Abstract
The magnetic field configuration-states of the magnetotails of the planets Uranus and Neptune are compared. Earth's case is also briefly treated, as well as some related aspects of the other three magnetic planets. In Uranus' case, due to the large tilt (59°) of the planet's magnetic dipole with respect to its spin axis and the unusual obliquity of that axis, the angle of attack (α) of the solar wind with respect to dipole alignment goes through all possible angles, 0° to 180°, yielding a very broad spectrum of configuration-states of its tail. Cases are discussed where the planetary magnetic dipole is either aligned with the Sun-planet line (“pole-on” state) or perpendicular to it and some intermediate states, for both Uranus and Neptune. Only Uranus experiences the pole-on state, which next occurs in November 1999 (± 2 months); last year (1993.2) it had the first “perpendicular” state since Voyager encounter which resembles Earth's case. Neptune never has a pole-on configuration, but it gets as close as α = 14° from it; the next occurrence is early in 2003. At Voyager encounter Neptune's magnetotail apparently rapidly migrated through a broad spectrum of field structures with near extreme states resembling an Earth-like case on the one hand and a cylindrically symmetric one on the other. Magnetopause “openness” should dramatically change in terms of the rapidly changing angle of attack throughout a planetary day for these two planets and this has important implications for their magnetotails. Any future magnetospheric mission plans for Uranus or Neptune should take into consideration the allowed range of values for α for the epoch of interest; this is especially of concern for Uranus which has a pole-on state and all possible αs, around the middle of 2014, 20 years from now.
Published Version
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