A KICK FROM THE SOLAR-WIND AS THE CAUSE OF COMET HALLEYS FEBRUARY 1991 FLARE

KNOWLEDGE of the interaction of comets with the solar wind 1,2 was greatly advanced during the 1986 passage of comet Halley, which was studied in situ and by remote sensing more thoroughly than any previous comet flyby 3,4. Of particular interest and novelty was the large flare 5 that Halley was seen to produce on 12 February 1991, when it was 14.3 AU from the Sun and 18-degrees below the ecliptic plane. There have been further outbursts, most recently on 17 March 6. We note that the Sun has been unusually active in recent months, and suggest that a shock wave, generated by the solar flare and propagating through the interplanetary medium, could have caused the comet to flare. For example, we show that a solar flare on 31 January could plausibly have produced a shock wave that would have reached Halley on 12 February, and would have been sufficiently strong to crack the comet's crust of fluffy ice.