Comet de Vico (122P) and latitude variations of plasma phenomena

Abstract

In situ measurements by the Ulysses spacecraft have unequivocally established differences in solar wind properties at different solar latitudes for 1992 through 1996, and comets are exposed to these different environments depending on latitude. In the polar regions, a less time-varying appearance and a plasma tail orientation corresponding to a higher solar wind speed are expected. In the equatorial regions, a more time-varying appearance is expected as well as a plasma tail orientation corresponding to a slower solar wind speed. Disconnection events (DEs) are produced at crossings of the heliospheric current sheet (HCS) as established by the association of the comet Halley DEs in 1985–1986 with the HCS and by magneto-hydrodynamic simulations. Thus, it is expected to see DEs when comets are in equatorial latitudes, but not when they are in polar latitudes. Comet de Vico (122P), with its inclination of 85.3°, good brightness, and plasma activity, was an excellent test comet for latitude variations. Images of de Vico have been collected as part of the Ulysses Comet Watch, and plasma structures are seen from 18 September 1995 to at least 25 October 1995, corresponding to a range of about 90° in ecliptic latitude. The analysis confirms the picture described above with the boundary between the equatorial and polar regions at 20°–25° latitude, in agreement with the in situ measurements by Ulysses. The tail orientation is consistent with the different velocity regimes; DEs are seen only in the equatorial region; the comet in the polar region has a distinctly less disturbed appearance.