Paper detail

On helium line polarization during the impulsive phase of an X1 flare

We analyze spectropolarimetric data of the He I 1083~nm multiplet ($1s2s~^3\!S_1 - 1s2p~^3\!P^o_{2,1,0}$) during the X1 flare SOL2014-03-29T17:48, obtained with the Facility Infrared Spectrometer (FIRS) at the Dunn Solar Telescope. While scanning active region NOAA 12017, the FIRS slit crossed a flare ribbon during the impulsive phase, when the helium line intensities turned into emission at $<$ twice the continuum intensity. Their linear polarization profiles are of the same sign across the multiplet including 1082.9 nm, intensity-like, at $<5$\% of the continuum intensity. Weaker Zeeman-induced linear polarization is also observed. Only the strongest linear polarization coincides with hard X-ray (HXR) emission at 30-70 keV observed by the Reuven Ramaty High Energy Solar Spectroscope Imager. The polarization is generally more extended and lasts longer than the HXR emission. The upper $J=0$ level of the 1082.9~nm component is unpolarizable, thus lower level polarization is the culprit. We make non-LTE radiative transfer calculations in thermal slabs optimized to fit only intensities. The linear polarizations are naturally reproduced, through a systematic change of sign with wavelength of the radiation anisotropy when slab optical depths of the 1082.9 component are $< 1$. Collisions with beams of particles are neither needed nor can they produce the same sign of polarization of the 1082.9 and 1083.0 nm components. The He I line polarization merely requires heating sufficient to produce slabs of the required thickness. Widely different polarizations of H$α$, reported previously, are explained by different radiative anisotropies arising from slabs of different optical depths.