Optimization of Saturn paraboloid magnetospheric field model parameters using Cassini equatorial magnetic field dataстатья
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Дата последнего поиска статьи во внешних источниках: 6 февраля 2017 г.
Аннотация: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.