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.

ESS 200CESS 200CLecture 12Lecture 12

Planetary MagnetospheresPlanetary 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

field with a dipole moment of ~300 nT RH

3 (3X1012 T m3) and a dipole tilt of ~100 (1 RH = 2440 km = 0.38 RE).

– 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 is probing the magnetosphere from orbit.

– Magnetic field changes consistent with field aligned currents have been reported.

– 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.

• Mars (1 RM = 3390 km = 0.53 RE)

• The magnetosonic Mach number ( ) goes from 6 near the Earth to 10 near Saturn.

• Plasma beta ( ) peaks at Mars and decreases in outer solar system.

• Bow shocks are stronger in the outer solar system.– The large overshoot in B just downstream is a signature of

strong shocks.

21

)( 22AsSWMS CCuM +=

)2( 02 μβ

Bp=

• 1 RJ = 11.2 RE

• 1 RS = 9.45 RE

• 1 RU = 4.00 RE

• 1 RN = 3.88 RE

Planet Distance (AU)

Magnetic Moment (ME)

Tilt Angle (degrees)

Magnetopause Distance

Km Rplanet

Earth 1.0 1 10.8 0.7X105 11 Jupiter 5.2 20,000 9.7 30-70X105 45-100 Saturn 9.5 580 <1 12x105 21 Uranus 19.2 49 59 6.9X105 27 Neptune 30.1 27 47 6.3X105 26

•Jupiter, Saturn, Uranus and Neptune are magnetized.

• Jupiter– Jupiter has a magnetic moment

of 1.53X1020Tm3 which is tilted by 9.70 and points toward 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.

03 202=λ

– 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. ( at Jupiter,

at Earth)

– End on Jupiter’s magnetosphere has a diameter of >20X106 km making it the largest object in the solar system.

88.0≈BSM RR75.0≈BSM RR

– 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.

figure 4Y-200 -100 0 100 2000

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• The Shape and Position of the Jovian Bow Shock and Magnetopause [Joy et al., 2002]

– It is very difficult to determine the 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.

– 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).

Solar Wind

Magnetosheath

Magnetosphere

JAP10OVG1O

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Figure 1

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– 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

– Green shows the magnetosheath

– Blue shows the solar wind.

– Samples were binned according to standoff distance and the fraction of time the spacecraft were within a given region was found.

– The boundaries were found to have a bimodal distribution with 2 preferred locations!

– 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.

–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.

– in the magnetosphere is in the direction to accelerate the plasma while in the ionosphere is in the direction to slow Jupiter’s rotation.

BJrr

×

BJrr

×

Reconnection in Jovian magnetosphere is driven by mass loading and rotation. Vasyliunas drew this cartoon to show expected reconnection geometry.

Galileo observations show plasmoids, and reconnection signatures.

Predominantly in the dusk local time sector.

Consistent with Vasyliunas picture.

– 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 near-infrared , 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.

Ganymede15 RJ orbit2631 km radius

Callisto26.3 RJ orbit2410 km radius

Io5.9 RJ orbit1821 km radius

Europa9.4 RJ orbit1561 km radius

– 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 ( ).

The flux tube at Io will be swept back by where uflow is the velocity of the corotating Jovian plasma.

ρμ 02BCA =

AAflow MCu ==αtan

– 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 ( ) rather than dynamic pressure ( ) that confines the magnetosphere

02 2μB

2uρ

– 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 Callisto has an internal magnetic field.

– As Jupiter rotates the magnetic field at Europa has a time varying amplitude of ~230nT (synodic period 11.1 hours) while that at Callisto is ~40nT (synodic period 10.1 hours).

– The time varying magnetic field will induce currents in the moons.

– Let the moons have a conducting shell near the surface then

where is the skin depth. It

characterizes the depth to which a wave can penetrate a conductor.

– Europa and Callisto are not thought to have sufficient atmospheres to support these currents. The skin depth would have to be

comparable to the planet radius.– A subsurface ocean is most likely.

With conductivity of sea water a depth of 10 km would suffice.

– The critical test occurred on the E26 orbit. If the interaction was with a permanent dipole the point would have been at the triangle.

21

)2(

1

0

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ωμσ

σμω

μσ

=

=∇

∇=∂∂

BiB

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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 diameterimpact crater

ridges

Area of “chaos terrain”, caused by partial melting of surface material? “Icebergs” are 1-10km across.

50km

Io

Europa

Callisto

Ganymede

– The perturbations to the spacecraft orbit determined by The perturbations to the spacecraft orbit determined by Doppler shifts of the radio signal give us density and some Doppler shifts of the radio signal give us density and some idea of how much mass is concentrated at the center. The idea of how much mass is concentrated at the center. The rest depends on our model assumptions.rest depends on our model assumptions.– The magnetometer tells us if there is a magnetic field in the The magnetometer tells us if there is a magnetic field in the core or if the inside is conducting. core or if the inside is conducting.

Two-layer: silicate shell, iron core

Io plus 800km of water/ice

Iron core, silicatemantle, thin water/ice shell

Undifferentiated ?Mixture of rock/ice/metal,with thin ice shell

– 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.

Planet Distance (AU)

Magnetic Moment (ME)

Tilt Angle (degrees)

Magnetopause Distance

Km Rplanet

Earth 1.0 1 10.8 0.7X105 11 Jupiter 5.2 20,000 9.7 30-70X105 45-100 Saturn 9.5 580 <1 12x105 21 Uranus 19.2 49 59 6.9X105 27 Neptune 30.1 27 47 6.3X105 26

• 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.

– 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.

Sittler et al., 2005

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.