We present the first in situ observations and a description of the large-scale behaviour of comet Halley's plasma environment. The PLASMAG-1 experiment, carried aboard the spacecraft Vega 1 and Vega 2, had the following aims: (1) to study the change of plasma parameters and distributions as a function of distance from the nucleus; (2) to investigate the existence and structure of the cometary bow shock; (3) to determine the change in chemical composition of the heavily mass-loaded plasma as the spacecraft approached the comet; and (4) to measure the neutral gas distribution along the spacecraft trajectory. We observe a discontinuity (the ‘cometopause’) between the solar-wind-controlled cometosheath and heavy-ion mantle and the magnetized cometary plasma region. From the measured neutral gas density distribution we estimate a total gas production rate of 1.3×1030 molecules s−1.

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  1. Space Research Institute, Profsoyuznaya 84/32, 117810, Moscow, GSP-7, USSR

    • K. I. Gringauz
    • , A. P. Remizov
    • , M. I. Verigin
    • , L. I. Denchikova
    • , A. V. Dyachkov
    • , I. N. Klimenko
    •  & G. A. Vladimirova
  2. Central Research Institute for Physics, PO Box 49, H-1525, Budapest, 114, Hungary

    • T. I. Gombosi
    • , I. Apáthy
    • , I. Szemerey
    • , A. J. Somogyi
    • , K. Szegő
    • , S. Szendrő
    • , M. Tátrallyay
    •  & A. Varga
  3. Max-Planck-Institut für Aeronomie, PO Box 20, D-3411, Katlenburg-Lindau, FRG

    • E. Keppler
    •  & A. K. Richter


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