Fig. 5.1. A model of Jupiter's magnetosphere (After Rogers 1995 ).
it is deflected. This sharp boundary is called the 'magnetopause'. Plasma in the solar wind is sharply deflected as it approaches the magnetopause, creating a 'bow shock'. The area between the bow shock and the magnetopause is called the j- ®
'magnetosheath' . We know that the distance of these boundaries from Jupiter ®
varies with the strength of the solar wind, which also varies over time . j| j-
The pressure balance at the magnetopause determines the size of a magneto- O 3 ¡5
sphere. The internal pressure is sustained by the planet's magnetic field and the 2
plasma (hot ionized gas) trapped inside the field. The external pressure is provided ¡5
by the solar wind - a fully ionized stream of plasma that flows continually, but ^ Ifl n variably, outwards from the Sun. The wind is highly supersonic, and its pressure variations tend to steepen into shock waves as they move away from the Sun. These interplanetary shocks produce sudden compressions and expansions of magnetospheres .
In 1995 as the Galileo spacecraft approached Jupiter, scientists eagerly awaited the detection of the magnetosphere by the spacecraft. The magnetosphere was finally detected by the spacecraft's magnetometer when a lull in the solar wind allowed Jupiter's magnetosphere to balloon outward. The bow shock of the magnetosphere actually washed back and forth over the spacecraft several times between November 16 and November 26, 1995 . Thus, this single-spacecraft event demonstrates that Jupiter's magnetopause moves inward or outward in response to the pressure of the solar wind.
At Earth, multiple-spacecraft observations have been used to understand how this motion occurs, since this type of observation can give us a snapshot view of a transient event in progress . According to Fairfield and Sibeck et al., variations in the size of Earth's magnetosphere have been reported for decades, some 1,821 observations .
At Jupiter, fewer but similar observations have been made with Pioneer and Voyager data, and it is well recognized that the magnetopause distance should be controlled, at least in part, by the pressure of the solar wind .
The Cassini and Galileo spacecraft obtained simultaneous observations during December 2000 and January 2001. The Cassini spacecraft was on its way to Saturn, and the Galileo spacecraft was in orbit around Jupiter at the same time. Scientists took advantage of the Cassini flyby, centered on December 30, 2000, while Galileo was still in its extended orbital mission around Jupiter. As Cassini approached the vicinity of Jupiter, it first encountered Jupiter's bow shock on December 28, 2000. Cassini would actually encounter the bow shock numerous times. The data indicate that Cassini actually encounters Jupiter's bow shock at least 16 times between December 28, 2000, and January 20, 2001, as the pressure of the solar wind increased and the bow shock washed back and forth over the spacecraft . During this encounter with Jupiter, Galileo was well sunward of Cassini. An analysis of the data taken by the two spacecraft indicate that the magnetopause was in a state of transition from a significantly inflated size at Cassini to a large but nominal size at Galileo. Based on the distance between the two spacecraft and the solar wind pressure, it would take some 5 hours for the pressure front to propagate from the position of Galileo to that of Cassini. Hence, there was clear evidence of a region of increasing pressure moving from Galileo to Cassini's position on timescales similar to what would be expected for a region of increased pressure in the solar wind . Thus, we see that Jupiter's magnetosphere is a constantly changing structure on a scale and timetable we can scarcely imagine, due to the influences of the solar wind.
Although a vacuum by our everyday standards, Jupiter's magnetosphere contains rarified plasma - that is, gas dissociated into positively charged ions and negatively g W charged electrons. Even larger than the magnetosphere is a rarefied nebula of
© .£ sodium atoms that envelops the entire Jovian system. Several spacecraft missions
C "]2 have reported the presence of large numbers of energetic ions and electrons q 3 jg surrounding Jupiter. Apparently, relativistic electrons are detectable for several
.¡5 2 astronomical units from the planet. Energetic neutral particles have also been
3 detected, neither the mass nor charge state of which can be determined, and so are u (A labeled energetic neutral atoms. Images have also shown the presence of sodium as a trace element. The Cassini spacecraft discovered a fast and hot atmospheric neutral wind extending more than 0.5 astronomical units from Jupiter, and the presence of energetic neutral atoms accelerated by the electric field in the solar wind. It is thought that these atoms originate in volcanic gases from the moon Io, and undergo significant changes through a number of electromagnetic interactions, then escape Jupiter's magnetosphere into the Jovian environment. This 'nebula' extends outward from Jupiter over hundreds of Jovian radii .
We are going to use the term 'plasma' a lot, so we should understand what plasma is. Plasma is highly ionized gas, consisting of almost equal numbers of free electrons and positive ions. The plasma sheet is low-energy plasma, largely concentrated within a few planetary radii of the equatorial plane, distributed throughout the magnetosphere throughout which concentrated electric currents flow . Because it is made up of low-density ionized gas, plasma is a very good electrical conductor with properties that are strongly affected by electric and magnetic fields. Individual ions and electrons interact with one another by both emission and absorption of low-frequency 'waves'. Plasma waves occur both as electrostatic oscillations - which are similar to sound waves - and as electromagnetic waves. Such waves are induced by instabilities within the plasma .
The magnetosphere owes its existence to Jupiter's magnetic field. Like that of Earth, Jupiter's magnetosphere consists of plasma populations that are mostly confined to certain regions of the magnetic field and are mostly pulled around with it as the planet rotates. In Jupiter's radiation belts, the trapped particles are ten times more energetic than those in the magnetic belts of Earth, and many times more abundant. Jupiter's magnetosphere contains an internal source of material in the volcanic activity of the moon Io . Io is an important source of material for the magnetosphere and most of the particles in the magnetosphere come from Io. The energy of the magnetosphere comes from Jupiter's rotation. Ions and electrons are initially energized as they are spun up to co-rotation by the magnetic field into the Io torus. The magnetosphere is very dynamic. The magnetic equator is offset 10° to the equatorial equator. The magnetic field itself is not fully symmetrical (Fig. 5.3) but generally can be described as a dipole, tilted 9.6° to the rotation axis . Therefore, being tilted the magnetosphere wobbles as it is whirled around by the planet's rotation. So, when viewed from a stationary point, it oscillates and twists. The outer fringe is affected by gusts from the solar wind and can change on a time scale of hours. There are also changes in the Io torus from year to year, perhaps due to changes in the moon's volcanic activity.
According to Rogers, the magnetic fields can be pictured as 'lines of force' like elastic cables running through space, anchored in the rotating planet. As the magnetic fields are stronger near the poles, the lines of force are closer together, the field lines converging towards the magnetic poles. Consequently, the field can be described as a magnetic bottle, and this is why it can accumulate plasma in the
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