Diagnostics of accelerated particle beams by spectral observations in Balmer series lines

Diagnostics of the lower atmosphere response to the particle beams still remains an unsolved problem of the solar flare physics. Up to the recent time the measurements of linear polarization caused by a particle beam bombardment has been considered as a perspective method of non-thermal processes detection. However, the late observations showed cases when appearance of HXR sources was followed by absence of any linear polarization. Even more difficulties appeared when theoretical simulations revealed a dependency of the linear polarization value on inclination of magnetic field lines. Thus, necessity of an additional and independent method of the particle beam diagnostics becomes evident.

Theoretical calculations of thermal mechanisms coupled with accelerated electron beams in flares showed that the H-alpha/H-beta line intensity ratio could be a possible indicator whether a mechanism of non-thermal excitation was present or absent.

Spectral observations of the 26 June 1999 flare in the H-alpha and H-beta lines were analyzed together with HXR and microwave data. We found peculiarities in the H-alpha/H-beta line intensity ratio (the "sidelobes") in emission kernels. We concluded that they appeared due to non-thermal electron beams. Consequently, the obtained results confirmed the theoretical predictions and bore a first witness for a detection of the spatial and temporal presence or absence of the non-thermal electron beams. However, as a more statistically probative evidence was needed, we extended our analysis on a set of flares where presence of non-thermal mechanisms of excitation was verified by HXR data. Conclusions of the analysis are discussed both from their observational and theoretical aspects.