Mario H. Acuna

The Cluster Magnetic Field Investigation

The Cluster mission provides a new opportunity to study plasma processes and structures in the near-Earth plasma environment. Four-point measurements of the magnetic field will enable the analysis of the three dimensional structure and dynamics of a range of phenomena which shape the macroscopic properties of the magnetosphere. Difference measurements of the magnetic field data will be combined to derive a range of parameters, such as the current density vector, wave vectors, and discontinuity normals and curvatures, using classical time series analysis techniques iteratively with physical models and simulation of the phenomena encountered along the Cluster orbit. The control and understanding of error sources which affect the four-point measurements are integral parts of the analysis techniques to be used. The flight instrumentation consists of two, tri-axial fluxgate magnetometers and an on-board data-processing unit on each spacecraft, built using a highly fault-tolerant architecture. High vector sample rates (up to 67 vectors s-1) at high resolution (up to 8 pT) are combined with on-board event detection software and a burst memory to capture the signature of a range of dynamic phenomena. Data-processing plans are designed to ensure rapid dissemination of magnetic-field data to underpin the collaborative analysis of magnetospheric phenomena encountered by Cluster.

The MESSENGER mission to Mercury: Scientific objectives and implementation

Abstract Mercury holds answers to several critical questions regarding the formation and evolution of the terrestrial planets. These questions include the origin of Mercurys anomalously high ratio of metal to silicate and its implications for planetary accretion processes, the nature of Mercurys geological evolution and interior cooling history, the mechanism of global magnetic field generation, the state of Mercurys core, and the processes controlling volatile species in Mercurys polar deposits, exosphere, and magnetosphere. The MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) mission has been designed to fly by and orbit Mercury to address all of these key questions. After launch by a Delta 2925H-9.5, two flybys of Venus, and two flybys of Mercury, orbit insertion is accomplished at the third Mercury encounter. The instrument payload includes a dual imaging system for wide and narrow fields-of-view, monochrome and color imaging, and stereo; X-ray and combined gamma-ray and neutron spectrometers for surface chemical mapping; a magnetometer; a laser altimeter; a combined ultraviolet–visible and visible-near-infrared spectrometer to survey both exospheric species and surface mineralogy; and an energetic particle and plasma spectrometer to sample charged species in the magnetosphere. During the flybys of Mercury, regions unexplored by Mariner 10 will be seen for the first time, and new data will be gathered on Mercurys exosphere, magnetosphere, and surface composition. During the orbital phase of the mission, one Earth year in duration, MESSENGER will complete global mapping and the detailed characterization of the exosphere, magnetosphere, surface, and interior.

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 field studies at jupiter by voyager 1: preliminary results.

Results obtained by the Goddard Space Flight Center magnetometers on Voyager 1 are described. These results concern the large-scale configuration of the Jovian bow shock and magnetopause, and the magnetic field in both the inner and outer magnetosphere. There is evidence that a magnetic tail extending away from the planet on the nightside is formed by the solar wind-Jovian field interaction. This is much like Earths magnetosphere but is a new configuration for Jupiters magnetosphere not previously considered from earlier Pioneer data. We report on the analysis and interpretation of magnetic field perturbations associated with intense electrical currents (approximately 5 x 106 amperes) flowing near or in the magnetic flux tube linking Jupiter with the satellite Jo and induced by the relative motion between Io and the corotating Jovian magnetosphere. These currents may be an important source of heating the ionosphere and interior of Io through Joule dissipation.

