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Register a new userCharacteristics of Sustained >100 MeV Gamma-ray Emission Associated with Solar Flares
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We characterize and provide a catalog of thirty >100 MeV sustained gamma-ray emission (SGRE) events observed by Fermi LAT. These events are temporally and spectrally distinct from the associated solar flares. Their spectra are consistent with decay of pions produced by >300 MeV protons and are not consistent with electron bremsstrahlung. SGRE start times range from CME onset to two hours later. Their durations range from about four minutes to twenty hours and appear to be correlated with durations of >100 MeV SEP proton events. The >300 MeV protons producing SGRE have spectra that can be fit with power laws with a mean index of ~4 and RMS spread of 1.8. Gamma-ray line measurements indicate that SGRE proton spectra are steeper above 300 MeV than they are below 300 MeV. The number of SGRE protons >500 MeV is on average about ten times more than then the number in the associated flare and about fifty to one hundred times less than the number in the accompanying SEP. SGRE can extend tens of degrees from the are site. Sustained bremsstrahlung from MeV electrons was observed in one SGRE event. Flare >100 keV X-ray emission appears to be associated with SGRE and with intense SEPs. From this observation, we provide arguments that lead us to propose that sub-MeV to MeV protons escaping from the flare contribute to the seed population that is accelerated by shocks onto open field lines to produce SEPs and onto field lines returning to the Sun to produce SGRE.
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We report the first detection of >100 MeV gamma rays associated with a behind-the-limb solar flare, which presents a unique opportunity to probe the underlying physics of high-energy flare emission and particle acceleration. On 2013 October 11 a GOES M1.5 class solar flare occurred ~ 9.9 degrees behind the solar limb as observed by STEREO-B. RHESSI observed hard X-ray emission above the limb, most likely from the flare loop-top, as the footpoints were occulted. Surprisingly, the Fermi Large Area Telescope (LAT) detected >100 MeV gamma-rays for ~30 minutes with energies up to GeV. The LAT emission centroid is consistent with the RHESSI hard X-ray source, but its uncertainty does not constrain the source to be located there. The gamma-ray spectra can be adequately described by bremsstrahlung radiation from relativistic electrons having a relatively hard power-law spectrum with a high-energy exponential cutoff, or by the decay of pions produced by accelerated protons and ions with an isotropic pitch-angle distribution and a power-law spectrum with a number index of ~3.8. We show that high optical depths rule out the gamma rays originating from the flare site and a high-corona trap model requires very unusual conditions, so a scenario in which some of the particles accelerated by the CME shock travel to the visible side of the Sun to produce the observed gamma rays may be at work.
Gamma-ray emission during Long-Duration Gamma-Ray Flare (LDGRF) events is thought to be caused mainly by $>$300 MeV protons interacting with the ambient plasma at or near the photosphere. Prolonged periods of the gamma-ray emission have prompted the suggestion that the source of the energetic protons is acceleration at a Coronal Mass Ejection (CME)-driven shock, followed by particle back-precipitation onto the solar atmosphere over extended times. We study the latter phenomenon using test particle simulations, which allow us to investigate whether scattering associated with turbulence aids particles in overcoming the effect of magnetic mirroring, which reflects particles as they travel sunwards. The instantaneous precipitation fraction, $P$, the proportion of protons that successfully precipitate for injection at a fixed height, $r_i$, is studied as a function of scattering mean free path, $lambda$ and $r_i$. We find that the presence of scattering helps back-precipitation compared to the scatter-free case, although at very low $lambda$ values outward convection with the solar wind ultimately dominates. Upper limits to the total precipitation fraction, $overline{P}$, are calculated for 8 LDGRF events for moderate scattering conditions ($lambda$=0.1 AU). Due to strong mirroring, $overline{P}$ is very small for these events, between 0.56 and 0.93% even in the presence of scattering. Time-extended acceleration and large total precipitation fractions, as seen in the observations, cannot be reconciled for a moving shock source according to our simulations. These results challenge the CME-shock source scenario as the main mechanism for gamma-ray production in LDGRFs.
