Jean-Claude Hénoux

Observatoire de Paris, DASOP/LPSH(UMR8645), 5 Place Jules Janssen, F-92195 Meudon Cedex, France

When they reach the cold chromosphere, energetic particles interact with neutral atoms. They appear as the dominant line formation process, dominating over the effects of local excitation and leading to enhanced line and continuum emission. So, investigating the properties of optical emission provides information on the non-thermal particles present in the lower solar atmosphere. The depth of formation of the intensity of the radiation observed depends on its frequency along the line profile. Therefore, the shape of a line profile is related to the depth variation of the non thermal line collisional excitation rate, i.e. to the depth dependence of the rate of energy deposit. Protons and electrons lead to different line shapes; this difference can be used to discriminate between these two kinds of particles. Another typical line shape is expected from lines emitted by accelerated ions, once they have captured, by charge exchange, a electron to local hydrogen atoms. This is also a possible diagnostics of ion bombardment not yet fully used. The anisotropy expected of the angular velocity distribution function of non-thermal particles can be detected by measuring the linear polarisation of lines formed by impact between local atoms and particle beams. Impact polarisation observations, together with hard X-ray and gamma-ray observations, allow to extend to low energies (< 1MeV) proton energy distribution estimations. Consequently, a flare polarimeter (PARIS) will be available for participating to joint observational programs with HESSI.