MESSENGER Observations of Magnetic Reconnection in Mercury’s Magnetosphere

MESSENGER from Mercury The spacecraft MESSENGER passed by Mercury in October 2008, in what was the second of three fly-bys before it settles into the planets orbit in 2011. Another spacecraft visited Mercury in the mid-1970s, which mapped 45% of the planets surface. Now, after MESSENGER, only 10% of Mercurys surface remains to be imaged up close. Denevi et al. (p. 613) use this near-global data to look at the mechanisms that shaped Mercurys crust, which likely formed by eruption of magmas of different compositions over a long period of time. Like the Moon, Mercurys surface is dotted with impact craters. Watters et al. (p. 618) describe a well-preserved impact basin, Rembrandt, which is second in size to the largest known basin, Caloris. Unlike Caloris, Rembrandt is not completely filled by material of volcanic origin, preserving clues to its formation and evolution. It displays unique patterns of tectonic deformation, some of which result from Mercurys contraction as its interior cooled over time. Mercurys exosphere and magnetosphere were also observed (see the Perspective by Glassmeier). Magnetic reconnection is a process whereby the interplanetary magnetic field lines join the magnetospheric field lines and transfer energy from the solar wind into the magnetosphere. Slavin et al. (p. 606) report observations of intense magnetic reconnection 10 times as intense as that of Earth. McClintock et al. (p. 610) describe simultaneous, high-resolution measurements of Mg, Ca, and Na in Mercurys exosphere, which may shed light on the processes that create and maintain the exosphere. Mercury’s magnetosphere responds more strongly to the influence of the Sun’s magnetic field than does Earth’s magnetosphere. Solar wind energy transfer to planetary magnetospheres and ionospheres is controlled by magnetic reconnection, a process that determines the degree of connectivity between the interplanetary magnetic field (IMF) and a planet’s magnetic field. During MESSENGER’s second flyby of Mercury, a steady southward IMF was observed and the magnetopause was threaded by a strong magnetic field, indicating a reconnection rate ~10 times that typical at Earth. Moreover, a large flux transfer event was observed in the magnetosheath, and a plasmoid and multiple traveling compression regions were observed in Mercury’s magnetotail, all products of reconnection. These observations indicate that Mercury’s magnetosphere is much more responsive to IMF direction and dominated by the effects of reconnection than that of Earth or the other magnetized planets.

An altitude-normalized magnetic map of Mars and its interpretation

Techniques developed for the reduction and analysisofterrestrialsatellitemagneticelddataareusedto better understand the magnetic eld observations made by Mars Global Surveyor. A global distribution of radial (Br) magnetic eld observations and associated uncertainties is invertedfor an equivalentsource magnetization distribution and then used to generate an altitude- normalized map of Br at 200 km. The observations are well-represented by a potential function of crustal origin, consistent with a rema- nent origin for the Martian magnetic features. The correla- tion between the 40546 Br observations andBr calculated from the magnetization solution at observation locations is 0.978. For a magnetization distribution connedto a 50 km layer,calculatedmagnetizationsrangefrom-22to+17A/m. We see correlations with tectonics that were only hinted at in earlier maps. Magnetic features appear to be truncated against Valles Marineris and Ganges Chasma, suggestive of a major change in crustal properties associated with fault- ing.

The Structure of Mercury's Magnetic Field from MESSENGER's First Flyby

During its first flyby of Mercury, the MESSENGER spacecraft measured the planets near-equatorial magnetic field. The field strength is consistent to within an estimated uncertainty of 10% with that observed near the equator by Mariner 10. Centered dipole solutions yield a southward planetary moment of 230 to 290 nanotesla RM3 (where RM is Mercurys mean radius) tilted between 5° and 12° from the rotation axis. Multipole solutions yield non-dipolar contributions of 22% to 52% of the dipole field magnitude. Magnetopause and tail currents account for part of the high-order field, and plasma pressure effects may explain the remainder, so that a pure centered dipole cannot be ruled out.

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.

Mercury's Magnetosphere After MESSENGER's First Flyby

Observations by MESSENGER show that Mercurys magnetosphere is immersed in a comet-like cloud of planetary ions. The most abundant, Na+, is broadly distributed but exhibits flux maxima in the magnetosheath, where the local plasma flow speed is high, and near the spacecrafts closest approach, where atmospheric density should peak. The magnetic field showed reconnection signatures in the form of flux transfer events, azimuthal rotations consistent with Kelvin-Helmholtz waves along the magnetopause, and extensive ultralow-frequency wave activity. Two outbound current sheet boundaries were observed, across which the magnetic field decreased in a manner suggestive of a double magnetopause. The separation of these current layers, comparable to the gyro-radius of a Na+ pickup ion entering the magnetosphere after being accelerated in the magnetosheath, may indicate a planetary ion boundary layer.

The AMPTE CCE Magnetic Field Experiment

The AMPTE CCE spacecraft carries a high-resolution Magnetic Field Experiment for the operational purpose of determining spacecraft attitude and to fulfill the scientific objectives of providing magnetic-field measurements necessary for the determination of particle pitch angles, identification of geospace boundaries, measurement of geomagnetic activity, and the study of magnetospheric current systems and plasma processes. This experiment includes a fluxgate-magnetometer system with the sensors mounted on a 2.3-m boom to reduce spacecraft-related measurement errors. Te experiment has 7 automatically switchable ranges from ±16 to ±65 536 nT (full scale) and resolutions commensurate with a 13-bit A/D converter in each range (±0.002-±8 nT). Approximately 8.6 vector-samples/s are acquired, with information on fluctuating fields in the 5-50-Hz range provided by a system of filters and peak detectors. This instrument was operated from the time of the launch of the CCE spacecraft and is presently working in all modes as designed. The details of this experiment and early data are presented in this paper.