J. E. P. Connerney

New models of Jupiter's magnetic field constrained by the Io flux tube footprint

Spherical harmonic models of the planetary magnetic field of Jupiter are obtained from in situ magnetic field measurements and remote observations of the position of the foot of the Io flux tube in Jupiters ionosphere. The Io flux tube (IFT) footprint locates the ionospheric footprint of field lines traced from Ios orbital radial distance in the equator plane (5.9 Jovian radii). The IFT footprint is a valuable constraint on magnetic field models, providing “ground truth” information in a region close to the planet and thus far not sampled by spacecraft. The magnetic field is represented using a spherical harmonic expansion of degree and order 4 for the planetary (“internal”) field and an explicit model of the magnetodisc for the field (“external”) due to distributed currents. Models fitting Voyager 1 and Pioneer 11 magnetometer observations and the IFT footprint are obtained by partial solution of the underdetermined inverse problem using generalized inverse techniques. Dipole, quadrupole, octupole, and a subset of higher-degree and higher-order spherical harmonic coefficients are determined and compared with earlier models.

The solar wind interaction with Mars: Locations and shapes of the bow shock and the magnetic pile-up boundary from the observations of the MAG/ER Experiment onboard Mars Global Surveyor

The Mars Global Surveyor spacecraft was inserted into an elliptical orbit around Mars on September 12, 1997. It includes the MAG/ER instrument with two magnetometers providing in-situ sensing of the ambient magnetic field and an electron reflectometer measuring the local distribution function of the electrons in the energy range of 10 eV to 20 keV. This statistical study deals with the identification and the position of the Bow Shock (BS) and of another plasma boundary, the Magnetic Pile-up Boundary (MPB), proved as permanent by MAG/ER. During the first year of the MGS mission, a total of 290 orbits have been considered to fit the geometric characteristics of these boundaries. The position and shape of these boundaries are compared with previous studies. Good agreement is found with the Phobos 2 observations, suggesting than the mean bow shock and MPB locations are independent of solar cycle phase. The great number of crossings shows that the Bow Shock position and nightside MPB position are highly variable.

The Global Magnetic Field of Mars and Implications for Crustal Evolution

The Mars Global Surveyor spacecraft obtained globally-distributed vector magnetic field measurements approximately 400 km above the surface of Mars. These have been compiled to produce the first complete global magnetic field maps of Mars. Crustal magnetization appears dichotomized, with intense magnetization mainly confined to the ancient, heavily cratered highlands in the south. The global distribution of sources is consistent with a reversing dynamo that halted early in Mars evolution. Intense crustal magnetization requires an increased oxidation state relative to mantle-derived rock, consistent with assimilation of an aqueous component at crustal depths.

Magnetic fields of the outer planets

It is difficult to imagine a group of planetary dynamos more diverse than those visited by the Pioneer and Voyager spacecraft. The magnetic field of Jupiter is large in magnitude and has a dipole axis within 10° of its rotation axis, comfortably consistent with the paleomagnetic history of the geodynamo. Saturns remarkable (zonal harmonic) magnetic field has an axis of symmetry that is indistinguishable from its rotation axis (≪1° angular separation); it is also highly antisymmetric with respect to the equator plane. According to one hypothesis, the spin symmetry may arise from the differential rotation of an electrically conducting and stably stratified layer above the dynamo. The magnetic fields of Uranus and Neptune are very much alike, and equally unlike those of the other known magnetized planets. These two planets are characterized by a large dipole tilts (59° and 47°, respectively) and quadrupole moments (Schmidt-normalized quadrupole/dipole ratio ≈1.0). These properties may be characteristic of dynamo generation in the relatively poorly conducting “ice” interiors of Uranus and Neptune. Characteristics of these planetary magnetic fields are illustrated using contour maps of the field on the planets surface and discussed in the context of planetary interiors and dynamo generation.

Images of Excited H3+ at the Foot of the lo Flux Tube in Jupiter's Atmosphere

The electrodynamic interaction between lo and the Jovian magnetosphere drives currents to and from the planets ionosphere, where H3+ emission is excited. Direct images of this phenomenon were obtained with the ProtoCAM infrared camera at the National Aeronautics and Space Administrations 3-m Infrared Telescope Facility. The emissions are localized to the instantaneous foot of the lo flux tube, ≈8� equatorward of the more intense auroral H3+ emission associated with higher magnetic latitudes. The foot of the lo flux tube leads that of (undisturbed) model magnetic field lines passing through lo by 15� to 20� in longitude and is less visible in the northern hemisphere at longitudes where the surface magnetic field strength is greatest. These data favor the unipolar inductor model of the lo interaction and provide insight into the source location and generation of Jovian decameter radio emission.

