H 2O + Ions in the Inner Plasma Tail of Comet Austin 1990 V

Abstract

Narrow-band images of Comet Austin 1990 V in the light of the red (0-8-O) H 2O + emission (6199 Å) are presented. The observations were carried out in the period April 30-May 7, 1990, with a focal reducer, Fabry-Perot interferometer, and a CCD camera attached to the 1-m telescope of Hoher List Observatory. The high spectral resolution of the images allowed a precise subtraction of the dust continuum even at the nucleus and a reliable absolute calibration. Thus, for the first time, the spatial distribution of water ions and its temporal behavior is observed in the cometary coma including the area close to the nucleus. Peak column densities between 10 11 and 2 × 10 11 cm -2 were obtained in the observational period. Close to the nucleus the column density distribution is strongly asymmetric. Toward the Sun the column density gradient is steep. In this direction the column density decreases by a factor of ≈4 over a distance of 10 4 km. In the tailward direction the distribution is flat. Frequently the column density maximum is shifted tailward. The contour passing through the nucleus usually extends more than 10 4 km tailward. This is caused by the mass loading of new ions into the tailward cometary plasma flow. We have integrated the column densities perpendicular to the tail direction and calculated the number of water ions per unit tail length. The derived profiles always have their maximum significantly shifted tailward. They are compared with a simple model of H 2O + production by photoionization of water. Good qualitative agreement is obtained but the mean ion velocities deduced from the model are too high by about a factor of 10. Probably a significant part of the ions in the flanks of the tail is not detected in our observations. The time interval of 22-23 min between successive images allows to study turns of the plasma tail and the formation of ray structures near the cometary nucleus. Even strong changes in morphology have only a minor influence on the ion content per unit tail length. This indicates that tail rays are usually not caused by a modulation of the ion production but that the tail ray phenomenon represents a redirection (rechanneling) of the ion flux out of the coma from old rays (main tail) into new rays. We compare our observations with MHD model calculations of magnetic field tangential discontinuities passing through the cometary coma.