The structure of a cometary Type I tail: Ground‐based and ice observations of P/Giacobini‐Zinner

The in situ magnetic field and plasma measurements from the International Cometary Explorer (ICE) Mission obtained on September 11, 1985 are compared with CCD images of P/Giacobini‐Zinner (G‐Z) acquired with the 3.6 m Canada‐France‐Hawaii Telescope (CFHT) on Mauna Kea, Hawaii and a slit spectrogram from Lick Observatory during the same period. The CFHT image at ∼3.5 hr after the ICE encounter showed a short central ion tail with a diameter of ∼3 × 10³ km and a length of ∼2 × 10⁴ km as observed in the H₂O⁺ bandpass centered at 7025 Å. There was no distinct evidence of fine structure or ray activity. The Lick spectrogram of the H₂O⁺ emissions taken ∼0.5 hr post‐encounter with the slit perpendicular to the sun‐comet line showed an ion tail with a diameter of 1.2 × 10⁴ km. The ICE observations revealed a well defined 9.6 × 10³ km diameter magnetotail composed of two magnetic lobes in pressure equilibrium with a high beta central plasma sheet. These differing measures of tail width are found to be mutually consistent if the ion emissions observed at Earth originate in a slab‐shaped plasma sheet whose orientation is controlled by the direction of the interplanetary magnetic field. The results of this study also suggest that some thinning and thickening of cometary type I tails, usually attributed to plasma instabilities, may be due to changes in the angle at which the plasma sheet is viewed as IMF direction varies.