Hubble Space Telescope imaging of Jupiter's UV aurora during the Galileo orbiter mission

Hubble Space Telescope (HST) Wide-Field Planetary Camera 2 (WFPC 2) images of Jupiters aurora have been obtained close in time with Galileo ultraviolet spectrometer (UVS) spectra and in situ particles, fields, and plasma wave measurements between June 1996 and July 1997, overlapping Galileo orbits G1, G2, G7, G8, and C9. This paper presents HST images of Jupiters aurora as a first step toward a comparative analysis of the auroral images with the in situ Galileo data. The WFPC 2 images appear similar to earlier auroral images, with the main ovals at similar locations to those observed over the preceding 2 years, and rapidly variable emissions poleward of the main ovals. Further examples have been observed of the equatorward surge of the auroral oval over 140–180° longitude as this region moves from local morning to afternoon. Comparison of the WFPC 2 reference auroral ovals north and south with the VIP4 planetary magnetic field model suggests that the main ovals map along magnetic field lines exceeding 15 RJ, and that the Io footprint locations have lead angles of 0–10° from the instantaneous magnetic projection. There was an apparent dawn auroral storm on June 23, 1996, and projections of the three dawn storms imaged with HST to date demonstrate that these appear consistently along the WFPC 2 reference oval. Auroral emissions have been consistently observed from Ios magnetic footprints on Jupiter. Possible systematic variations in brightness are explored, within factor of 6 variations in brightness with time. Images are also presented marked with expected locations of any auroral footprints associated with the satellites Europa and Ganymede, with localized emissions observed at some times but not at other times.

Far-Ultraviolet Imaging of Jupiter's Aurora and the Io “Footprint”

Far-ultraviolet images of Jupiter from the Hubble Space Telescope Wide Field Planetary Camera 2 reveal polar auroral emissions at 300 kilometer resolution and three times higher sensitivity than previously achieved. Persistent features include a main oval containing most of the emission and magnetically connected to the middle magnetosphere, diffuse and variable emissions poleward of the main oval, and discrete emission from Ios magnetic footprint equatorward of the oval. The auroral emissions are variable, exhibit magnetic conjugacy, and are visible above the planet limb. All emissions approximately co-rotate with Jupiter except the Io “footprint,” which is fixed along Ios magnetic flux tube.

Morphological differences between Saturn's ultraviolet aurorae and those of Earth and Jupiter

It has often been stated that Saturns magnetosphere and aurorae are intermediate between those of Earth, where the dominant processes are solar wind driven, and those of Jupiter, where processes are driven by a large source of internal plasma. But this view is based on information about Saturn that is far inferior to what is now available. Here we report ultraviolet images of Saturn, which, when combined with simultaneous Cassini measurements of the solar wind and Saturn kilometric radio emission, demonstrate that its aurorae differ morphologically from those of both Earth and Jupiter. Saturns auroral emissions vary slowly; some features appear in partial corotation whereas others are fixed to the solar wind direction; the auroral oval shifts quickly in latitude; and the aurora is often not centred on the magnetic pole nor closed on itself. In response to a large increase in solar wind dynamic pressure Saturns aurora brightened dramatically, the brightest auroral emissions moved to higher latitudes, and the dawn side polar regions were filled with intense emissions. The brightening is reminiscent of terrestrial aurorae, but the other two variations are not. Rather than being intermediate between the Earth and Jupiter, Saturns auroral emissions behave fundamentally differently from those at the other planets.

Magnetic fields at the solar wind termination shock

A transition between the supersonic solar wind and the subsonic heliosheath was observed by Voyager 1, but the expected termination shock was not seen owing to a gap in the telemetry. Here we report observations of the magnetic field structure and dynamics of the termination shock, made by Voyager 2 on 31 August–1 September 2007 at a distance of 83.7 au from the Sun (1 au is the Earth–Sun distance). A single crossing of the shock was expected, with a boundary that was stable on a timescale of several days. But the data reveal a complex, rippled, quasi-perpendicular supercritical magnetohydrodynamic shock of moderate strength undergoing reformation on a scale of a few hours. The observed structure suggests the importance of ionized interstellar atoms (‘pickup protons’) at the shock.

Jupiter's magnetic field and magnetosphere

Among the planets of the solar system, Jupiter is unique in connection with its size and its large magnetic moment, second only to the suns. The Jovian magnetic field was first detected indirectly by radio astronomers who postulated its existence to explain observations of nonthermal radio emissions from Jupiter at decimetric and decametric wavelengths. Since the early radio astronomical studies of the Jovian magnetosphere, four spacecraft have flown by the planet at close distances and have provided in situ information about the geometry of the magnetic field and its strength. The Jovian magnetosphere is described in terms of three principal regions. The inner magnetosphere is the region where the magnetic field created by sources internal to the planet dominates. The region in which the equatorial currents flow is denoted as the middle magnetosphere. In the outer magnetosphere, the field has a large southward component and exhibits large temporal and/or spatial variations in magnitude and direction in response to changes in solar wind pressure.