Abstract
Abstract. The paraboloid model of Saturn's magnetosphere describes the magnetic field as being due to the sum of contributions from the internal field of the planet, the ring current, and the tail current, all contained by surface currents inside a magnetopause boundary which is taken to be a paraboloid of revolution about the planet-Sun line. The parameters of the model have previously been determined by comparison with data from a few passes through Saturn's magnetosphere in compressed and expanded states, depending on the prevailing dynamic pressure of the solar wind. Here we significantly expand such comparisons through examination of Cassini magnetic field data from 18 near-equatorial passes that span wide ranges of local time, focusing on modelling the co-latitudinal field component that defines the magnetic flux passing through the equatorial plane. For 12 of these passes, spanning pre-dawn, via noon, to post-midnight, the spacecraft crossed the magnetopause during the pass, thus allowing an estimate of the concurrent subsolar radial distance of the magnetopause R1 to be made, considered to be the primary parameter defining the scale size of the system. The best-fit model parameters from these passes are then employed to determine how the parameters vary with R1, using least-squares linear fits, thus providing predictive model parameters for any value of R1 within the range. We show that the fits obtained using the linear approximation parameters are of the same order as those for the individually selected parameters. We also show that the magnetic flux mapping to the tail lobes in these models is generally in good accord with observations of the location of the open-closed field line boundary in Saturn's ionosphere, and the related position of the auroral oval. We then investigate the field data on six passes through the nightside magnetosphere, for which the spacecraft did not cross the magnetopause, such that in this case we compare the observations with three linear approximation models representative of compressed, intermediate, and expanded states. Reasonable agreement is found in these cases for models representing intermediate or expanded states.
Highlights
The availability of empirically determined models of magnetospheric magnetic fields is a valuable resource in many areas of related research, providing knowledge in particular of the mapping of features and phenomena along field lines between the magnetosphere and the planetary ionosphere
In addition to the internal planetary field, taken to be the three-term axisymmetric model derived by Burton et al (2010) in the most recent work, this model contains representations of the field due to the ring current and the tail current, all contained within the paraboloid magnetopause by the surface current flowing on that boundary, to which a penetrating field due to the IMF can be added
While the procedure described above is appropriate to the Revs which span the dawn, noon, and dusk sectors, for which the orbits intersect the magnetopause as shown in Fig. 1, it is clearly not appropriate to the nightside passes on Revs 23– 26 for which this is not the case. In these cases we compare these data with a set of models representative of compressed, intermediate, and expanded conditions, using updated model parameters determined in Sect. 5 from fitting to the data derived from Revs 17–19, 145, 146, and 163
Summary
The availability of empirically determined models of magnetospheric magnetic fields is a valuable resource in many areas of related research, providing knowledge in particular of the mapping of features and phenomena along field lines between the magnetosphere and the planetary ionosphere. We focus on the global “paraboloid model” of Saturn’s magnetosphere, in which the outer magnetopause boundary is taken to form a paraboloid of revolution about the planet-Sun line (Alexeev et al, 2006). This model has been used, in particular, to investigate the dependence of the magnetospheric magnetic structure and the origins of the aurorae on the direction and strength of the interplanetary mag-. In this paper we significantly expand the comparison of the model with observations by employing Cassini magnetic field data from 18 passes through the magnetosphere from apoapsis to periapsis, or vice versa, that cover all principal local time (LT) regimes. The results are used to empirically determine best-fit equations of how the model parameters depend on system size, taken for simplicity to depend linearly on the distance to the subsolar magnetopause
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