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

Abstract

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

IMF dependence of Saturn's auroras: modelling study of HST and Cassini data from 12–15 February 2008

“…In this paper we have further studied the relationship between the dynamics of Saturn's auroral emissions and variations in the IMF in the upstream solar wind, following discussions of the January 2004 joint Cassini-HST imaging campaign data by Belenkaya et al (2007Belenkaya et al ( , 2008. While the latter campaign provided an important initial data set for study, the large ∼ 1300 R S displacement between the spacecraft and the planet with a related ∼ 17 h radial propagation delay results in considerable uncertainties in the identification of detailed correspondences between the interplanetary data and the auroral images.…”

“…We note, however, that the propagated solar wind dynamic pressures presented by Clarke et al (2009) indicate that somewhat more moderate values, ∼ 0.02-0.04 nPa, are appropriate towards the beginning and end of the interval considered. To test the sensitivity of our results to the magnetosphere model employed, we have therefore repeated the calculations using a model derived by Belenkaya et al (2008) which is appropriate to an intermediate pressure of 0.03 nPa, corresponding to more typical values of the magnetopause subsolar radius of ∼ 22 R S . The modified calculations show that the results are not sensitively dependent on which model is employed, such that only the compressed model results will be shown here.…”

“…As indicated above, the location of the open-closed field line boundary in the ionosphere for given interplanetary conditions is determined here via field line calculations using the Saturn magnetospheric model described by Alexeev et al (2006) and Belenkaya et al (2006bBelenkaya et al ( , 2007Belenkaya et al ( , 2008, in which the magnetopause is taken to be a paraboloid of revolution about the Saturn-Sun line. The model is consequently expressed in KSM coordinates.…”

“…The interval is 12-15 February 2008, corresponding to day of year (DOY) 43-46 inclusive. The Cassini magnetic field observations are used to provide suitably lagged IMF data which are employed within the Saturn paraboloid magnetosphere model (Alexeev et al, 2006;Belenkaya et al, 2006aBelenkaya et al, , b, 2007Belenkaya et al, , 2008, to calculate the position of the open-closed field line boundary in Saturn's ionosphere. The calculated boundary is then compared with the Southern Hemisphere UV auroral images obtained by the HST.…”

Field‐aligned currents in Saturn's northern nightside magnetosphere: Evidence for interhemispheric current flow associated with planetary period oscillations

“…The main magnetospheric contribution to the azimuthal field instead comes from the magnetopause and tail current system, which bends field lines out of meridian planes from the dayside toward the tail on both sides of the noon-midnight meridian. These fields have been estimated using the paraboloid model of Saturn's magnetic field, specifically the "intermediate" model for a subsolar magnetopause distance of 22 R s [Belenkaya et al, 2008]. These values have thus been routinely computed for each Cassini pass employed here and subtracted from the initial data, such that all future reference in the paper to the "azimuthal field" and products derived therefrom implies use of these The northern inbound and southern outbound segments of these trajectories are projected along model magnetic field lines into the northern and southern ionospheres, respectively, the trajectories being divided at the near-periapsis point on the left of each plot where they achieve their largest colatitude with respect to both northern and southern poles.…”