Abstract
Positrons play a major role in the emission of solar gamma-rays at energies from a few hundred keV to >1 GeV. Although the processes leading to positron production in the solar atmosphere are well known, the origin of the underlying energetic particles that interact with the ambient particles is poorly understood. With the aim of understanding the full gamma-ray spectrum of the Sun, I review the key emission mechanisms that contribute to the observed gamma-ray spectrum, focusing on the ones involving positrons. In particular, I review the processes involved in the 0.511 MeV positron annihilation line and the positronium continuum emissions at low energies, and the pion continuum emission at high energies in solar eruptions. It is thought that particles accelerated at the flare reconnection and at the shock driven by coronal mass ejections are responsible for the observed gamma-ray features. Based on some recent developments I suggest that energetic particles from both mechanisms may contribute to the observed gamma-ray spectrum in the impulsive phase, while the shock mechanism is responsible for the extended phase.
Highlights
Positrons, the antiparticle of electrons, were proposed theoretically by Dirac and were first detected by Anderson in 1933 [1]
Astrophysical processes involving positrons have been found in the interstellar medium [3], and galactic bulge and disk [4]
In the remaining 1.2% of cases, π0 decays into an electron positron pair and a gamma-ray
Summary
The antiparticle of electrons, were proposed theoretically by Dirac and were first detected by Anderson in 1933 [1]. Electromagnetic radiation from radio waves to gamma-rays produced by the energized electrons and protons by various processes is known as a flare. CMEs carry the released energy as the kinetic energy of the expelled magnetized plasma with a mass as high as 1016 g and speeds exceeding 3000 km/s Such fast CMEs drive fast-mode magnetohydrodynamic shocks that can energize ambient electrons and ions to very high energies. Atoms 2020, 8, 14 acceleration and shock drift acceleration are the mechanisms by which particles are energized at the shock (see [7] for a review on acceleration mechanisms during solar eruptions). Particles from the shock propagate away from the Sun and are detected as energetic particle events in space. The dark shock propagate away from the Sun and are detected as energetic particle events in space.
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