In addition to the bremsstrahlung from energetic electrons, spectral lines are observed at photon energies between around 400 keV and 8 MeV. These spectral lines originate from the interaction of energetic ions with ions in the thermal plasma. Typically an accelerated ion collides with an atomic nucleus in the solar atmosphere. The excited nucleus rapidly decays, emitting a photon at a discrete energy characteristic of the nucleus. The emission of these gamma-ray lines is stimulated by ions with energies exceeding 1 MeV. The most intense radiation arises from low in the solar atmosphere where the plasma density is high.
Example of how a gamma-ray photon is produced
Gamma-ray lines are our primary tracer of ion acceleration in flares. Also, flare-accelerated ions that escape into interplanetary space can be directly observed with particle detectors on a satellite if the satellite is located within the path of the ions. These observations provide critical information about ion acceleration in flares. They also provide valuable information about the relative abundances of both accelerated ions and ions in the thermal plasma. We cannot understand particle acceleration in flares without determining the relative amount of energy that goes into electrons, protons, and other ions.
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