atoms Review Positron Processes in the Sun Nat Gopalswamy Solar Physics Laboratory, NASA Goddard Space Flight Center, Greenbelt, MD 20771, USA; [email protected] Received: 19 March 2020; Accepted: 16 April 2020; Published: 22 April 2020 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. Keywords: solar flares; coronal mass ejections; shocks; positrons; positronium; positron annihilation; pion decay 1. Introduction Positrons, the antiparticle of electrons, were proposed theoretically by Dirac and were first detected by Anderson in 1933 [1]. Positrons are extensively used in the laboratory for a myriad of purposes (see review by Mills [2]). Astrophysical processes involving positrons have been found in the interstellar medium [3], and galactic bulge and disk [4]. Positrons are commonly found in the Sun [5]. The proton–proton chain, which accounts for most of the energy release inside the Sun involves the emission of a positron when two protons collide to form a deuterium nucleus. There are plenty of electrons present in the solar core, so the positrons are immediately annihilated with electrons and produce two gamma-ray photons. In one proton–proton chain, two positrons are emitted and hence contribute a total of four photons in addition to the two photons emitted during the formation of a helium-3 (3He) nucleus from the fusion of deuterium nucleus with a proton. High-energy particles exist in the solar atmosphere, energized during solar eruptions. Solar eruptions involve flares and coronal mass ejections (CMEs). A process known as magnetic reconnection taking place in the solar corona is thought to be the process which converts energy stored in the stressed solar magnetic fields into solar eruptions (see Figure1 for a schematic of a typical eruption). One part of the released energy heats the plasma in the eruption region, while another goes to energize electrons and ions. Electromagnetic radiation from radio waves to gamma-rays produced by the energized electrons and protons by various processes is known as a flare. Acceleration in the reconnection region is referred to as flare acceleration or stochastic acceleration. CMEs also 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 also energize ambient electrons and ions to very high energies. Diffusive shock Atoms 2020, 8, 14; doi:10.3390/atoms8020014 www.mdpi.com/journal/atoms Atoms 2020, 8, 14 2 of 12 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). The flare and shock accelerations were referred to as first- and second-phase accelerations during an eruption by Wild et al. [8]. Accelerated particles have access to open and closed magnetic structures associated with the eruption resulting in a number of electromagnetic emissions via different emission mechanisms. Energetic particles escaping along interplanetary magnetic field lines are detected as solar energetic particle (SEP)Atoms 2019 events, 7, x FOR by PEER particle REVIEW detectors in space and on ground. These particles, originally2 of known 12 as solar cosmic rays, were first detected by Forbush [9] in the 1940s. 33 34 Figure 1.FigureSchematic 1. Schematic of a solar of a solar eruption eruption and and the the sites sites of of particleparticle acceleration acceleration (e,p, (e,p, …):::: one): in one the current in the current 35 sheet formedsheet formed low in low the in corona the corona and and the the other other on on thethe surface of of the the shock shock driven driven by the by coronal the coronal mass mass 36 ejectionejection (CME) (CME) flux rope. flux rope. The The arrows arrows toward toward the the currentcurrent sheet sheet indicate indicate the thereconnection reconnection inflow inflow while while 37 the onesthe diverging ones diverging away away indicate indicate the the outflow. outflow. The The redred ellipses in inthe the photosphere photosphere represent represent the feet the feet 38 of flare loops where accelerated particles precipitate and produce flare radiation. Particles from the of flare loops where accelerated particles precipitate and produce flare radiation. Particles from the 39 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. The dark 40 ellipse inside the flux rope represents a prominence that erupted along with the CME (adapted from 41 ellipse insideGopalswamy the flux [6]). rope represents a prominence that erupted along with the CME (adapted from Gopalswamy [6]). 