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The Viability of Energetic Protons as an Agent for Atmospheric Heating during the Impulsive Phase of Solar Flares

Recent observations of γ-ray line intensities in solar flares results have led to the suggestion that the energy content in accelerated protons may rival that of accelerated electrons. However, simply having a large energy content in accelerated protons is not sufficient to ensure an important role for protons in driving the response of the atmosphere to flare energy input: a viable model must also deposit a large fraction of the particle energy at the top of the chromosphere to account for the large increase in soft X-ray emission measure that characterizes the main phase of a flare. While hard X-ray producing electrons indeed deposit a sufficient amount of energy at the appropriate depths in the atmosphere, only protons with a characteristic energy of order 1 MeV do likewise. It is intriguing that a characteristic energy of just this value has been obtained through independent analysis of γ-ray line intensities. However, it must be noted that protons of this energy do not have a signature in hard X-rays, leading us to conclude that a substantial fraction of the flare energy budget must still reside in deka-keV electrons. In order to complete the global energetic picture, further corroborating observational diagnostics for MeV protons are desirable. In this Letter, we discuss such suitable observations.