Observations of the plasma flow in comet P/Swift-Tuttle

We present direct ground based observations of the plasma flow sunward and tailward of the nucleus of comet P/Swift-Tuttle. The observations are long-slit high resolution spectra of the H₂O⁺ emission centered at 6199Å with a velocity resolution of about 7 km s⁻¹ (FWHM) and a spatial resolution of about 10⁴ km at the comet. Emission is visible from just inside the predicted position of the cometopause on the sunward side of the nucleus out to 5×10⁵ km on the tailward side. The deceleration of the solar plasma on the sunward side is clearly observed as is the acceleration of cometary ions into the tail. These observations show the effectiveness of ground based methods for the systematic study of cometary plasmas and point to the need for a better theoretical understanding of their acceleration mechanisms.