P43A0492 PROPERTIES OF SATURNS MAGNETOPAUSE II'

1
CASSINI MAGNETOMETER TEAM
P43A-0492 PROPERTIES OF SATURNS MAGNETOPAUSE II.
TIME-DEPENDENT MORPHOLOGY AND DYNAMICS
N. Achilleos1, C. S. Arridge1, C Bertucci1, M. K.
Dougherty1, K. K. Khurana2, M. E. Burton3, C. T.
Russell2, E. J. Smith3 , B. T. Tsurutani3 1 The
Blackett Laboratory, Imperial College London, SW7
2AZ, UK 2 UCLA Institute of Geophysics and
Planetary Physics, Los Angeles, CA 90024, USA 3
Jet Propulsion Laboratory / Caltech, 4800 Oak
Grove Drive, Pasadena, CA 91109 United
States Contact christopher.arridge_at_imperial.ac.uk
n.achilleos_at_imperial.ac.uk
INTRODUCTION The shape of Saturns magnetopause
(MP), and those of other planets, has a complex,
time-dependent geometry. The location of the
magnetopause surface is determined by the balance
between the magnetic pressure of the
magnetosphere and the thermal and dynamic
pressure of the solar wind. The shape of the
magnetopause depends crucially on the
distribution of stress in the magnetosphere
itself. We refer here to two existing models of
Saturns magnetopause. Namely, the empirical
model by Slavin et al. 1 and the theoretical
model by Maurice et al. 2. We present the new
technique of Arridge et al. (2005, in progress)
for fitting a magnetopause model without the need
for simultaneous upstream dynamic pressure
measurements. This type of modelling is important
for the Cassini mission during mission segments
where observed solar wind plasma parameters are
scarce, and thus proxies for these quantities
would be valuable. This technique has been
applied in order to generate a new model for the
Kronian magnetopause - we describe the dependence
of the size and shape of this model on solar wind
dynamic pressure DP. UNITS Unless otherwise
indicated, distances are in units of Saturn radii
(1 RS 60330 km) and pressures are in
nano-Pascals (nPa). CO-ORDINATES The KSM
(Kronocentric Solar Magnetospheric) system has
Saturn at the origin, X positive towards the Sun,
and Z perpendicular to X and oriented so that
Saturns magnetic/rotation axis lies in the XZ
plane.

DYNAMICS
MAGNETOPAUSE (MP) MODEL FUNCTIONAL FORM
EFFECT OF DIPOLE TILT
BEST-FIT SHAPE MODEL
? The first six Cassini orbits were sampled at 5
min intervals, including the regions where the MP
was traversed. The best fit MP shape (with a4 set
to 0) was applied in order to determine the
probability of a particular standoff distance
being inside or outside the magnetosphere. The
resulting distributions are shown below, along
with corresponding solar wind pressure from the
model. We intend to add further orbits to the
analysis, in order to improve the estimates of
quantities such as the mean and standard
deviation for the standoff distance (? and ?). No
bimodality is evident as yet for the sample,
unlike the case for Jupiter in the original
analysis of Joy et al 4.
The dipole tilt angle ? between Saturns magnetic
axis and the ZKSM direction is -20? for the
Cassini era. As a result, the MP becomes
distorted or hinged its tail is deflected
along the North-South direction and the nose is
pushed in the opposite direction. To try and
correct for this effect, Arridge et al (2005)
apply a distance - dependent rotation to
unhinge the position ri of each MP crossing. RH
15 RS is a fixed hinging distance. For ri ltlt
RH rotation is through angle ? and for ri gtgt RH
dipole tilt has negligible assumed effect.
DP 0.01nPa
?Functional Forms In these formulae, r
represents radial distance from Saturn r0 is the
magnetopause standoff distance (i.e. the radial
location of the subsolar nose of the MP
surface) ? is the angle subtended at the planet
between the subsolar direction and the position
of a point on the MP surface K is a flaring
parameter DP is solar wind dynamic pressure
and the ai are constants. This MP shape was
originally introduced by Shue et al. 3 for
terrestrial modelling. The difference between
observed and modelled r is minimized for fitting
ai coefficients
DP 0.1nPa
Model Parameters a1 10.39?0.9 a2
0.2184?0.02 a3 0.6763?0.02 a4 -1.496?0.2 ?
1/ a2 4.579 ?0.5
STANDOFF DISTANCE VERSUS SOLAR WIND DYNAMIC
PRESSURE MAGNETODISC EFFECT
COMPARISON OF MODELS
OBTAINING DYNAMIC PRESSURE (DP)
Because there are no consolidated measurements of
solar wind dynamic pressure DP at the MP
crossings, this quantity has been derived by
applying the following Newtonian pressure balance
Magnetic Pressure
Dynamic Pressure
Static Pressure
? here is the angle between the model MP surface
normal and the solar wind flow and BMS is the
magnetospheric field strength measured by the
Cassini magnetometer at each crossing.
The inverse index ? in this relation is 4.6 -
significantly smaller than 6, the vacuum dipole
value. This strongly suggests that the internal
source of magnetospheric plasma operating at
Saturn is affecting the magnetopause shape. In
addition, the centrifugal force on the cold
(sub)-corotating plasma from this source distorts
the magnetic field into a disk-like
configuration in the equatorial outer
magnetosphere. The MP surface apparently bends
more strongly around this obstacle with
increasing dynamic pressure (since a4 lt 0,
flaring parameter K decreases with increasing DP).
Cassini outside magnetosphere
REFERENCES 1. Slavin, J. A. et al., Solar wind
flow about the outer planets Gas dynamic
modeling of Jupiter and Saturn bow shocks, JGR
A, 90, 6275 (1985). 2. Maurice, S. et al.,
Geometry of Saturns magnetopause model, JGR,
101, 27053 (1996) 3. Shue, J.-H. et al., A new
functional form to study the solar wind control
of the magnetopause size and shape, JGR A, 102,
9497 (1997) 4. Joy, S. P. et al., Probabilistic
models of the Jovian magnetopause and bow shock
locations, JGR A, 107, 1309 (2002) 5. Jackman,
C. M. et al, Interplanetary conditions and
magnetospheric dynamics, JGR A, 110, A102212
(2005)
(Model by Arridge et al this work)
Cassini inside magnetosphere