Forward-Scattering Enhancement of Comet Brightness. I. Background and Model

摘要

At small scattering angles, θ (= 180° - phase angle), micron-size particles forward-scatter sunlight hundreds to thousands of times more strongly than at side angles, enabling comets to become visible in daylight and even cast shadows at night. In this paper, we comprehensively review forward scattering as it relates to comet dust. We introduce a "compound Henyey-Greenstein (HG)" function, modified by the coma's dust-to-gas light ratio, to model scattering in cometary light curves - the first time this function has ever been applied to comets. To date, only five comets have actually been measured photometrically in forward-scattering geometry. Three of them - C/1927 XI (Skjellerup-Maristany), C/1975 VI (West), and C/1980 Yl (Bradfield) - were assayed by the "gold standard" method of visible-thermal photometry to obtain the coma's energy balance and the scattering (or "phase") function. That of C/1927 XI, reduced from Lampland's unpublished daylight radiometry in Lowell Observatory archives, is presented here in provisional form for the first time. Scattering curves for the other two comets - 96P/Machholz and C/2004 F4 (Bradfield) - are derived from non-thermal SOHO-satellite C3-coronograph photometry (Grynko 2005). Compared to brightness at 6 = 90°, the composite light curve shows enhancements of approximately 10-fold at 9 = 30° and approximately 100-fold by 9 = 13°5, the smallest angle at which a comet scattering function has yet been derived. The compound-HG scattering model developed here successfully fits the composite scattering function of these five comets for 9 < 150°. We show that it also successfully fits scattering data for comets in back-scattering geometry (150°≤ θ ≤< 180°), justifying its use for all scattering angles. In application, it correctly predicted the 9-fold brightness surge and naked-eye daylight visibility of comet C/2006 PI (McNaught) in forward-scattering geometry. This model should prove useful for forecasting brightness enhancements of comets that enter forward-scattering geometry in the future, as well as for analyzing forward scattering in historical apparitions - the subject of subsequent papers in this series.