Heliospheric Latitude Variations of Properties of Cometary Plasma Tails: A Test of the Ulysses Comet Watch Paradigm

Abstract

Images of comets de Vico in 1995, Hyakutake in 1996, and Hale–Bopp in 1997 taken by observers in the Ulysses Comet Watch clearly show plasma tail properties reflecting the demarcation of the solar wind into distinct equatorial and polar regions with the boundary determined by the maximum extent of the heliospheric current sheet (HCS). Generally, (1) comet plasma tails in the polar region appear relatively undisturbed (as expected from a steady solar wind), while comet tails in the equatorial region appear disturbed (as expected from a highly varying solar wind); (2) disconnection events (DEs) are observed only in the equatorial region where comets pierce the HCS; (3) the position angle of the plasma tail is consistent with a solar wind speed of 750 km s −1 in the polar region and an average solar wind speed of 450 km s −1 in the equatorial region. While the paradigm seems firm, it was established during a limited range of the solar cycle, and an extension to other ranges is desirable. We test this paradigm using the published record for essentially the entire 20th century. The catalogs of M. J. S. Belton and J. C. Brandt (1966, Astrophys. J. Suppl. 13, 125–332)—giving comet tail orientations and descriptive data—and M. B. Niedner (1981, Astrophys. J. Suppl. 46, 141–157)—giving DE data—were the principal sources. When combined with the DEs in Comet Halley (J. C. Brandt et al. 1999, Icarus 137, 69–83) and the Ulysses-era comets, the data set is extensive. Results for the test are as follows: (1) Images of Comet Mrkos (1957d) clearly show the change of appearance from polar to equatorial regions. We discuss the unusual case of Comet Borrelly (1903c) and present an evaluation of the descriptive notes in the Belton and Brandt catalog. (2) The observed latitude envelope of DEs as a function of solar cycle is consistent with the maximum extent of the HCS. The HCS extends to higher latitudes at solar maximum (cf. J. T. Hoeksema 1991, Adv. Space Res. 11, (1)15–(1)24; S. T. Suess 1993, Adv. Space Res. 13, (9)31–(9)42) and the observed DEs trace this extension. (3) The orientations of the plasma tail can be used to infer the solar-wind speed. Most of the suitable comets in the Belton and Brandt catalog are in the equatorial region, one is in the polar region, and a few cross the equatorial–polar boundary. In all cases, the orientation is consistent with a high-speed solar wind in the polar region and a lower-speed, variable solar wind in the equatorial region. The data available for comets throughout the 20th century are consistent with the Ulysses Comet Watch paradigm. Thus, detailed studies of plasma tails can be used to map the structure of the heliosphere, specifically the location of the HCS and the location of the boundary between the equatorial and the polar regions. We note that data for comets in the polar region are still relatively rare.