Kinematics of the ion tail of comet P/Swift-Tuttle

Abstract

We have obtained long-slit high resolution spectra of the H_2O^+ 6199 A complex in the near tail of comet P/Swift-Tuttle. The observations were made using the Hamilton echelle spectrometer fed by the Lick Observatory 0.6 m coude auxiliary telescope. For most of our observations, the spectral slit was aligned along the Sun-tail axis and the cometary nucleus was placed at one end of the slit, giving us spectra having the spatial and spectral resolution needed to measure the radial velocity and velocity dispersion continuously down the cometary tail out to a distance of 4 x 10^5 km. The radial velocities confirm the earlier more restricted observations by Rauer & Jockers [Icarus, 102, 117 (1993)] and by Wyckoff & Lindholm (1994) showing that the tail motions are indeed bulk flows in the antisolar direction. Out to 3 x 10^5 km in the tail typical bulk flows are at a speed of -30 km s^(-1). The velocity dispersion, (σ_r), of the H_2O^+ lines follows a pattern that is quite systematic; σ_r is smallest near the cometary nucleus, and steadily increases down the tail. The highest velocity dispersions are found ahead of the nucleus and off the tail axis. These velocity dispersions are equivalent to ion temperatures ranging from 10^5 to 10^6 K. We note a clear anticorrelation between the H_2O^+ line intensities (related to the ion density) and the bulk flow and dispersion velocities; direct mass loading of the solar wind by the observed water ions may be responsible. We discuss several approximate equipartition methods used to infer local magnetic fields induced by the interaction of the cometary ions with the solar wind particle/field stream. Typical fields derived are near 50 nT. The measured tailward accelerations are consistent with this order of magnitude B field.