Sustained Gamma-ray Emission Research Articles

Solar flares with sustained gamma-ray emission and some characteristics of high-energy proton fluxes

We describe the characteristics of long-term gamma fluxes with quantum energies >100 MeV, obtained from Fermi/LAT data during the impulsive phase of the most energetic flare phenomena. We compare GOES data on proton fluxes with energies >500 MeV with Fermi/LAT data on gamma fluxes for the period 2010–2018. The results of the analysis of all data obtained on 32 gamma-ray flares from the Fermi/LAT catalog show that the flare phenomena can be classified into three different types: type 1 — gamma fluxes accompanied by energetic proton fluxes; type 2 — gamma rays recorded in the absence of increases in proton fluxes; type 3 — gamma fluxes not recorded during observed increases in energetic proton fluxes. The burst character of energy release in the hard X-ray range was noted in some flares.

Speed and Acceleration of Coronal Mass Ejections Associated with Sustained Gamma-Ray Emission Events Observed by Fermi/LAT

The sustained gamma-ray emission (SGRE) from the Sun is a prolonged enhancement of >100 MeV gamma-ray emission that extends beyond the flare impulsive phase. The origin of the >300 MeV protons resulting in SGRE is debated, with both flares and shocks driven by coronal mass ejections (CMEs) being the suggested sites of proton acceleration. We compared the near-Sun acceleration and space speed of CMEs with “Prompt” and “Delayed” (SGRE) gamma-ray components. We found that “Delayed”-component-associated CMEs have higher initial accelerations and space speeds than “Prompt Only”-component-associated CMEs. We selected halo CMEs (HCMEs) associated with type II radio bursts (shock-driving HCMEs) and compared the average acceleration and space speed between HCME populations with or without SGRE events, major solar energetic particle (SEP) events, metric, or decameter-hectometric (DH) type II radio bursts. We found that the SGRE-producing HCMEs associated with a DH type II radio burst and/or a major SEP event have higher space speeds and especially initial accelerations than those without an SGRE event. We estimated the radial distances and speeds of the CME-driven shocks at the end time of the 2012 January 23 and March 7 SGRE events using white-light images of STEREO Heliospheric Imagers and radio dynamic spectra of Wind WAVES. The shocks were at the radial distances of 0.6–0.8 au and their speeds were high enough (≈975 km s−1 and ≈750 km s−1, respectively) for high-energy particle acceleration. Therefore, we conclude that our findings support the CME-driven shock as the source of >300 MeV protons.

The Common Origin of High-energy Protons in Solar Energetic Particle Events and Sustained Gamma-Ray Emission from the Sun

Abstract We report that the number of >500 MeV protons (Ng ) inferred from sustained gamma-ray emission (SGRE) from the Sun is significantly correlated with that of protons propagating into space (N SEP) as solar energetic particles (SEPs). Under the shock paradigm for SGRE, shocks driven by coronal mass ejections (CMEs) accelerate high-energy protons sending them toward the Sun to produce SGRE by interacting with the atmospheric particles. Particles also escape into the space away from the Sun to be detected as SEP events. Therefore, the significant N SEP–N g correlation (correlation coefficient 0.77) is consistent with the common shock origin for the two proton populations. Furthermore, the underlying CMEs have properties akin to those involved in ground level enhancement events indicating the presence of high-energy (up to ∼GeV) particles required for SGRE. We show that the observed gamma-ray flux is an underestimate in limb events (central meridian distance >60°) because SGRE sources are partially occulted when the emission is spatially extended. With the assumption that the SEP spectrum at the shock nose is hard and that the 100 MeV particles are accelerated throughout the shock surface (half width in the range 60°–120°) we find that the latitudinal widths of SEP distributions are energy dependent with the smallest width at the highest energies. Not using the energy-dependent width results in an underestimate of N SEP in SGRE events occurring at relatively higher latitudes. Taking these two effects into account removes the apparent lack of N SEP–N g correlation reported in previous studies.

