LAUSR

Abstract Polarization measurements made with the STEREO/SECCHI COR1 and COR2 coronagraphs during the perihelion passage of sungrazing comet C/2012 S1 (ISON) in November 2013 are presented and compared with earlier measurements made of Kreutz sungrazing comet C/2011 W3 (Lovejoy). Comet ISON was an Oort Cloud object, probably entering the solar system for the first time. Strong evidence was found for a radical change in the polarization properties once the tail particles approached within 10 solar radii of Sun center. Above this apparent threshold, the polarization properties of Comet ISON appeared very similar to those seen previously for Comet Lovejoy, with high positive polarization, and a broad negative branch reaching out to an inversion point between 40°–50°. Below 10 solar radii the polarization levels drop dramatically; an effect not seen for Comet Lovejoy. We speculate that radiation processes close to the Sun, and possibly starting as far out as 0.25 AU (50 solar radii), are responsible for the high levels of polarization seen in both comets. This may be due either to the disassociation of fluffy aggregates into the component monomers, or the sublimation of organic refractory mantles from silicate grain cores, and subsequent crystallization of the silicate cores. These two scenarios may happen in conjunction with each other. The mechanism behind the precipitous drop in Comet ISON’s polarization below 10 solar radii is unclear, but may be due to new much larger particles being produced from the breakup of macroscopic debris material embedded within the tail, possibly caused by the sublimation of olivines.