42 High-energy particles exist in the solar atmosphere, energized during solar eruptions. Solar 43 Invokingeruptions the involve copious flares production and coronal of energetic mass ejections particles (CMEs). from the A Sun,process Morrison known [as10 ]magnetic suggested that the44 active reconnection Sun must taking be a source place in of the gamma-rays. solar corona Heis thought listed electron-positronto be the process which annihilation converts energy as one of the processes45 stored expected in the stressed to produce solar gamma-raymagnetic fields emission into solar from eruption cosmics (see sources Figure 1 [ 10for]. a Elliot schematic [11] of suggested a 46 that “positivetypical electrons”eruption). One from part muon of the decay released should energy lead heats to detectable the plasma 0.5in MeVthe eruptio gamma-rayn region, line while emission. 47 another goes to energize electrons and ions. Electromagnetic radiation from radio waves to gamma- Lingenfelter and Ramaty [12] performed detailed calculations of gamma-ray emission processes from 48 rays produced by the energized electrons and protons by various processes is known as a flare. the49 Sun. Acceleration Chupp et al.in the [13 reconnection] identified region for the is firstreferred time to as the flare positron acceleration annihilation or stochastic radiation acceleration. at 0.5 MeV along50 withCMEs other also nuclearcarry the linesreleased during energy the as intensethe kinetic solar energy flares of the of expelled 1972 August magnetized 4 and plasma 7 using with data a from the51 Gamma-ray mass as Monitorhigh as 10 onboard16 g and NASA’s speeds exceeding Seventh Orbiting3000 km/s. Solar Such Observatory fast CMEs (OSO-7)drive fast mission.-mode 52 magnetohydrodynamic shocks that can also energize ambient electrons and ions to very high 2.53 Mechanisms energies. ofDiffusive Positron shock Production acceleration and shock drift acceleration are the mechanisms by which 54 particles are energized at the shock (see [7] for a review on acceleration mechanisms during solar 55 Positronseruptions). are The predominantly flare and shock producedaccelerations by were three referred processes: to as first (i)- and emission second- fromphase radioactiveaccelerations nuclei, (ii)56 pair productionduring an eruption by nuclear by Wild deexcitation, et al. [8]. Accelerated and (iii) particles decay have of positively access to open charged and closed pions magnetic (π+) that take place57 whenstructures ions acceleratedassociated with in the eruption corona interactresulting within a number the ions of in electromagnetic the photosphere emissions/chromosphere. via Kozlovsky58 different et al. emission [14] list mechanisms. 156 positron-emitting Energetic particles radioactive escaping along nuclei interplanetary resulting magnetic from the field interaction lines of 59 are detected as solar energetic particle (SEP) events by particle detectors in space and on ground. protons and α-particles (helium nuclei) with 12 different elements and their isotopes. The most 60 These particles, originally known as solar cosmic rays, were first detected by Forbush [9] in the 1940s. important61 positron-emittingInvoking the copious radioactive production of nuclei energetic that particles result from from the the Sun, interaction Morrison [10] of protonssuggested (p) and 12 13 14 15 16 18 α-particles62 that with the active carbon Sun ( mustC, beC), a source nitrogen of gamma ( N, -rays.N), andHe listed oxygen electron ( O,-positronO) are annihilation listed in Table as one1. In the interaction63 of the of pprocesses and α withexpected16O, to the produce oxygen gamma nucleus,-ray emission ends up from in the co excitedsmic sources state [10] (16O*). Elliot of 6.052[11] MeV; 64 suggested that “positive electrons” from muon decay should lead to detectable 0.5 MeV gamma-ray 65 line emission. Lingenfelter and Ramaty [12] performed detailed calculations of gamma-ray emission Atoms 2020, 8, 14 3 of 12 this nucleus deexcites by emitting an electron-positron pair with a lifetime of 0.096 ns. Another such excited nucleus is 40Ca* with a lifetime of 3.1 ns. Kozlovsky et al. [15] list another set of 23 positron emitters produced when accelerated 3He interact with targets such as 12C, 14N, 16O, 20Ne, 24Mg, 28Si, and 56Fe.