Spatial distribution of the neutral carboneous compounds glow in the sunward Halley comet coma Original Research Article

This work presents a study of the C2, C3, CH, and CN glow in the Halley’s coma in sunward direction. For that purpose, 1035 spectra in the near UV and visible region registered by the Three-Channel Spectrometer (TKS) on board Vega-2 station on 9 March 1986 are used. An improved method of the dust continuum extraction in the visible region is applied. The “dust regions” in the Halley’s comet spectra are re-examined. The spectral index u and a normalization coefficient of the continuum are computed for each spectrum by the least squares method on the basis of a linear regression. The index u is obtained to lay in the interval 3–3.4. For all spectra, the average u and du values are: uav = 3.2126, duav = 0.1197. Thus, the dust continuum evaluation is reliable, which conduces to a correct separation of the gas emissions. The radial profiles, presenting the carboneous compounds column intensities as a function of the distance p of the line of sight to the nucleus, termed projected distance or p-parameter, are examined. The obtained profiles correspond very well to the Haser’s distribution when the optical thickness in the inner coma environment is taken into account. The deviation from Haser’s model for CN and C3 in the p < 1000 km region, obtained in previous investigations, is not seen now. Possibly, this deviation had been the result of a dust continuum in this zone, not perfectly subtracted. Some estimates of the production rates Q and scale lengths L and their dependences on the distance to the nucleus R are made. We obtain image for 0.83 AU on 9 March 1986, which presupposes some changes of the used parameters. For the CH radical, best results are obtained when the dependencies LCH ∼ R2 and Q ∼ R−8 are used. A slight discrepancy between the measurements and the theoretical curve is obtained in the case of C3 and CH at p > 4000 km. The peculiarities of the C2, C3, CH, and CN glow, observed until now, are confirmed. Thanks to the TKS scanning, composed intensity distributions for each carboneous compound are constructed, covering a larger space region. The obtained distributions are alike and similar to the dust one and relatively stable in the measurement period. The intensities decrease with the increase of the distance to the nucleus. The two jets and the jet-free zone as well as the jet structure are examined.