P43A0945 PROPERTIES OF SATURNS MAGNETOPAUSE I'

1
CASSINI MAGNETOMETER TEAM
P43A-0945 PROPERTIES OF SATURNS MAGNETOPAUSE I.
MAGNETIC STRUCTURE AND GEOMETRY
M. E. Burton3, N. Achilleos1, C. S. Arridge1, C
Bertucci1, M. K. Dougherty1, K. K. Khurana2, 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 n.achilleos_at_imperial.ac.uk
christopher.arridge_at_imperial.ac.uk
c.bertucci_at_imperial.ac.uk
INTRODUCTION We present some of the observations
taken by the magnetometer onboard the Cassini
spacecraft, at times when it traversed Saturns
magnetopause during its initial orbits. The
large-scale shape of Saturns magnetopause (MP)
has been empirically modelled by Slavin et al.
1 (using Voyager and Pioneer data) and
theoretically by Maurice et al. 2. More
recently, geometrical modelling by Arridge et al.
(described in P43A-0942, this meeting) has
produced a shape model whose degree of flaring
at the magnetospheric flanks is anti-correlated
with (inferred) solar wind dynamic pressure. Here
we compare geometrical normal vectors derived
from these MP models with those obtained by
minimum variance analysis (MVA) of the rapid
rotations in the magnetic data, which probably
indicate magnetopause current layers (MPCL) (see
e.g. Sonnerup and Scheible 3). The models which
are more streamlined away from the subsolar
region have normals which generally well-aligned
with the MVA normals. We also indicate probable
magnetic signatures of wave activity and MP
boundary layers. Further confirmation of the
possible origins of these structures such as
mirror mode instability, plasma depletion layers,
mixing of particle populations at the MP
await correlative studies with other Cassini
datasets. Instances of strong variability in MVA
normal orientation between successive MP
encounters could indicate surface waves
propagating along the MP surface for example,
surface distortions arising from the
Kelvin-Helmholtz instability, acting at the MP
boundary which may separate plasmas of very
different bulk velocity. 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.

COMPARISON OF OBSERVED AND MODEL MAGNETOPAUSE
GEOMETRY
ANATOMY OF MAGNETOPAUSE (MP) ENCOUNTERS
BOUNDARY LAYERS AND WAVES
? The axisymmetric MP surface models by Slavin et
al. and by Arridge et al. (with and without
anti-correlation between flaring and solar wind
dynamic pressure) are shown in cylindrical KSM
coordinates. Also shown are the projected MVA
normals (coloured lines) determined for a
representative set of magnetopause encounters at
locations indicated by circular symbols. The
surface models have similar subsolar curvatures
but differ at the flank of the magnetosphere.
? MVA analysis of MP encounter 1
? Magnetic data from inbound leg of Cassini orbit
B.
Magnetometer data shown here for the inbound leg
of Orbit B illustrates some of the multiple MP
encounters of the Cassini dataset. Such periods
with frequent encounters are usually indicative
of a rapidly moving magnetopause surface and / or
ripples or waves propagating along the MP
boundary (e.g. 4). The results of a minimum
variance analysis (MVA) of the first MP crossing
(marked 1) are shown above right. The
components B1, B2 and B3 are associated with the
directions of maximum, intermediate and minimum
variance for the time interval indicated.
Hodograms of the field vector are shown in the
B1-B2 plane (ideally parallel to the MP surface)
and B2-B3 plane. The crossings analysed here for
the indicated orbits are generally low-shear
crossings (angle between magnetosheath and
magnetospheric field 20-30?). For downstream
local times further from noon, this reflects the
close alignment of the magnetosheath field with
that of Saturns magnetospheric lobe.
Angle between MVA and model normal (deg)
?Magnetic Topography The ratio of MVA
eigenvalues are shown on a colour scale as a
function of time, and of the length (in seconds)
of the MVA interval. ?1 ?3 and ?2 are the
respective maximum, minimum and intermediate
variances of the field. The ratio ?2 /?3 gt 10
indicates a well-determined minimum variance
direction, identified with the direction normal
to the magnetopause. Well-determined MVA
structures for MP encounters often appear over a
wide range of time / length scales, as seen by
the vertical smearing of the brighter colours.
The ?2 /?3 panel shows the correlation between
the well-determined structures and the multiple
MP encounters shown by the high gradients in
total field strength (yellow trace). The bottom
panels indicate angle between average field and
MVA normal (90? for a well-defined MP crossing),
and ratio of normal to average field.
increasing time
? Further analysis is required to confirm whether
any apparent oscillations in deviation angle
are due to waves propagating along the
magnetopause surface (e.g. Huddleston et al 4),
which may arise from Kelvin-Helmholtz
instability(for example).
? Inbound intervals of magnetic data from Orbit
(Rev) A show the signatures of possible boundary
layers near the magnetopause, the magnetopause
current layer (MPCL) and possible magnetospheric
plasma boundary structures. Plasma depletion
layers (PDL) could be responsible for intervals
of elevated field strength on the magnetosheath
side of MPCL structures. Correlative studies
with plasma data are desirable for confirming the
nature of these extended boundaries.
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. Sonnerup, B. U. O. and
Scheible, M. in Analysis Methods for
Multi-Spacecraft Data, ed. Paschmann and Daly,
ISSI, Bern, 1998 4. Huddleston. D. E. et al,
Magnetopause structure and role of reconnection
at the outer planets, JGR, 102, 24289 (1997)