Scattering Polarization in Solar Flares

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Introduction

Fast particles can excite atomic states, resulting in linearly polarized line emission. The detection of any linear polarization during a solar flare could therefore provide key evidence for the existence of a particle beam, in the so-called thick-target model. This polarization, in principle, should appear in the chromosphere, where the thick-target particle beams interact according to the model.

Accordingly, spectropolarimetric observations of strong chromospheric spectral lines (such as the hydrogen H-alpha, Na D2, or Ca II K) have long been sought, with controversial results. Some observations suggest that there is a significant linear polarization of these lines of the order of several percent observed at the edges of the flare ribbons (Reference [1]). Other observers claim no observable signatures at all (Reference [2]). The theory is complicated, but any successful observation would provide a giant step forward for the thick-target model.

We call the direct excitation of polarization "impact polarization." Like any process creating spectral line polarization, impact polarization results from an anisotropy of the medium in which the lines are formed. In particular, the impact-polarization models assume that the distribution of velocities of particles is not isotropic. If the atoms of the plasmas collide with these anisotropically moving particles, the subsequent photons emitted from the atoms are linearly polarized.

Since we have other evidence for the bombardment of the chromosphere byenergetic electron and proton beams, possibly originating in the coronal reconnection site, such an explanation of the observed polarization seems to be natural. However, there are theoretical reasons to believe that this process may be not as efficient as previously thought, because of the role of the radiation transfer effects and of the depolarizing collisions with the isotropic component of the particles velocity distribution. The modeling to date has been one-dimensional, relying on the particle transport the vertical (beam) direction to produce the polarization.

The model

There is another natural mechanism that can also provide a good explanation of the most common observational findings and that can perhaps explain even more than the impact polarization hypothesis. Similarly to the process of impact approximation, the atoms can become polarized after their exposure to anisotropic illumination. The resulting scattering polarization is a well-known phenomenon in solar physics and its most familiar manifestation is the existence of the so-called Second Solar Spectrum (.pdf file). This is, for example, a linearly polarized spectrum of the continuum and spectral lines observed near the solar limb which is caused by the anisotropy of radiation due to limb-darkening.

Fig. 1: The anisotropy of radiation is highest at the edges of the flare ribbons. Multi-dimensional radiative transfer calculation is needed for determination of the amount of this anisotropy.