Fire Under Fire: Proton Probabilities at Perihelion

This paper computes the probabilities that the near-Sun flyby mission, Fire, might be bombarded by high energy protons from solar-activity related proton acceleration lying beneath Fire’s perihelion altitude of three solar radii. Although the very largest interplanetary proton events are commonly believed to be caused by interplanetary shocks, this paper assumes the working hypothesis that there is a second proton acceleration process associated with progressively spectral hardening hard X-ray flares studied by Kiplinger (1 995), Those studies found that progressively hardening spectral behavior was highly associated with interplanetary protons seen near Earth. Progressive hardening is easily distinguished from the spectral evolutionary behavior of common “impulsive flares” that generally have no associated interplanetary protons. This paper develops an equation to assess the probability that such an event might occur while the spacecraft orbits directly above the source and it analyzes two moderate proton events in order to estimate the proton dosages that may be received. It is concluded that, potentially, some events could pose a serious risk to Fire, but that the probability of a direct hit is only one in every few thousand missions.