A kick from the solar wind as the cause of comet Halley's 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 flare⁵ that Halley was seen to produce on 12 February 1991, when it was 14.3 AU from the Sun and 18° below the ecliptic plane. There have been further outbursts, most recently on 17 March⁶. 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.