Dependence of the open-closed field line boundary in Saturn's ionosphere on both the IMF and solar wind dynamic pressure: comparison with the UV auroral oval observed by the HST

摘要

We model the open magnetic field region in Saturn's southern polar ionosphere during two compression regions observed bythe Cassini spacecraft upstream of Saturn in January 2004, and compare thesewith the auroral ovals observed simultaneously in ultraviolet imagesobtained by the Hubble Space Telescope. The modelling employs the paraboloidmodel of Saturn's magnetospheric magnetic field, whose parameters are variedaccording to the observed values of both the solar wind dynamic pressure andthe interplanetary magnetic field (IMF) vector. It is shown that the openfield area responds strongly to the IMF vector for both expanded andcompressed magnetic models, corresponding to low and high dynamic pressure,respectively. It is also shown that the computed open field region agreeswith the poleward boundary of the auroras as well as or better than thosederived previously from a model in which only the variation of the IMFvector was taken into account. The results again support the hypothesis thatthe auroral oval at Saturn is associated with the open-closed field lineboundary and hence with the solar wind interaction.