ESS 200C
Lecture 12
Planetary Magnetospheres
• We have studied the Earth’s magnetosphere in great detail for
over 40 years and think we have developed an understanding
of the fundamental physical processes active here. The next
step is to test those ideas by applying them to other parameter
regimes. Fortunately we have a number of other candidates.
• Mercury, Jupiter, Saturn, Uranus and Neptune have an
interaction similar to that at Earth - a supersonic solar wind
interacts with a magnetic field to form a magnetospheric cavity
but the nature of the obstacle differs greatly as do the solar wind
parameters.
• Jupiter’s moon Ganymede has an intrinsic magnetic field
however it interacts with a plasma wind within Jupiter’s vast
magnetosphere rather than the solar wind.
• Jupiter’s moon Io provides the main source of plasma for
Jupiter’s magnetosphere. Saturn’s moon Enceladus may be a
major source for Saturn’s magnetosphere.
• The Moon has a remanent magnetic field.
• Mars too has localized field concentrations.
• Asteroids may have a strong interaction with the solar wind.
• The ionospheres of Venus and Titan (when outside Saturn’s
magnetosphere) interact with the solar wind flow to form an
induced magnetospheric cavity.
• The small size and large amount of gas that evaporates from a
comet make its interaction with the solar wind unique.
• Europa and Callisto have induced magnetospheres possibly
related to a subsurface ocean. (Ganymede too may have an
induced field but it is small compared to the intrinsic magnetic
field.)
• Mercury
– Mercury has an intrinsic magnetic – Magnetic field changes
field with a dipole moment of ~300
consistent with field aligned
3
12
3
nT RM (3X10 T m ) and a dipole
currents have been reported.
tilt of ~100.
– The magnetic field is strong
enough to stand off the solar wind
at a radial distance of about 2RM.
– Mercury’s magnetosphere
contrasts that at the Earth
because it has no significant
atmosphere or ionosphere.
– Mariner 10 flew through the tail of
Mercury’s magnetosphere and
found evidence of substorm
activity although this is
controversial. MESSENGER will
probe the magnetosphere from
orbit. It flow by a month ago.
• Mars
– Mars does not have a global magnetic field but is
thought to have had one in the distant past.
– Mars Global Surveyor found evidence of crustal
magnetization mainly in ancient cratered Martian
highlands.
– The magnetic signatures are thought to be caused by
remanent magnetism (when a hot body cools below
the Curie temperature in the presence of a strong
magnetic field the body can become magnetized).
– The surface magnetic field is organized in a series of
quasi-parallel linear features of opposite polarity.
– One explanation of this is tectonic activity similar to
sea floor spreading and crustal genesis at Earth. The
field reversals result from reversals in Mar’s magnetic
field.
– The north-south dichotomy is not understood.
• Jupiter
– Jupiter has a magnetic moment
of 1.53X1020Tm3 which is tilted
by 9.70 and points toward 3  202 0
in System 3 coordinates.
– System 3 is a left handed
coordinate system based on
radio measurements.
– Jupiter’s rotation period is 9h
55m 29.7s
.
– Near Jupiter the dipole is not a
good approximation. The
contour plot shows the
magnetic field strength looking
from the north and south poles.
The complex pattern indicates
that higher order multipoles are
important.
– Pioneer 10 encountered the
bow shock at r =109RJ and
the magnetopause at 97RJ.
 Unlike the Earth at Jupiter we
rarely have a solar wind
monitor to help us determine
the dependence of the bow
shock and magnetopause to
the solar wind. We have some
data from the Pioneers and
Voyagers and simulations.
 The position of the subsolar
magnetopause varies with
solar wind dynamic pressure as
p-0.22
 The bow shock and
magnetopause are much closer
together at Jupiter than at
Earth. ( RM RBS  0.88 at Jupiter,
RM RBS  0.75 at Earth)
– End on Jupiter’s
magnetosphere has a
diameter of >20X106 km
making it the largest
object in the solar
system.
– Flow streamlines and velocity magnitude in the magnetosheath.
– These are results from a global magnetohydrodynamic simulation.
– Jupiter’s bow shock is relatively closer to the magnetopause than the
Earth’s.
a
100
100
Y0
Y0
-100
-100
-200
location of a planetary boundary from
fly-by data. The boundary is only
observed along the trajectory of the
spacecraft. Orbiters are better but
only give a limited number of actual
boundary observations.
