NO TWO MAGNETOSPHERES ARE ALIKE

Earth’s is one of seven known magnetospheres in the solar system, all of which are nestled within the magnetosphere of the Sun, the heliosphere. The smallest planetary magnetosphere is Mercury’s. The largest is Jupiter’s. In fact, by volume Jupiter’s magnetosphere is the largest object in the solar system, large enough to accommodate a thousand Suns within its confines. (The Sun itself is large enough to hold a million Earths.) Inside Jupiter’s huge magnetosphere lurks another—the tiny magnetosphere of its moon, Ganymede; this mini-magnetosphere within a magnetosphere was a surprising and fascinating discovery made by the Galileo spacecraft during its 8-year tour of Jupiter’s magnetosphere. Jupiter’s magnetosphere is quite different from Earth’s, not just in size, but also in two fundamental ways. First, a major source of the plasma that it contains is the moon Io, whose volcanos spew sulfur dioxide gas into the magnetosphere where it is transformed into sulfur and oxygen ions. Second, the two magnetospheres have quite different energy sources. While the dynamic variability of Earth’s magnetosphere is generated by the solar wind, Jupiter’s magnetosphere is powered by energy extracted from the gas giant’s rapid rotation. (Jupiter completes one rotation roughly every 10 hours.) Jupiter’s rotational energy is transferred to the magnetosphere by electrical currents that couple the co-rotating ionosphere to the magnetospheric plasma supplied by Io. Although the jovian magnetosphere is rotationally driven, the solar wind also exercises some degree of

Auroras are observed at Jupiter and Saturn as well as at Earth (left, viewed from above the north pole). Jupiter’s ultraviolet aurora is the most powerful in the solar system, radiating several terawatts of power.



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Understanding the Sun and Solar System Plasmas: Future Directions in Solar and Space Physics NO TWO MAGNETOSPHERES ARE ALIKE Earth’s is one of seven known magnetospheres in the solar system, all of which are nestled within the magnetosphere of the Sun, the heliosphere. The smallest planetary magnetosphere is Mercury’s. The largest is Jupiter’s. In fact, by volume Jupiter’s magnetosphere is the largest object in the solar system, large enough to accommodate a thousand Suns within its confines. (The Sun itself is large enough to hold a million Earths.) Inside Jupiter’s huge magnetosphere lurks another—the tiny magnetosphere of its moon, Ganymede; this mini-magnetosphere within a magnetosphere was a surprising and fascinating discovery made by the Galileo spacecraft during its 8-year tour of Jupiter’s magnetosphere. Jupiter’s magnetosphere is quite different from Earth’s, not just in size, but also in two fundamental ways. First, a major source of the plasma that it contains is the moon Io, whose volcanos spew sulfur dioxide gas into the magnetosphere where it is transformed into sulfur and oxygen ions. Second, the two magnetospheres have quite different energy sources. While the dynamic variability of Earth’s magnetosphere is generated by the solar wind, Jupiter’s magnetosphere is powered by energy extracted from the gas giant’s rapid rotation. (Jupiter completes one rotation roughly every 10 hours.) Jupiter’s rotational energy is transferred to the magnetosphere by electrical currents that couple the co-rotating ionosphere to the magnetospheric plasma supplied by Io. Although the jovian magnetosphere is rotationally driven, the solar wind also exercises some degree of Auroras are observed at Jupiter and Saturn as well as at Earth (left, viewed from above the north pole). Jupiter’s ultraviolet aurora is the most powerful in the solar system, radiating several terawatts of power.

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Understanding the Sun and Solar System Plasmas: Future Directions in Solar and Space Physics Powerful electrical currents couple the moons Io, Ganymede, and Europa with Jupiter’s ionosphere, producing auroral emissions at the footpoints of the magnetic field lines linking the moons to the ionosphere. influence on magnetospheric processes. The extent of this influence is not known. The Galileo orbiter has provided a wealth of new information about the structure and dynamics of Jupiter’s magnetosphere. However, because the spacecraft was in an equatorial orbit, it was unable to make measurements in the polar magnetosphere, the region where the electrical currents that couple the ionosphere and magnetosphere flow, producing Jupiter’s spectacular aurora. The Survey Committee considers observations in this region to be of such importance that it recommends a dedicated space physics mission to Jupiter’s polar magnetosphere as its third-highest-priority moderate mission, after the MMS and the LWS geospace probes. The Jupiter Polar Mission (JPM) will place a spacecraft in an elliptical polar orbit about Jupiter to study the processes that couple the ionosphere and magnetosphere, and transfer rotational energy to the magnetosphere. JPM will determine the relative contributions of planetary rotation and the solar wind as sources of magnetospheric energy, and will assess the role of Io’s volcanism in providing the mass that drives the flow of plasma within the jovian magnetosphere. JPM will also identify the charged particles responsible for Jupiter’s powerful aurora and determine how those particles are energized. Why are such measurements so important? First, because of what they will reveal about the workings of a magnetosphere that is profoundly different from our own. And, second, because of what we may learn from them about the physics of magnetospheres belonging to other rapidly rotating astrophysical bodies, such as pulsars and protostellar disks. PLANETARY MAGNETOSPHERES The seven magnetospheres are Mercury’s, Earth’s, Jupiter’s, Ganymede’s, Saturn’s, Uranus’s, and Neptune’s. The magnetospheres of Uranus and Neptune were briefly sampled during the Voyager flybys in 1986 and 1989; Galileo has just completed an extensive survey of the Jupiter system. Saturn’s is the next magnetosphere to be explored. The Cassini spacecraft, which began its orbital tour of the Saturn system in July 2004, carries instruments to study the sources and sinks of Saturn’s magnetospheric plasma and its interaction with the rings and with Saturn’s moons, in particular with Titan. Characterization of Mercury’s tiny magnetosphere will be one of the objectives of the MESSENGER mission. MESSENGER, the first spacecraft to visit Mercury since the Mariner flybys in the mid-1970s, will enter orbit around Mercury in 2009 following two flybys in 2007 and 2008.