The velocity, ion density, flow direction and ion temperature of the solar-wind plasma were measured during Sakigake's approach to comet Halley from 10 to 13 March, with a distance of closest approach of 7 × 106 km. Interplanetary magnetic field (IMF) observations show that Sakigake travelled through the solar plasma sheet and also through the lobes of the heliomagnetosphere (or through the magnetic cloud). At the time of the measurements, interplanetary space was undergoing a small disturbance caused by the two-stream solar wind with a 27-day periodicity. Although it is difficult to isolate an effect of the comet on solar-wind parameters from background disturbances, it remains possible that the solar wind was decelerated by the cometary atmosphere.
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