EVOLUTION OF THE C2 SPECTRUM IN HALLEY INNER COMA - EVIDENCE FOR A DIFFUSE SOURCE

The spectra of Halley's comet obtained on March 9, 1986, by the Three- Channel Spectrometer (TKS) embarked on board the Vega 2 spacecraft, al low a study of the variation of the shape of the C2 Swan band sequence (DELTAnu = 0) fluorescence spectrum. The spectra obtained, which do n ot separate the different bands of this sequence, reveal nevertheless an important variation in the ratio of two intensities obtained at wav elengths close to the (1, 1) and (0,0) bandheads. The spatial repartit ion of this ratio shows low values near the nucleus (less than a few t housand kilometers), high values in the jets, and intermediate values outside the jets and far from the nucleus (above about 15 000 km). The se facts are interpreted, employing a study of the C2 fluorescence spe ctrum based on a Monte-Carlo method, by some different excitation temp eratures, possibly due to two different origins for the C2 molecules: a main one in the nucleus, and a secondary one in a dust jet of CHON g rains. The long time needed by the C2 molecules to reach their fluores cent equilibrium (about 3000 s), and two different initial population distributions, would explain the TKS observations.