KINEMATICS OF THE ION TAIL OF COMET P SWIFT-TUTTLE/

We have obtained long-slit high resolution spectra of the H2O+ 6199 An gstrom complex in the near tail of comet P/Swift-Tuttle. The observati ons were made using the Hamilton echelle spectrometer fed by the Lick Observatory 0.6 m coude auxiliary telescope. For most of our observati ons, the spectral slit was aligned along the Sun-tail axis and the com etary nucleus was placed at one end of the slit, giving us spectra hav ing the spatial and spectral resolution needed to measure the radial v elocity and velocity dispersion continuously down the cometary tail ou t to a distance of 4x10(5) km. The radial velocities confirm the earli er more restricted observations by Rauer and Jockers [Icarus, 102, 117 (1993)] and by Wyckoff and Lindholm (1994) showing that the tail moti ons are indeed bulk flows in the antisolar direction. Out to 3X10(5) k m in the tail typical bulk flows are at a speed of similar to 30 km s( -1). The velocity dispersion, (sigma(r)), of the H2O+ lines follows a pattern that is quite systematic; sigma(r) is smallest near the cometa ry nucleus, and steadily increases down the tail. The highest velocity dispersions are found ahead of the nucleus and off the tail axis. The se velocity dispersions are equivalent to ion temperatures ranging fro m 10(5) to 10(6) K. We note a clear anticorrelation between the H2O+ l ine intensities (related to the ion density) and the bulk flow and dis persion velocities; direct mass loading of the solar wind by the obser ved water ions may be responsible. We discuss several approximate equi partition methods used to infer local magnetic fields induced by the i nteraction of the cometary ions with the solar wind particle/field str eam. Typical fields derived are near 50 nT. The measured tailward acce lerations are consistent with this order of magnitude B field.