Energetic Proton Back-Precipitation onto the Solar Atmosphere in Relation to Long-Duration Gamma-Ray Flares

Abstract

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 hypothesis using test particle simulations, which allow us to investigate whether scattering associated with turbulence aids particles in overcoming the effect of magnetic mirroring, which impedes back-precipitation by reflecting 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 $r_i$. Upper limits to the total precipitation fraction, $\overline{P}$, were calculated for eight LDGRF events for moderate scattering conditions ($\lambda$=0.1 au). 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. For eight LDGRF events, due to strong mirroring, $\overline{P}$ is very small, 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. Therefore, it is not possible to obtain both long duration $\gamma$ ray emission and efficient precipitation within this scenario. These results challenge the CME shock source scenario as the main mechanism for $\gamma$ ray production in LDGRFs.

arXiv commentAccepted for publication in A&A

References in corpus (17)

• Impulsive and Long Duration High-Energy Gamma-ray Emission From the Very Bright 2012 March 7 Solar Flares
• The First Ground Level Enhancement Event of Solar Cycle 24: Direct Observation of Shock Formation and Particle Release Heights
• Height of Shock Formation in the Solar Corona Inferred from Observations of Type II Radio Bursts and Coronal Mass Ejections
• Characteristics of Sustained >100 MeV Gamma-ray Emission Associated with Solar Flares
• Solar Energetic Particle drifts in the Parker spiral
• Fermi-LAT Observations of High-energy Behind-the-limb Solar Flares
• First detection of >100 MeV gamma rays associated with a behind-the-limb solar flare
• Fermi-LAT observations of the 2017 September 10${}^{th}$ solar flare
• Drift-induced perpendicular transport of Solar Energetic Particles
• Probing the Puzzle of Behind-the-Limb $\gamma$-ray Flares: Data-driven Simulations of Magnetic Connectivity and CME-driven Shock Evolution
• Interplanetary magnetic structures guiding solar relativistic particles
• Comparing Long-Duration Gamma-Ray Flares and High-Energy Solar Energetic Particles
• Evidence for a Coronal Shock Wave Origin for Relativistic Protons Producing Solar Gamma-Rays and Observed by Neutron Monitors at Earth
• Radio, Hard X-Ray, and Gamma-Ray Emissions Associated with a Far-Side Solar Event
• 3D propagation of relativistic solar protons through interplanetary space
• The Acceleration of Energetic Particles at Coronal Shocks and Emergence of a Double Power Law Feature in Particle Energy Spectra
• A Statistical Study to Determine the Origin of Long-Duration Gamma-ray Flares

Cited by in corpus (1)

• Modelling shock-like injections of solar energetic particles with 3D test particle simulations