-100
400
X
0
-200
100
-200
-100
400
X
0
100
b
e
300
300
Z
Z
200
200
100
100
0
-200
-100
400
X
0
0
100
-200
-100
400
X
0
100
c
f
300
300
Z
Z
200
200
100
100
0
-200
-100
0
Y
100
200
0
-200
Dawn-Dusk
– Joy et al. used MHD simulations to
determine the shapes of the
boundaries as a function of dynamic
pressure (10th, 50th and 90th
percentile modes are at the left).
-200
Magnetopause
d
Noon-Midnight
– It is very difficult to determine the
200
Equatorial
• The Shape and Position of
the Jovian Bow Shock and
Magnetopause [Joy et al.,
2002]
Bow Shock
200
-100
0
Y
100
200
figure
– Joy et al. used all of the data at
Jupiter to determine the boundaries
by developing probalistic models.
– Red shows when the spacecraft
were within the magnetosphere
300
– Green shows the magnetosheath
200
28
ULY O
29
– Blue shows the solar wind.
– The boundaries were found to
have a bimodal distribution with 2
preferred locations!
100
Y
– Samples were binned according to
standoff distance and the fraction of
time the spacecraft were within a
given region was found.
30
27
31
CAS
0
P11 O
VG1 i
VG2 i
ULY i
-100
P10 i
1
P11 i
Solar Wind
VG2 O
Magnetosheath
-200
Magnetosphere
0
VG1 BS/MP
VG1 O
-200
P10 O
-100
JA
0
X
100
Figure 1
– That Jupiter had a magnetic
field and therefore a
magnetosphere was known
before the first spacecraft.
 Decimetric emissions were
discovered in 1958 and shown
to be synchrotron radiation
emitted by energetic electrons.
– The first spacecraft to probe
Jupiter’s magnetosphere was
Pioneer 10.
– The outer magnetosphere (r >
60RJ) is extremely variable with
a more dipolar structure than the
middle magnetosphere.
– The middle magnetosphere
(60RJ<r<20RJ) has a strong
equatorial current sheet. The
field is magnetotail like.
– The main source of plasma
for this plasma sheet is in
the inner magnetosphere
(r<20RJ).
 The region near Io contains
a dense donut shaped ring
of heavy ion (sulfur and
oxygen) plasma - the Io
torus.
 Near Jupiter strong radiation
belts are found.
– The outer magnetosphere of
Jupiter is highly variable.
 On the dayside the bow shock was
detected at 86RJ-113RJ and the
magnetopause at 46RJ-110RJ.
– The magnetic field in the outer
magnetosphere is very weak,
complex and continuously
changing
 The magnetic field has Bz<0.
 The magnetic field is much more
dipole like than in the middle
magnetosphere.
– The plasma is very hot and tenous.
 30-40 keV
 10-3 to 10-2 cm-3
– This hot rarefied plasma is mainly
responsible for holding off the solar
wind
– An equatorial current sheet
that is rotating and a few RJ
thick dominates the region
between roughly 20RJ and
60RJ.
– The rotating flow carries an
azimuthal current that
stretches the magnetic field
into a tail-like configuration.
– Observations of the magnetic field from near the equator in the
middle magnetosphere.
– The Galileo spacecraft moved repeatedly through an equatorial
current sheet (left).
–One current sheet crossing (right).
– Since Jupiter’s dipole is tilted with respect to the rotation axis, at
a given position the current sheet moves up and down. It does
not move rigidly. Since information travels at a finite speed the
outer magnetosphere lags behind the rotating planet giving a
warped rotating surface.
–In addition to the
magnetopause and tail
currents Jupiter has an
equatorial current sheet.
–That the equatorial
currents have spiral
streamlines indicates the
presence of radial
currents.
–The middle Jovian
magnetosphere is dominated by
the azimuthal current sheet and
plasma sheet.
–A frictional torque in the
ionosphere accelerates the
plasma to corotation.
–The ionospheric torque is
transmitted to the
magnetosphere via field-aligned
currents that close through
radial currents.
 
– J  B in the magnetosphere is
in the direction toaccelerate the
plasma while J  B in the
ionosphere is in the direction to
slow Jupiter’s rotation.
– The inner magnetosphere is
the region of intense
energetic (>MeV) ions and
electrons. These have their
peak at r~1.9RJ.
– The energetic electrons
generated the synchrotron
radiation that was the first
evidence of Jupiter’s
magnetosphere.
– The radial distribution of
high-energy particles has
large decreases at the orbits
of the moons.
