Characteristics of Sustained &gt;100 MeV Gamma-ray Emission Associated with Solar Flares

Share, G. H.; Murphy, R. J.; Tolbert, A. K.; Dennis, B. R.; White, S. M.; Schwartz, R. A.; Tylka, A. J.

doi:10.48550/arxiv.1711.01511

Cited by 3 publications

(3 citation statements)

References 51 publications

(95 reference statements)

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“…The Large Area Telescope (LAT) on board the Fermi γ-ray Space Telescope (Fermi : Atwood et al 2009) has increased the number of observed solar flares with photon emission above 100 MeV by an order of magnitude compared to all previous instruments (Share et al 2017). One prominent characteristic of these flares is the long-duration emission extending hours past the impulsive phase, long after other flare associated electromagnetic emissions at longer wavelengths have decayed (e.g., Ajello et al 2014).…”

Section: Introductionmentioning

confidence: 99%

Probing the Puzzle of Behind-the-limb γ-Ray Flares: Data-driven Simulations of Magnetic Connectivity and CME-driven Shock Evolution

Jin

Petrosian

Liu

et al. 2018

ApJ

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Recent detections of high-energy γ-rays from behind-the-limb (BTL) solar flares by the Fermi γ-ray Space Telescope pose a puzzle and challenge on the particle acceleration and transport mechanisms. In such events, the γ-ray emission region is located away from the BTL flare site by up to tens of degrees in heliogrpahic longitude. It is thus hypothesized that particles are accelerated at the shock driven by the coronal mass ejection (CME) and then travel from the shock downstream back to the front side of the Sun to produce the observed γ-rays. To test this scenario, we performed data-driven, global magnetohydrodynamics simulations of the CME associated with a well-observed BTL flare on 2014 September 1. We found that part of the CME-driven shock develops magnetic connectivity with the γ-ray emission region, facilitating transport of particles back to the Sun. Moreover, the observed increase in γ-ray flux is temporally correlated with (1) the increase of the shock compression ratio and (2) the presence of a quasi-perpendicular shock over the area that is magnetically connected to -2the γ-ray emitting region, both conditions favoring the diffusive shock acceleration (DSA) of particles. These results support the above hypothesis and can help resolve another puzzle, i.e., long-duration (up to 20 hours) γ-rays flares. We suggest that, in addition to DSA, stochastic acceleration by plasma turbulence may also play a role, especially in the shock downstream region and during the early stage when the shock Alfvén Mach number is small.

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Section: Introductionmentioning

confidence: 99%

Probing the Puzzle of Behind-the-limb γ-Ray Flares: Data-driven Simulations of Magnetic Connectivity and CME-driven Shock Evolution

Jin

Petrosian

Liu

et al. 2018

ApJ

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“…The Sun is a bright source of multi-GeV γ-rays, with emission observed both from its halo -due to cosmic-rays electrons interacting with solar photons -and its disk -due to hadronic cosmic rays (mostly protons) interacting with solar gas. (Emission from solar particle acceleration is only bright during flares and has not been observed above 4 GeV [1][2][3][4][5][6][7][8].) Although the halo emission [9] agrees with theory [10][11][12], the disk emission does not, and hence is our focus.…”

mentioning

confidence: 99%

Evidence for a New Component of High-Energy Solar Gamma-Ray Production

et al. 2018

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The observed multi-GeV gamma-ray emission from the solar disk -sourced by hadronic cosmic rays interacting with gas, and affected by complex magnetic fields -is not understood. Utilizing an improved analysis of the Fermi-LAT data that includes the first resolved imaging of the disk, we find strong evidence that this emission is produced by two separate mechanisms. Between 2010-2017 (the rise to and fall from solar maximum), the gamma-ray emission is dominated by a polar component. Between 2008-2009 (solar minimum) this component remains present, but the total emission is instead dominated by a new equatorial component with a brighter flux and harder spectrum. Most strikingly, although 6 gamma rays above 100 GeV are observed during the 1.4 years of solar minimum, none are observed during the next 7.8 years. These features, along with a 30-50 GeV spectral dip which will be discussed in a companion paper, were not anticipated by theory. To understand the underlying physics, Fermi and HAWC observations of the imminent Cycle 25 solar minimum are crucial.

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“…In addition to the preliminary analysis in Ack17 mentioned at the outset, there have been similar determination of electron spectra based on HXRs (Share et al 2017;Plotnikov et al 2017) assuming both thin and thick-target, based only on electron energy spectra, and without consideration of the energy dependence of the escape time. As expected the electron indexes derived in these papers are different than those presented here, which not only are for a thin target model but also include the energy dependence of the escape time.…”

Section: Combined Electron Spectramentioning

confidence: 98%

Implications of a Loop-top Origin for Microwave, Hard X-Ray, and Low-energy Gamma-Ray Emission from Behind-the-limb Flares

Petrosian

2018

ApJ

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Fermi has detected hard X-ray (HXR) and gamma-ray photons from three flares, which according to STEREO occurred in active regions behind the limb of the Sun as delineated by near Earth instruments. For two of these flares RHESSI has provided HXR images with sources located just above the limb, presumably from the loop top (LT) region of a relatively large loop. Fermi-GBM has detected HXRs and gamma-rays, and RSTN has detected microwaves emissions with similar light curves. This paper presents a quantitative analysis of these multiwavelength observations assuming that HXRs and microwaves are produced by electrons accelerated at the LT source, with emphasize on the importance of the proper treatment of escape of the particles from the acceleration-source region and the transrelativistic nature of the analysis. The observed spectra are used to determine the magnetic field and relativistic electron spectra. It is found that a simple power-law in momentum (with cut off above a few 100 MeV) agrees with all observations, but in energy space a broken power law spectrum (steepening at ∼ mc 2 ) may be required. It is also shown that the production of the > 100 MeV photons detected by Fermi-LAT at the LT source would require more energy compared to photospheric emission. These energies are smaller than that required for electrons, so that the possibility that all the emissions originate in the LT cannot be ruled out on energetic grounds. However, the differences in the light curves and emission centroids of HXRs and > 100 MeV gamma-rays favor a different source for the latter.

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Characteristics of Sustained >100 MeV Gamma-ray Emission Associated with Solar Flares

Cited by 3 publications

References 51 publications

Probing the Puzzle of Behind-the-limb γ-Ray Flares: Data-driven Simulations of Magnetic Connectivity and CME-driven Shock Evolution

Probing the Puzzle of Behind-the-limb γ-Ray Flares: Data-driven Simulations of Magnetic Connectivity and CME-driven Shock Evolution

Evidence for a New Component of High-Energy Solar Gamma-Ray Production

Implications of a Loop-top Origin for Microwave, Hard X-Ray, and Low-energy Gamma-Ray Emission from Behind-the-limb Flares

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