MercuryESS 200CESS 200CLecture 12Lecture 12Planetary 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 magneticfield.)•Mercury– Mercury has an intrinsic magnetic field with a dipole moment of ~300 nT RM3 (3X1012T m3) and a dipole 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.– Magnetic field changes consistent with field aligned currents have been reported.•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 crustalmagnetization 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 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.03202=λ– Pioneer 10 encountered the bow shock at r =109RJand 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≈BSMRR75.0≈BSMRR– 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.figureY-200 -100 0 100 2000100200300400fZDawn-DuskY-200 -100 0 100 2000100200300400cZX-200 -100 0 1000100200300400eZNoon-MidnightX-200 -100 0 1000100200300400bZX-200 -100 0 100-200-1000100200dM agnetopauseYEquatorialX-200 -100 0 100-200-1000100200aBow Shoc kY• 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, 50thand 90thpercentile modes are at the left).So l ar Wi n dMagnetosheathMagnetosphereJAP10OVG1OVG2OP11iVG2iP10iUL YiVG1iP11OUL YO2831270VG1 B S/MPCASFi112930XY-200 -100 0 100-200-1000100200300– 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!– 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.– 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 electons.– 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 outer magnetosphere of Jupiter is highly