To Galileo Science Instruments Contents Page
Jupiter's magnetosphere is the largest structure in the solar system. It is millions of kilometers across and tens of millions of kilometers long. If it were visible to us, it would appear in the night sky bigger than the moon.
The magnetometer instruments onboard the Galileo Orbiter will measure magnetic fields throughout Jupiter's enormous magnetosphere, from its boundaries where interactions with the solar wind will be studied to the inner regions where properties of the planet itself will be investigated.
Planets having large magnetic fields, including the Earth, are surrounded by magnetospheres--"bubbles" in the solar wind in which the planet's magnetic field dominates and controls the behavior of charged particles. As the solar wind streams around this area, a magnetic tail is formed on the far side of the planet. Jupiter's magnetosphere can be visualized as a giant, tattered wind sock, with a bulbous end toward the Sun and an elongated, flapping tail stretched away from the Sun. Most of this magnetosphere is filled with gases of charged particles.
The Pioneer and Voyager missions showed that the size, shape, and internal structure of the Jovian magnetosphere changes, but they remained near Jupiter too briefly to study the process of change. As it orbits Jupiter for nearly two years, Galileo will record changes in the magnetosphere and help us understand their causes. For example, at Earth, large-scale instabilities that trigger explosive responses throughout the magnetosphere, including ionospheric effects such as aurora and radio noise, are initiated in the tail. The Galileo tour will include a large looping orbit that provides two months in the near-planetary regions of the tail where evidence of similar processes will be sought. (Jupiter's tail may extend as far as Saturn, over 650 million kilometers (390 miles) distant, but Galileo will go only about 11 million kilometers (7 million miles) down the tail.)
Jupiter's four largest satellites--Io, Europa, Ganymede, and Castillo--orbit within Jupiter's magnetosphere. As the magnetized planet rotates, it sets the gases in the magnetosphere into rotation. The rotating charged particles interact with the satellites in ways that differ depending on whether or not a satellite is conducting, magnetized, or has an ionosphere. The Orbiter's tour through the Jupiter system will yield information about magnetosphere-satellite interactions and about the satellites themselves. For example, measurements will reveal if the satellites have magnetic fields and thus provide critical information about their interiors.
Currents flowing along the magnetic field in the Jovian magnetosphere play a crucial role in coupling the magnetosphere with the upper atmosphere. In recent years, such currents have been measured at Earth, but their importance at Jupiter was recognized much earlier. These currents stimulate radio emissions from the ionosphere (both at Jupiter and Earth) and may play a role in producing auroras much like Earth's Northern and Southern Lights. Neutral atoms coming from Io or other moons are "spun up" to the rotation rate of the surrounding charged particles by forces transmitted from the ionosphere along the magnetic field of the magnetosphere. Exchange of mass and energy between the planet and its magnetosphere may occur along these field lines.
During the long journey to Jupiter, the magnetometer instrument will study the properties of interplanetary fields, including fast streams and interplanetary shocks. Long-term measurements of the solar wind magnetic field at great distances from the Sun will be studied to find changes as solar activity increases in the next solar cycle.
The instrument consists of six sensing circuits, data handling circuits, and power circuits. Two clusters of three sensors each are mounted on an 11-meter (36-foot) boom that unfurls from the spinning section of the Orbiter after IUS separation. One set of sensors is mounted at the tip of the boom, while the other is about 6.7 meters (22 feet) from the spacecraft spin axis. The sensors must be placed at some distance from the main body of the spacecraft to minimize magnetic effects from the spacecraft. On the spacecraft, the sensed magnetic field is converted from an analog voltage to a digital signal. Data indicating the orientation of the spacecraft are added and the measurements are analyzed by the instrument's data processor. Effects caused by spacecraft-generated fields or the electronics circuits can be identified, measured, and separated during the data analysis so that only physically useful data need be transmitted to Earth. The challenge associated with pushing the onboard data-processing capability of the instrument to its limit has been met successfully in the magnetometer design.
The basic magnetic measuring device, the ring-core sensor, was fabricated by the Naval Surface Weapons Center, White Oak, Silver Spring, MD. The entire assembly and associated electronics were designed by the University of California, Los Angeles, and fabricated by the Westinghouse Electric Company. An RCA microprocessor is used.
The principal investigator for the magnetometer experiment is Dr. Margaret Kivelson (photo), professor of space physics in the Earth and space sciences department at UCLA. She is supported by four coinvestigators from UCLA.
To Galileo Science Instruments Contents Page