Imaging polarimetry of Hα kernels of solar flares with a ferroelectric liquid crystal polarimeter

Impact polarization of the kernels of solar flares in the chromospheric lines is one of the important diagnostics of high-energy particles in solar flares. Even though so far the detection of polarization up to 10% have been reported by various authors, generally the polarization of the flare kernels is small. To detect the impact polarization, high-cadence imaging polarimetry with the precision of the order of 0.1% is required. The seeing-induced error is the greatest cause degrading the polarimetric precision, but it can be reduced by high-frequency polarization modulation. We developed a polarimeter with ferroelectric liquid crystals (FLCs) combining with a high-speed CCD camera, and the modulation frequency as high as 1 kHz realized the precision of the polarimetry in the Hα line of the order of 0.1%. Many flares have been observed with this polarimeter, and we found that just small flares sometimes show significant polarization. However, the majority of the flares do not show polarization signals larger than 0.1%. Our results suggest that there are some unknown parameters to produce the polarization of flare kernels.