Optimization of Saturn paraboloid magnetospheric field model parameters using Cassini equatorial magnetic field data
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 parabol...
| Main Authors: | , , , , , , , |
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| Format: | Article |
| Language: | English |
| Published: |
Copernicus Publications
2016-07-01
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| Series: | Annales Geophysicae |
| Online Access: | https://www.ann-geophys.net/34/641/2016/angeo-34-641-2016.pdf |
| Summary: | 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 <i>R</i><sub>1</sub> 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 <i>R</i><sub>1</sub>, using least-squares linear
fits, thus providing predictive model parameters for any value of <i>R</i><sub>1</sub> 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. |
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| ISSN: | 0992-7689 1432-0576 |