– The volcanic moon Io (r=5.9RJ)
and the Io plasma torus
(5RJ<r<8RJ) dominate inner
magnetosphere physics.
 The plasma torus is the source
of most of the plasma in the
Jovian magnetosphere and is
its densest part.
 The densest part of the torus
(the cold torus) lies inside of Io
(5.7RJ) and contains ~few eV
ions and electrons. It is thought
to be formed by inward diffusing
particles. The ions are S+ and
O+.
 The outer torus (r>5.9RJ)
contains warm plasma (5-10eV
electrons, 10-100eV ions). The
ions are O+,O+2,S+,S2+,S3+, and
SO2+.
 The source strength is
between 6X1027s-1 and
1.7X1028s-1.
 Neutral atoms are sputtered
(the ejection of atoms by
impact of magnetospheric
particles) off Io.
 The neutrals become
ionized by interaction with
electrons of the torus.
– Jovian aurora are as bright as the
brightest seen on Earth.
– Aurora are best observed in the far
ultra-violet (UV) where hydrogen
atoms and molecules radiate but
they also are observed in the nearinfrared , visible and X-ray
wavelengths.
– At high northern and southern
latitudes an auroral oval analogous
to the Earth’s auroral oval can be
found.
– At lower latitudes three lines of
auroral emissions are evident. This
aurora is the ionospheric signature
of the interaction between Jovian
plasma and the moons,
Ganymede, Europa and Io.
– The high latitude aurora map to the
Jovian magnetosphere.
Ganymede
Callisto
Io
Europa
– Io has a strong interaction with
the Jovian plasma. Io is known to
supply the plasma that fills the
Jovian magnetosphere.
 Io most likely behaves like a
conductor.
– When the Jovian plasma
reaches Io it slows down. That
information is sent to Jupiter by
Alfvén waves that propagate
along the field line at the Alfvén
velocity ( CA  B 2  0 ).
 The flux tube at Io will be swept
back by tan   u flow C A  M A
where uflow is the velocity of the
corotating Jovian plasma.
– The Alfvén waves carry field
aligned current between Io
and the Jovian ionosphere.
–Jovian auroral oval and aurorae associated with
Jupiter’s interaction with Io, Europa and Ganymede.
– Ganymede has an internal magnetic field and a magnetosphere.
 The magnetic moment is 1.4X1013Tm3 with an equatorial field
strength of ~750nT.
 The dipole is tilted by ~100 relative to the spin axis and points to 2000
Ganymede east longitude (00 faces Jupiter).
 Ganymede’s magnetic field is thought to be generated in a molten
core.
– Ganymede’s magnetic field is strong enough to stand off Jupiter’s
magnetic field and plasma.
 At Ganymede’s orbit (14.97RJ) the Alfvén Mach number is <1
implying that it is magnetic pressure ( B 2 2 0 ) rather than dynamic
2
pressure (  u ) that confines the magnetosphere
– Ganymede’s field is
approximately opposite to
that of Jupiter so it is thought
to be reconnecting.
 The field lines going upward
and downward are
equivalent to the lobe fields
at the Earth.
 The closed field region is
small.
 The properties vary with the
10.5 hours synodic period of
Jupiter’s rotation. This is
predictable at Ganymede
unlike the variations in
planetary magnetospheres.
 Note the reconnection site
is always near the equator.
– On its G8 orbit Galileo
passed onto the closed field
lines of Ganymede.
– Trapped energetic electrons
like those found in the
Earth’s magnetosphere have
been observed in
Ganymede’s
magnetosphere.
– The distribution has loss
cones near small pitch
angles (00 and 1800) and a
depression at 900. At Earth
this is called a”butterfly”
distribution and is consistent
with electrons drifting in the
inferred magnetosphere.
Pitch Angle
– Most likely neither Europa nor
– Europa and Callisto are not
Callisto has an internal magnetic
thought to have sufficient
field.
atmospheres to support these
currents.
– As Jupiter rotates the magnetic
 The skin depth would have to be
field at Europa has a time varying
comparable to the planet radius.
amplitude of ~230nT (synodic
period 11.1 hours) while that at
– A subsurface ocean is most likely.
Callisto is ~40nT (synodic period
 With conductivity of sea water
10.1 hours).
a depth of 10 km would
– The time varying magnetic field will
suffice.
induce currents in the moons.
