On the Shock Source of Sustained Gamma-Ray Emission from the Sun

Gopalswamy, Ν.; Mäkelä, P.; Yashiro, S.; Lara, A.; Akiyama, S.; Xie, H.

doi:10.1088/1742-6596/1332/1/012004

Cited by 17 publications

(42 citation statements)

References 38 publications

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“…Evidence for the shock shown in Figure 5 is the interplanetary type II radio burst that lasted until the end of the SGRE event and a bit beyond. The estimated duration of the type II burst (about 7.5 h) is in agreement with the linear relation found between the two durations [33].…”

Section: Pion Continuumsupporting

confidence: 89%

“…The Large Area Telescope (LAT) on the Fermi satellite has detected dozens of such time-extended gamma-ray events from the Sun at energies >100 MeV, thanks to the detector's high sensitivity [31]. The average duration of these gamma-ray events is about 9.7 h and with six events lasting for more than 12 h [32,33], including an event that lasted for almost a day. The time-extended events are known by different names "long duration gamma-ray flare (LDGRF)" [34,35], "sustained gamma-ray emission (SGRE)" [33,36], and "late-phase gamma-ray emission (LPGRE)" [32].…”

Section: Pion Continuummentioning

confidence: 99%

“…The average duration of these gamma-ray events is about 9.7 h and with six events lasting for more than 12 h [32,33], including an event that lasted for almost a day. The time-extended events are known by different names "long duration gamma-ray flare (LDGRF)" [34,35], "sustained gamma-ray emission (SGRE)" [33,36], and "late-phase gamma-ray emission (LPGRE)" [32]. The Fermi/LAT observations have revived the interest in the origin of the high-energy particles in these events because the accelerator needs to inject >300 MeV ions toward the chromosphere/photosphere to produce the pions required for the gamma-ray events.…”

Section: Pion Continuummentioning

confidence: 99%

“…In this scenario, the largest spatial extent of the gamma-ray source is the size of the post-eruption arcade (or flare area) discerned from coronal images taken in extreme ultraviolet wavelengths. In the shock scenario, the gamma-ray source is spatially extended because the angular extent of the shock is much larger than that of the flare structure [38,39]; shock acceleration is naturally time-extended evidenced by type II radio bursts and SEP events [33] Gopalswamy et al [40] demonstrated the spatially extended nature of the gamma-ray source during the 2014 September 1 event (see also [36,39,41]). They used multiview data from the Solar and Heliospheric Observatory (SOHO) and the Solar Terrestrial Relations Observatory (STEREO) missions to obtain a detailed picture of the eruption, including magnetic structures that extend beyond the flare structure (post-eruption arcade, PEA) as described in Figure 5.…”

Section: Pion Continuummentioning

confidence: 99%

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Positron Processes in the Sun

Gopalswamy

2020

Atoms

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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.

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Section: Pion Continuumsupporting

confidence: 89%

Section: Pion Continuummentioning

confidence: 99%

Section: Pion Continuummentioning

confidence: 99%

Section: Pion Continuummentioning

confidence: 99%

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Positron Processes in the Sun

Gopalswamy

2020

Atoms

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“…14 and Webb et al 2000;Gopalswamy 2009). The large extent of the flux rope is readily inferred from the fact that all SGRE events with duration >3 hr are associated with halo CMEs (Gopalswamy et al 2018a;2019) and halo CMEs are inferred to be fast and wide from quadrature observations (Gopalswamy et al 2013c).…”

Section: The Magnetic Structure Associated With Sgrementioning

confidence: 99%

Source of Energetic Protons in the 2014 September 1 Sustained Gamma-ray Emission Event

et al. 2020

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We report on the source of >300 MeV protons during the SOL2014-09-01 sustained gamma-ray emission (SGRE) event based on multi-wavelength data from a wide array of space-and groundbased instruments. Based on the eruption geometry we provide concrete explanation for the spatially and temporally extended γ-ray emission from the eruption. We show that the associated flux rope is of low inclination (roughly oriented in the east-west direction), which enables the associated shock to extend to the frontside. We compare the centroid of the SGRE source with the location of the flux rope's leg to infer that the high-energy protons must be precipitating between the flux rope leg and the shock front. The durations of the SOL2014-09-01 SGRE event and the type II radio burst agree with the linear relationship between these parameters obtained for other SGRE events with duration ≥3 hrs. The fluence spectrum of the SEP event is very hard, indicating the presence of high-energy (GeV) particles in this event. This is further confirmed by the presence of an energetic coronal mass ejection (CME) with a speed >2000 km/s, similar to those in ground level enhancement (GLE) events. The type II radio burst had emission components from metric to kilometric wavelengths as in events associated with GLE events. All these factors indicate that the high-energy particles from the shock were in sufficient numbers needed for the production of γ-rays via neutral pion decay.

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Gradual SEP Events

Reames

2021

Solar Energetic Particles

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Gradual solar energetic-particle (SEP) events are “big proton events” and are usually much more “gradual” in their decay than in their onset. As their intensities increase, particles streaming away from the shock amplify Alfvén waves that scatter subsequent particles, increasing their acceleration, eventually limiting ion flow at the “streaming limit.” Waves generated by higher-speed protons running ahead can also throttle the flow of lower-energy ions, flattening spectra and altering abundances in the biggest SEP events. Thus, we find that the A/Q-dependence of scattering causes element-abundance patterns varying in space and time, which define source-plasma temperatures T, since the pattern of Q values of the ions depends upon temperature. Differences in T explain much of the variation of element abundances in gradual SEP events. In nearly 70% of gradual events, SEPs are shock-accelerated from ambient coronal plasma of ~0.8–1.6 MK, while 24% of the events involve material with T ≈ 2–4 MK re-accelerated from residual impulsive-suprathermal ions with pre-enhanced abundances. This source-plasma temperature can occasionally vary with solar longitude across the face of a shock. Non-thermal variations in ion abundances in gradual SEP events reaccelerated from the 2–4 MK impulsive source plasma are reduced, relative to those in the original impulsive SEPs, probably because the accelerating shock waves sample a pool of ions from multiple jet sources. Late in gradual events, SEPs become magnetically trapped in a reservoir behind the CME where spectra are uniform in space and decrease adiabatically in time as the magnetic bottle containing them slowly expands. Finally, we find variations of the He/O abundance ratio in the source plasma of different events.

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On the Shock Source of Sustained Gamma-Ray Emission from the Sun

Cited by 17 publications

References 38 publications

Positron Processes in the Sun

Positron Processes in the Sun

Source of Energetic Protons in the 2014 September 1 Sustained Gamma-ray Emission Event

Gradual SEP Events

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