Structure and dynamics of Saturn's outer magnetosphere and boundary regions

Abstract

In 1979–1981, the three USA spacecraft Pioneer 11 and Voyagers 1 and 2 discovered and explored the magnetosphere of Saturn to the limited extent possible on flyby trajectories. Considerable variation in the locations of the bow shock (BS) and magnetopause (MP) surfaces were observed in association with variable solar wind conditions and, during the Voyager 2 encounter, possible immersion in Jupiter's distant magnetic tail. The limited number of BS and MP crossings were concentrated near the subsolar region and the dawn terminator, and that fact, together with the temporal variability, makes it difficult to assess the three‐dimensional shape of the sunward magnetospheric boundary. The combined BS and MP crossing positions from the three spacecraft yield an average BS to MP stagnation point distance ratio of 1.29±0.10. This is near the 1.33 value for the earth's magnetosphere, implying a similar sunward shape at Saturn. Study of the structure and dynamical behavior of the outer magnetosphere, both in the sunward hemisphere and the magnetotail region using combined plasma and magnetic field data, suggest that Saturn's magnetosphere is more similar to that of earth than that of Jupiter. Also, evidence was found by Voyager 1 for tailward flowing plasma near the pre‐dawn MP, a phenomenon well known for the cases of both earth and Jupiter. That this was not observed by Voyager 2 at Saturn may have been related to the possible immersion of Saturn in Jupiter's magnetotail during a significant portion of the Voyager 2 encounter period, since the plasma flux in the Jovian tail is markedly lower than that in the solar wind on average.