Solar flares and energetic particles

Abstract

Solar flares are now observed at all wavelengths from g-rays to decametre radio waves. They are commonly associated with efficient production of energetic particles at all energies. These particles play a major role in the active Sun because they contain a large amount of the energy released during flares. Energetic electrons and ions interact with the solar atmosphere and produce high-energy X-rays and g-rays. Energetic particles can also escape to the corona and interplanetary medium, produce radio emissions (electrons) and may eventually reach the Earth's orbit. I shall review here the available information on energetic particles provided by X-ray/g-ray observations, with particular emphasis on the results obtained recently by the mission Reuven Ramaty High-Energy Solar Spectroscopic Imager. I shall also illustrate how radio observations contribute to our understanding of the electron acceleration sites and to our knowledge on the origin and propagation of energetic particles in the interplanetary medium. I shall finally briefly review some recent progress in the theories of particle acceleration in solar flares and comment on the still challenging issue of connecting particle acceleration processes to the topology of the complex magnetic structures present in the corona.

Small electron acceleration episodes in the solar corona

“…There is ample evidence for acceleration processes in the solar corona that result in nonthermal particle distributions. These include the radiative signatures and direct particle detections in large eruptive events that result in flares and coronal mass ejections (CMEs) (Zharkova et al 2011;Vilmer 2012;Aschwanden 2012;Miller et al 1997;Kontar et al 2011;Holman et al 2011;Aschwanden et al 2017). The underlying driver for such particle acceleration events is generally understood to be the excess energy stored in stressed magnetic fields that is released via the process of magnetic reconnection.…”

Forced magnetic reconnection and plasmoid coalescence

“…Solar flares release up to ∼ 10 25 J of magnetic energy over the course of minutes to hours (Benz 2017;Fletcher et al 2011). Of this energy, up to 50% can be transported away by ∼ 10 39 energetic charged particles (Aschwanden et al 2016;Vilmer 2012) and the mechanism responsible is still an area of contention. The primary means by which magnetic energy stored in the corona is converted into kinetic energy is widely accepted to be magnetic reconnection, due to the broad range of observed phenomena that it accounts for (Janvier et al 2015;Shibata & Magara 2011;Priest 2014).…”

“…The reconnection of magnetic field lines comes in many flavours, each of which is relevant in some particular coronal magnetic field topology. In the case of solar flares, a significant factor in the acceleration of particles is the direct electric field at the site of the reconnection (Zharkova et al 2011) -in the standard flare model this is described as being part of a 'monolithic' current sheet above the flaring loop, which is borne out by observations of a hard X-ray source at the loop-top (Su et al 2013;Vilmer 2012).…”

Astrophysical processes on the Sun

“…Finally, Vilmer [33] describes solar flares with a particular emphasis on particle acceleration, including both the latest observational and theoretical highlights.…”