Power Spectral Analysis of Enhanced Scintillation of Quasar 3C459 Due to Comet Halley

The radio source 2314+038 (3C459) showed enhanced scintillations on three days at a solar elongation of about 90� as the plasma tail of Halley's Comet swept across it on six days during 16-21 December 1985. If we assume that the plasma velocities in the tail were not constant everywhere, but increased linearly from about 50 kms-1 at the tail axis to the normal average solar wind velocity of 400 kms-1 at the edges where the tail merged with the solar wind, a power spectral analysis of the scintillations shows two ranges of the rms electron density variation t:..N and scale size a. In particular, these are a fine scale zone near the axis where a is in the range 9 to 27 km and t:..N in the range 2 to 5 cm -3 and a zone near the edges with a and t:..N in the ranges 100 to 265 km and 0�4 to 0�8 cm-3 respectively. The assumption of a single velocity of 100 kms-1 throughout the tail shows similar fine scales near the tail axis and large scales near the edges. The scale sizes in that case range from about 18 km at the axis to about 70 km at the edges, corresponding to t:..N of 3�3 and 0�85 cm-3 respectively. A comparison with the results obtained by Slee et al. (1987) shows that there is no radial variation of t:..N. The tail-lag is seen to playa crucial role in determining the correct occulting geometry and the path of the source through the tail.