The Spatial Distribution of Gaseous Atomic Sodium in the Comae of Comets: Evidence for Direct Nucleus and Extended Plasma Sources

Sodium D-line emission (5890 and 5896 Å) has been observed in bright comets at small to moderate heliocentric distances for many years. We present here the first in depth study of a set of spatial profiles of the sodium D-line emission constructed from long-slit spectroscopic observations of Comets Bennett C/1969 Y1, Kohoutek C/1973 D1, and 1P/Halley. Preliminary analysis of these data lead to the suggestion by Combiet al.(1996,A Plasmagenic Source for Gaseous Sodium in Comets.Presented at Asteroids, Comets, Meteors) that a major fraction of the gaseous sodium was produced by an extended source in the tail and that the source was likely to be some charged species. Dissociative recombination of a molecular ion was suggested. The spatial profiles of sodium are not like typical neutral species. The inner region from the nucleus (<2 × 10⁴km) can be explained in terms of a model that accounts for collisional entrainment in the expanding coma and the heliocentric velocity dependent fluorescence rate and radiation pressure acceleration. This source comes either directly from the nucleus or has a very short-lived parent (?10³s). Away from the nucleus, down the tail and to the sides, the spatial profile slope flattens, indicating a second extended source. The striking similarity of the extended region of sodium spatial profiles with those of ions (H₂O⁺), both along and perpendicular to the tail, is highly suggestive that an ion source is responsible for the production of the extended component of gaseous sodium in the coma. The production rate of the highly variable extended source when present is four to five times that of the direct nucleus source. Observations (Schneideret al., 1991,Science253,1394–1397) and quantitative model analyses (Wilson and Schneider, 1994,Icarus111,31–34) have shown that a dissociative recombination of a sodium bearing molecular ion (NaX⁺) produces a peculiar component of the neutral sodium near Io. It displays a variable spatial morphology consistent with that of a molecular ion source “picked-up” in the plasma torus corotating with Jupiter's magnetic field. The rapid onset of the appearance of gaseous sodium in bright comets, its spatial distribution in the extended coma and near tail, and recent observations of sodium tails are all consistent with our original suggestion of this plasma source for sodium in comets.