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

We report on the source of {>},300~mbox{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 ground-based instruments. Based on the eruption geometry we provide concrete explanation for the spatially and temporally extended gamma -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 geq 3~mbox{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 with a speed {>},2000~mbox{km},mbox{s}^{-1}, 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 gamma -rays via neutral pion decay.

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

It has recently been shown that the spatially and temporally extended γ-ray emission in solar eruptions are caused by >300 MeV protons precipitating on the Sun from shocks driven by coronal mass ejections (CMEs). The γ-rays result from the decay of neutral pions produced in the proton-proton interaction when the >300 MeV protons collide with those in the chromosphere. The evidence comes from the close correlation between the durations of the sustained gamma-ray emission (SGRE) and the associated interplanetary (IP) type II radio bursts. In this paper, we provide further evidence that support the idea that protons accelerated in IP shocks driven by CMEs propagate toward the Sun, precipitate in the chromosphere to produce the observed SGRE. We present the statistical properties of the SGRE events and the associated CMEs, flares, and type II radio bursts. It is found that the SGRE CMEs are similar to those associated with ground level enhancement events. The CME speed is well correlated with the SGRE fluence. High CME speed is an important requirement for the occurrence of SGRE, while the flare size is not. Based on these results, we present a schematic model illustrating the spatially and temporally extended nature of SGRE related to the CME flux rope-shock structure.

Magnetic Flux Reconnection in Flaring Active Regions with Sustained Gamma-Ray Emission

Abstract Characteristics of sustained >100 MeV γ-ray emission (SGRE) events observed by the Large Area Telescope on Fermi were recently reported by Share et al. Their spectra are consistent with the decay of pions produced by >300 MeV protons and appear spectrally and spatially distinct from preceding associated flares. The source(s) of the sustained production of the >300 MeV protons is uncertain, but acceleration in coronal/interplanetary shock waves driven by coronal mass ejections, followed by a return of the protons back to the Sun, is favored. This scenario requires proton transport through converging magnetic fields behind the shock, which might result in considerable reflection of the protons back into space, and 1 au observations of the associated solar energetic proton (SEP) events do not always include a population of E > 300 MeV protons. Alternative source candidates that involve trapping or continued acceleration of SEPs in coronal loops have been considered. The energy release rates from magnetic reconnection in flaring active regions (ARs) have been measured with a new technique, and in this work we compare those measured flux reconnection rates with emission profiles in 11 SGRE events. In general, the magnetic reconnection event is nearly or completely finished before the bulk of the γ-ray emission, which argues against scenarios of continued proton acceleration in the flaring ARs.

Interplanetary Type II Radio Bursts from Wind/WAVES and Sustained Gamma-Ray Emission from Fermi/LAT: Evidence for Shock Source

Abstract We present quantitative evidence that interplanetary type II radio bursts and sustained gamma-ray emission (SGRE) events from the Sun are closely related. Out of about 30 SGRE events reported in Share et al. we consider 13 events that had a duration exceeding ∼5 hr to exclude any flare-impulsive phase gamma-rays. The SGRE duration also has a linear relation with the ending frequency of the bursts. The synchronism between the ending times of SGRE and the type II emission strongly supports the idea that the same shock accelerates electrons to produce type II bursts and protons (>300 MeV) that propagate from the shock to the solar surface to produce SGRE via pion decay. The acceleration of high-energy particles is confirmed by the associated solar energetic particle (SEP) events detected at Earth and/or at the Solar Terrestrial Relations Observatory spacecraft. Furthermore, the presence of >300 MeV protons is corroborated by the fact that the underlying coronal mass ejections (CMEs) had properties identical to those associated with ground-level enhancement events: they had speeds of >2000 km s−1 and all were full-halo CMEs. Many SEP events did not have detectable flux at Earth in the >300 MeV energy channels, presumably because of poor magnetic connectivity.