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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 flareSOL2014-03-29T17:48, obtained with the Facility Infrared Spectrometer (FIRS) atthe Dunn Solar Telescope. While scanning active region NOAA 12017, the FIRSslit crossed a flare ribbon during the impulsive phase, when the helium lineintensities turned into emission at $<$ twice the continuum intensity. Theirlinear polarization profiles are of the same sign across the multipletincluding 1082.9 nm, intensity-like, at $<5$\% of the continuum intensity.Weaker Zeeman-induced linear polarization is also observed. Only the strongestlinear polarization coincides with hard X-ray (HXR) emission at 30-70 keVobserved by the Reuven Ramaty High Energy Solar Spectroscope Imager. Thepolarization 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 lowerlevel polarization is the culprit. We make non-LTE radiative transfercalculations in thermal slabs optimized to fit only intensities. The linearpolarizations are naturally reproduced, through a systematic change of signwith wavelength of the radiation anisotropy when slab optical depths of the1082.9 component are $< 1$. Collisions with beams of particles are neitherneeded nor can they produce the same sign of polarization of the 1082.9 and1083.0 nm components. The He I line polarization merely requires heatingsufficient to produce slabs of the required thickness. Widely differentpolarizations of H$\alpha$, reported previously, are explained by differentradiative anisotropies arising from slabs of different optical depths.

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