Results from Jupiter's Magnetosphere

 

Overview ..
The main scientific objectives of the Galileo Magnetometer team are to investigate the structure and dynamics of Jupiter's magnetosphere, investigate plasma and field interactions in the near vicinity of the four Galilean moons, and measure the internal magnetic fields of these bodies. (The following lists close passes Galileo has made of the large moons of Jupiter through mid-1997.)

Prior to orbital insertion:

Close pass by Io, December 7, 1995.

Following orbital insertion:

Ganymede 1: June 27, 1996 COMPLETE
Ganymede 2: September 6, 1996 COMPLETE
Callisto 3: November 4, 1996 COMPLETE
Europa 4: December 19, 1996 COMPLETE
Europa 6: No magnetometer data COMPLETE
Ganymede 7: April 5, 1997 COMPLETE
Ganymede 8: May 7, 1997 COMPLETE
Callisto 9: June 28, 1997 COMPLETE
Callisto 10: September 17, 1997 COMPLETE
Europa 11: November 7, 1997  

Between the close encounters, the fields and particles instruments have acquired continuous low time-resolution data for long time intervals providing insight into the spatial structure and the temporal variations of the magnetosphere.
Principal Results ..
The Galilean Moons

Passes by the moons have revealed that Ganymede has an intrinsic magnetic field -- large enough to carve a mini-magnetosphere out of Jupiter's magnetosphere. This surprising result has created considerable interest among planetary scientists who are trying to understand the source of this magnetic field as no other moon in the solar system is known have an intrinsic magnetic field

A large magnetic perturbation was measured near Io. The magnetometer team has concluded that it is likely, but yet uncertain, that Io also has an intrinsic magnetic field. The ambiguity arises because strong currents flow in the plasma near Io modifying the magnetic field. There, then, is some possibility that plasma currents could explain these observations.

In late 1999, Galileo will pass over Io's south pole at low altitude. The magnetometer team believes that data from this pass will unambiguously establish the source of the magnetic signature.

A pass by Callisto showed no intrinsic magnetic field. Europa has, at most, a very small intrinsic magnetic field; additional passes will probably help constrain its magnitude and its symmetry properties.

The Magnetosphere

Low time resolution data, taken on the inbound pass, established that the magnetosphere was first encountered at an unusually large distance from Jupiter (with the bow shock standing off very far from Jupiter). The inbound data were useful in comparing properties of the magnetosphere near the dawn meridian during the Galileo epoch with the same region 22 years earlier during the Pioneer 10 pass. Data from the subsequent orbits are revealing both a strong dependence of the magnetic field configuration on "local time" (i.e., the Sun-Jupiter-Galileo angle) and rapid temporal variations that may have features in common with terrestrial geomagnetic activity.

Modeling

In trying to understand the properties of the moons and the magnetosphere, the magnetometer team relies heavily on analysis of computer models. One set of models attempts to extend our knowledge of how a rapidly rotating magnetized planet (Jupiter) interacts with the solar wind plasma that flows radially outward from the Sun at supersonic speed. Another set of models seeks to learn about the way in which a subsonic flow (the plasma of Jupiter's magnetosphere flowing onto the Galilean moons) interacts with both magnetized and unmagnetized bodies. These simulations of physical processes help considerably in interpretation of the data.

Data

The Galileo magnetometer operates in several different data-taking modes:

1... During intervals when the spacecraft is not tracked from the Earth and at other certain other critical times during the mission, the magnetometer uses its internal computer to average measured fields, thus enabling it to acquire data at very low time resolution (1 minute to 16 hours). This capability was used during the interplanetary cruise and inbound pass in November and December of 1995. Additionally, it will be used during the remainder of the tour when the rate at which data is returned to the ground is low.
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2. During mission-critical events like passes by the Galilean moons, data are placed on the tape recorder at a rate of 0.33 s/vector. These data are read back from the tape recorder at low rates over many hours.
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3. When data are not being written onto the tape recorder, the magnetometer data are returned as 24 s averages or, at higher rate, when data are being acquired on the ground at 20 bps or higher.
Last Updated: October 09, 1997