March 13th, 2015: SOHO and STEREO satellite comet photometry: Contributions from sodium and heat

The light curves of the small Kreutz sungrazing comets in different filters in the coronagraphs aboard the SOHO and STEREO satellites are distinctive and different.  Most famous is the brightness excess in the SOHO C2 and C3 orange filters, which Biesecker et al. (2002) and Knight et al. (2010) assign to the prominent yellow-orange sodium D line doublet emission at 5889 Å and 5895 Å.  I have reviewed and synthesized comet sodium emission data in the literature spanning a wide range of heliocentric distances, r.  Based on this synthesis, the temporary steep r^−8 brightness “kink” over r = 0.16 AU to 0.12 AU – superimposed upon the otherwise generally standard r^−4 brightening trend – can be explained by rapidly increasing atomic sodium resonant fluorescence emission relative to the dust continuum brightness.  At r ˂ ~0.05 AU, heat from the coma dust grains glowing at T > 1200º K should bleed into IR, red, and clear filter channels, resulting in particularly severe contamination if the albedo of coma dust is extremely low, as suggested by stellar occultation data (e.g., Lacerda & Jewitt 2012).  I present a model to extract the scattered sunlight signal from sodium and heat emission in SOHO and STEREO photometry.

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