– The critical test occurred on the
– Let the moons have a conducting
E26 orbit. If the interaction was
shell near the surface then
with a permanent dipole the point

B
1

2B
would have been at the triangle.
t   0

2
 B  i  0 B
d  (  0 2)
1
2
where d is the skin depth. It
characterizes the depth to which a
wave can penetrate a conductor.
Kivelson et al. 2000
– Images of Europa’s surface
also are consistent with an
ocean at some time.
 Impact craters are rare
(young surface age). Ridges
and other linear features are
common (caused by tidal
deformation?)
27km diameter
ridges
impact crater
 Area of “chaos terrain”,
caused by partial melting
of surface material?
“Icebergs” are 1-10km
across.
50km
– The perturbations to the spacecraft orbit determined by
Doppler shifts of the radio signal give us density and some
idea of how much mass is concentrated at the center. The rest
depends on our model assumptions.
– The magnetometer tells us if there is a magnetic field in the
core or if the inside is conducting.
 Two-layer:
silicate shell, iron
core
Europa
 Iron core,
silicate
mantle, thin water/
ice shell
Io
Callisto
 Io plus 800km
of water/ice
Ganymede
Undifferentiated ?
Mixture of rock/ice/
metal,with thin ice
shell
• Saturn, Uranus and Neptune are magnetized.
Planet
Distance
(AU)
Magnetic Tilt
Magnetopause
Moment Angle
Distance
(ME)
(degrees) Km
Rplanet
Earth
Jupiter
Saturn
Uranus
Neptune
1.0
5.2
9.5
19.2
30.1
1
20,000
580
49
27
10.8
9.7
<1
59
47
0.7X105
30-70X105
12x105
6.9X105
6.3X105
11
45-100
21
27
26
– Saturn has an axially symmetric inner magnetosphere while
Jupiter’s 100 tilt spreads out the Io torus.
– At present Uranus has an Earth-like magnetosphere since
the 600 tilt is from a rotation axis pointing at the Sun.
– At Neptune the dipole axis relative to the solar wind
undergoes large variations.
• Because of the zero tilt Saturn’s
magnetosphere is simple?
– The outer magnetosphere
rotates with the planet.
– There is a plasma torus with H+
associated with Titan however
the Titan source is not
continuous since it spends time
in the solar wind.
– The highest density is at 6RS
and has a contribution from ring
material.
– However the biggest surprise
from Cassini is that the biggest
source may be a moon.
– The profile of radiation belt
particles shows strong losses at
the orbits of Saturn’s moons.
Gombosi and Hansen, 2005
Cassini Observations of Saturn’s Current Sheet
It has an 11 hour periodicity!
– Cassini plasma observations
show that water group ions
dominate.
Sittler et al., 2005
– The highest densities were
found near Enceledus.
– The ions are corotating.
– On the first Enceladus fly-by
magnetic field observations
showed that the field was “piled
up” on the moon.
– Images show a large plume of
material coming from the
southern pole.
– Enceladus seems to be a major
source for plasma at Saturn.
From PDS
• Uranus magnetosphere looks very
much like that of Earth.
– There are two ideas why Uranus’
magnetic axis is so far from the
rotation axis.
– We measured the magnetic field
while Uranus was undergoing a
field reversal.
– Uranus dynamo operates in a
different location than Earth,
Jupiter, Saturn etc.
– Uranus has radiation belts.
• During one rotation Neptune’s
configuration chances greatly.
– The spin axis is inclined by 280 with
respect to the ecliptic.
– The inclination of the dipole axis
with respect to the plane of the
ecliptic varies from 140 to 720.
– Neptune has a weak radiation belt
near Triton and appears to be the
solar system’s least active.
Uranus
Neptune
Neptune’s Magnetosphere
• Simple pressure balance
arguments give the stand off
distances at Earth, Saturn,
Uranus and Neptune but fail at
Jupiter because of the strong
internal source of plasma.
– Jupiter’s magnetosphere is
“sharper” than the others
because of the rotating plasma.
– A shock forms at the nose of a
supersonic airplane. Similarly
the shock forms close to
Jupiter’s magnetopause.
• The magnetosonic Mach number (
)
goes from 6 near the Earth to 10 near Saturn.
p


• Plasma beta (
( B 2 20 ) ) peaks at Mars and
decreases in outer solar system.
• Bow shocks are stronger in the outer solar system.
1
M MS  uSW (Cs2  C A2 ) 2
– The large overshoot in B just downstream is a signature of
strong shocks.