The Gamma-Ray Imager/Polarimeter for Solar flares (GRIPS) is a balloon-borne telescope designed to study solar-flare particle acceleration and transport. We describe GRIPSs first Antarctic long-duration flight in Jan 2016 and report preliminary calibration and science results. Electron and ion dynamics, particle abundances and the ambient plasma conditions in solar flares can be understood by examining hard X-ray (HXR) and gamma-ray emission (20 keV to 10 MeV) with enhanced imaging, spectroscopy and polarimetry. GRIPS is specifically designed to answer questions including: What causes the spatial separation between energetic electrons producing HXRs and energetic ions producing gamma-ray lines? How anisotropic are the relativistic electrons, and why can they dominate in the corona? How do the compositions of accelerated and ambient material vary with space and time, and why? GRIPSs key technological improvements over the Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI) include 3D position-sensitive germanium detectors (3D-GeDs) and a single-grid, multi-pitch rotating modulator (MPRM) collimator. The 3D-GeDs have spectral FWHM resolution of a few hundred keV and spatial resolution $<$1 mm$^3$. For photons that Compton scatter, usually $gtrsim$150 keV, the energy deposition sites can be tracked, providing polarization measurements as well as enhanced background reduction. The MPRM single-grid design provides twice the throughput of a bi-grid imaging system like RHESSI. The grid is composed of 2.5 cm thick W/Cu slats with 1-13 mm variable slit pitch, achieving quasi-continuous FWHM angular coverage over 12.5-162 arcsecs. This resolution is capable of imaging the separate magnetic loop footpoint emissions in a variety of flare sizes. (Abstract edited down from source.)
The {gamma}-ray sky can be decomposed into individually detected sources, diffuse emission attributed to the interactions of Galactic cosmic rays with gas and radiation fields, and a residual all-sky emission component commonly called the isotropic diffuse {gamma}-ray background (IGRB). The IGRB comprises all extragalactic emissions too faint or too diffuse to be resolved in a given survey, as well as any residual Galactic foregrounds that are approximately isotropic. The first IGRB measurement with the Large Area Telescope (LAT) on board the Fermi Gamma-ray Space Telescope (Fermi) used 10 months of sky-survey data and considered an energy range between 200 MeV and 100 GeV. Improvements in event selection and characterization of cosmic-ray backgrounds, better understanding of the diffuse Galactic emission, and a longer data accumulation of 50 months, allow for a refinement and extension of the IGRB measurement with the LAT, now covering the energy range from 100 MeV to 820 GeV. The IGRB spectrum shows a significant high-energy cutoff feature, and can be well described over nearly four decades in energy by a power law with exponential cutoff having a spectral index of $2.32pm0.02$ and a break energy of $(279pm52)$ GeV using our baseline diffuse Galactic emission model. The total intensity attributed to the IGRB is $(7.2pm0.6) times 10^{-6}$ cm$^{-2}$ s$^{-1}$ sr$^{-1}$ above 100 MeV, with an additional $+15$%/$-30$% systematic uncertainty due to the Galactic diffuse foregrounds.
Solar flares convert magnetic energy into thermal and non-thermal plasma energy, the latter implying particle acceleration of charged particles such as protons. Protons are injected out of the coronal acceleration region and can interact with dense plasma in the lower solar atmosphere, producing mesons that subsequently decay into gamma rays and neutrinos at O(MeV-GeV) energies. We present the results of the first search for GeV neutrinos emitted during solar flares carried out with the IceCube Neutrino Observatory. While the experiment was originally designed to detect neutrinos with energies between 10 GeV and a few PeV, a new approach allowing for a O(GeV) energy threshold will be presented. The resulting limits allow us to constrain some of the theoretical estimates of the expected neutrino flux.
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