[en] The use of impact linear polarization of chromospheric lines as a diagnostic of 100 keV protons is proposed. Observations of hard X-ray emission are examined, noting the possibility that 100 keV proton beams bombarding the atmosphere could create a hot thermal plasma at the origin of X-ray emission. The application of impact linear line polarization as a diagnostic of the existence of energetic particles with anisotropic velocity distribution functions is described. The mechanisms leading to an anisotropic proton distribution in the solar chromosphere are discussed and the required initial energy and energy flux for accelerated protons in the corona are derived. It is suggested that it is possible to infer the original proton anisotropy in the corona and the effect of this anisotropy on the proton energy and proton energy flux required to reach the H-alpha-forming layers. 28 refs

[en] The rate of direct collisional excitation of Lyα by (superthermal) photoelectrons is compared to the thermal rates. These photoelectrons are produced via ionization of chromospheric material (mainly helium) by XUV radiation from the overlying flare source. We found that for intense XUV flares, with area not larger than approximately 10¹⁸ cm². There exists a layer in the atmosphere where direct excitation effects are significant, which leads to enhanced emerging intensity in the Lyα wings (for Δν > 3.5 νsub(D)). These results were established by using a-priori given models for hydrogen and thermal electrons in flares, but the ionization of helium was independently calculated. We then calculated the velocity distribution of the photoelectrons, and found substantial anisotropy (the photoelectrons moving preferentially near the horizontal) increasing with the depth in the atmosphere and with the compactness of the XUV source. The Lyα photons resulting from photoelectrons excitation alone are linearly polarized, with a degree increasing with the line of sight inclination, up to a value of 4.5%, but the polarization is lost during the travel outwards, unless practically 100% coherence in photon scattering is assumed. This does not preclude the possibility for other lines to be polarized, however. The other possible mechanism of direct excitation (collisions by electron beams) is shown to produce the same qualitative effects: in particular excitation is also due predominantly to electrons moving at small angle with the horizontal. (orig.)

[en] Linear polarization was observed in the S I 1437 A line in bright flaring points during the soft X-ray emission from a flare on 1980 July 15. The degree of polarization is about 25% and is detected at a signal-to-noise ratio of 2.9. The polarized electric vector is directed toward disk center to within 3₀

[en] The transport of fast (about 1 MeV) protons in the corona is considered when their density and anisotropy are sufficiently large to excite Alfven waves. The Alfven waves scatter the protons and make them almost isotropic. The Alfven wave instability remains in a marginally stable state in which the proton anisotropy is just large enough to excite Alfven waves to a level which would eliminate any greater anisotropy. The protons propagate diffusively down a coronal loop and have a path length of order 100 times their length in the absence of Alfven waves. This increases their Coulomb losses, and much higher energy protons (2-20 MeV) are required initially to produce H-alpha impact line linear polarization in the chromosphere as compared to the case in the absence of Alfven waves (about 200 keV) for loop lengths about 10 to the 9th cm. The interpretation of the observed polarization becomes less direct because it depends on more poorly known parameters than in the case of the absence of Alfven waves. 6 refs

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