Comet Hale-Bopp: a Probe of the Polar and Equatorial Solar Wind
J. C. Brandt (U. Colorado), S. Larson (U. Arizona), C. C. Petersen (Sky Publishing), M. Snow, Y. Yi (U. Colorado), The Ulysses Comet Watch Team (UCW)
The Ulysses spacecraft has established a two-part solar wind with distinctly different properties in the polar and equatorial regions. Cometary plasmas in these regions should mirror these differences including tail orientation, appearance and occurrence of disconnection events (DEs). Comet Hale-Bopp with its orbital inclination of 89.4 and exceptional brightness is a good test of the developing picture.
Extensive imaging has been gathered that documents properties of the comet in the polar region, primarily March and April 1997. As the comet approached the heliospheric current sheet (where DEs are expected) in early May 1997, observing conditions were less favorable. Nevertheless, sufficient images are available to show that a major DE occurred which is clearly visible on May 7 & 8, 1997.
We summarize the differing properties of comets in the different solar-wind regions by supplementing our findings for comet Hale-Bopp with results for comets de Vico and Hyakutake. Comets in the equatorial region show a disturbed appearance, experience DEs, and have a plasma tail orientation corresponding to an average solar-wind speed of approximately 450 km/sec. Comets in the polar region show a less-disturbed appearance, do not experience DEs, and have a plasma tail orientation corresponding to a solar-wind speed of 750 km/sec.