Spectropolarimetric insight into plasma sheet dynamics of a solar flare

CoMP captured linearly polarized light from two emission lines of Fe XIII at 1074.7 and 1079.8 nm, from 1.03 to 1.5 solar radii. We focus here on the hot plasma sheet lying above the bright flare loops and beneath the ejected coronal mass ejection.

The polarization has a striking and coherent spatial structure, with unexpectedly small polarization aligned with the plasma sheet. By elimination, we find that small-scale magnetic field structure is needed to cause such significant depolarization, and suggest that plasmoid formation during reconnection (associated with the tearing-mode instability) creates magnetic structure on scales below instrument resolution of 6 Mm.