Solar flare neutrons observed on the ground and in space
From RHESSI Wiki
Trickier to detect than gamma-rays, energetic neutrons nonetheless offer a valuable window on solar ion acceleration. Free neutrons are produced when accelerated ions in the MeV energy range and above collide with ambient nuclei. Some slow down and thermalise in the solar atmosphere, contributing to the observed flux in the 2.223 MeV deuterium formation line if they capture on protons. Others escape completely from the solar atmosphere, potentially to be detected in space or even on Earth.
The possibility of detecting energetic neutrons from flares was first aired at the very start of the 1950s by Ludwig Biermann, but the first detection was only achieved some decades later, using the Gamma-Ray Spectrometer instrument on the Solar Maximum Mission.
Free neutrons are unstable, beta decaying with a mean lifetime of 880 s. A 150 MeV neutron travels 1 AU in a lifetime so the probability of survival to Earth drops dramatically below this energy.
