Observations of comet Halley at Hα and 6300 Å

Abstract

A small aperture telescope has been used with the Fabry‐Perot interferometer at the Arecibo Observatory to make high spectral resolution measurements of Comet Halley emissions at 6562.72 Å (Hα) and 6300.30 Å (O¹D). Observations in March, 1986, are characterized by a highly structured spectral signature at Hα. The Hα spectra imply non‐isotropic outflow of hydrogen atoms following dissociation of the parent species. The Hα surface brightness for a 5′.9 field of view centered on the comet head was 59 ± 13 rayleighs(R) on March 12, 1986, decreasing to 25 ± 6R on March 23 (after correction for atmospheric extinction). Calculation of atomic hydrogen production rates and outflow velocities is not straightforward due to the non‐isotropic nature of the atomic hydrogen outflow. The O(¹D) emission at 6300.3 Å was accompanied by a feature at 6300.8 Å that we attribute to NH2. The brightness of the O(¹D) emission for a 6′ field of view was 260 ± 50 R on March 15 and 17. This brightness decreased by a factor of about 2 when the comet was viewed with a 5′.9 field of view centered 6′ off the comet nucleus (sunward or tailward). The width of the O(¹D) emission line was 3.9 ± 1.5 kms−1 on March 15, and 7.4 ± 2.2 km s−1 on March 17. The O(¹D) spectral line profiles on March 17 were skewed to the red side of line center.