X-Ray, Radio and SEP Observations of Relativistic Gamma-Ray Events

Abstract

The rather frequent occurrence, and sometimes long duration, ofray events at photon energies above 100 MeV challenges our understanding of particle acceleration processes at the Sun. The emission is ascribed to pion-decay photons due to protons with energies above 300 MeV. We study the X-ray and radio emissions and the solar energetic particles (SEPs) in space for a set of 25 Fermi -ray events. They are accompanied by strong SEP events, including, in most cases where the parent activity is well-connected, protons above 300 MeV. Signatures of energetic electron acceleration in the corona accompany the impulsive and early post-impulsive -ray emission. -ray emission lasting several hours accompanies in general the decay phase of long-lasting soft X-ray bursts and decametric-tokilometric type II bursts. We discuss the impact of these results on the origin of the -ray events. IntroductionThe advent of the Fermi mission showed that the Sun is an occasional, but unexpectedly frequent, emitter of -ray photons above 100 MeV. These are understood to be produced by pion decays in nuclear interactions involving protons or He-nuclei at energies above 300 MeV/nucleon. One did not expect that the Sun was able to accelerate relativistic protons and nuclei even in seemingly modest flares. These particles are rarely detected in space (<1 event per year). Furthermore, the duration, several hours, of some -ray events is much longer than that of hard X-ray signatures of electron acceleration in the impulsive flare phase.The question of how the -ray emission is related to other signatures of particle acceleration and energy release in the corona is crucial to understanding the origin of the high-energy protons. It might also be expected that such high-energy populations interacting at the Sun are accompanied by particularly energetic solar energetic particle (SEP) events. This chapter is based on 25 events. The Fermi/LAT temporal data were made available to the HESPERIA project by G. Share prior to their publication in a comprehensive paper ). The present chapter introduces the relevant process of emission and pre-Fermi observations of piondecay -rays (Sect. 8.2), and gives an overview of the Fermi/LAT observations (Sect. 8.3). Section 8.3 was prepared by Gerald Share and Ron Murphy. Related X-ray and radio observations and associated SEP events are presented in Sects. 8.4 and 8.5, respectively. Preliminary conclusions on the interpretation of the -ray events are in Sect. 8.6. Theory and Early Observations of Gamma-Ray Emission at Photon Energies >60 MeVOn 1982 Jun 3 the gamma-ray spectrometer on the Solar Maximum Mission satellite observed emission from 0.3 to 100 MeV from a X8.0 GOES-class flare (Forrest et al. 1986). The impulsive flare lasted about 1 min and was followed by a distinct harder emission phase that peaked in about 1 min and lasted for over 15 min. The energy spectrum of this sustained emission displayed a characteristic hump at photon energies above 60 MeV (Fig. 8.1a), which appeared to be consistent with th...

The Acceleration of Energetic Particles at Coronal Shocks and Emergence of a Double Power-law Feature in Particle Energy Spectra

“…The shock-streamer interaction configuration in this work may also have important implications for sustained γ-ray emission (SGRE) events, in which γ-ray emission above 100 MeV can last for hours after the flare impulsive phase (see, e.g., Kahler et al 2018;Klein et al 2018;Share et al 2018, and references therein). The >100 MeV γ-ray emission is thought to be produced by the decay of pions from interactions of >300 MeV protons with background ions in the solar atmosphere.…”

INTEGRAL serendipitous observations of solar and terrestrial X-rays and gamma rays

“…Within the European Union's FP7 SEPServer project (Vainio et al, 2013), ACS light-curves have been extracted and studied for a set of 24 solar flares which occurred between 2002(Rodríguez-Gasén et al, 2014. In the frame of the EU's H2020 HESPERIA project (Malandraki and Crosby, 2018), 25 more solar flare light curves measured with SPI/ACS between 2011 and 2014 were provided to the solar physicist community (Klein et al, 2018). These two European projects were dedicated to the study of Solar Energetic Particle (SEP) events with the aim of providing a public database of solar events and forecasting tools for space weather, using a large set of SEP data and associated electromagnetic emissions at the Sun from radio to gamma-ray frequencies.…”

“…During the impulsive and early post-impulsive phase of solar flares, high-energy protons and α-particles >300 MeV/nucleon can be accelerated concomitantly with electrons and generate a characteristic hump at photon energies above 60 MeV by pion production and decay. But the late-phase of >100 MeV gamma-ray emission observed by the Fermi Large Area Telescope (LAT) lasts much longer than the impulsive hard X-ray emission phase traced by SPI/ACS (Share et al, 2018;Klein et al, 2018), and the origin of these long-duration gamma-ray events is still debated. Associated to data from the Helioseismic and Magnetic Imager (HMI) on board the Solar Dynamics Observatory, the ACS data are also used to study the generation of helioseismic waves (also referred to as "sunquakes") during strong solar flares (Sharykin and Kosovichev, 2018).…”

Fermi, Wind, and SOHO Observations of Sustained Gamma-Ray Emission from the Sun

“…While only a couple of such SGRE events were reported in the past [1][2], observations from the Large Area Telescope (LAT) on board the Fermi satellite [3] have shown that these events are rather common. Since its launch, Fermi has detected about 30 SGRE events until the end of 2017 [4][5][6][7][8]. Two possible sources of these protons have been discussed in the literature: protons accelerated/trapped in large-scale flare structures or by MHD shocks as reviewed in [9][10].…”

“…Furthermore, even eruptions occurring on the backside of the Sun produce gamma-ray emission on the disk, indicating a particle source that extends over tens of degrees unlike flares [4,[11][12][13], analogous to the gammaray line emission from backside eruptions [14]. The shock possibility has been bolstered by the association between SGRE events and type II radio bursts in the decameterhectometric (DH) wavelengths [5][6]. It is well known that the DH type II bursts are caused by interplanetary (IP) shocks driven by energetic coronal mass ejections (CMEs) characterized by high speeds and wide angular extent [15].…”