break speed v 0 in equation (4) vary from event to event [Lin, 1974[Lin, , 2011White et al, 2011]. This section studies the effects on remote f p emission of varying p and v 0 separately.…”