height (hmax) significantly above the surface, as found in more-typical planetary ionospheres. A photoionization source in the ultraviolet region of the spectrum will act similarly, and thus both processes act to some (unknown) degree to produce Europa's ionosphere. Preliminary results from Galileo suggest an Nmax~ 104 e- cm-3 just above the surface.46
Once produced, the ionospheric plasma has a short residence time at Europa since there is no intrinsic magnetic field or dense atmosphere to keep it bound to the satellite. Rather, the ions and electrons will immediately begin to gyrate about the jovian magnetic field. Since the magnetic field is coupled to Jupiter's rapid rotation, the ionospheric plasma becomes entrained in the corotating magnetospheric plasma that impinges on the trailing side of Europa (upstream in the magnetospheric flow) and is swept away downstream, ahead of the satellite in its orbit.
The current state of knowledge of Europa's neutral atmosphere and its embedded ionosphere is extremely rudimentary. The day-to-day variability of both and their responses to the stresses caused by electrodynamical interactions with the magnetosphere in which they reside are essentially unknown at this time. However, in contrast to the terrestrial situation, the atmosphere and ionosphere on Europa are highly representative of surface materials, and therefore the detection of all possible species is important.
Europa is located in the inner magnetosphere of Jupiter (at a radial distance of ~ 9.5 RJ), a region populated mainly by plasma derived from the Io plasma torus. The plasma there consists of protons, oxygen and sulfur ions, and their corresponding electrons. The background magnetic field of Jupiter is quite strong near Europa's orbit (~ 500 nT), and the fluxes of energetic electrons and ions are among the highest found in the solar system. The plasma is nearly corotational, being dragged around Jupiter by the magnetic field as Jupiter rotates; with the magnetic field rotating at a faster rate than Europa revolves around Jupiter, the plasma hits Europa with a relative velocity of ~ 120 km/s on Europa's orbital trailing hemisphere. Most of the plasma resides in a thin plasma sheet (with a half-thickness ~ 2RJ), located approximately in the plane of Jupiter's magnetic equator. Because Jupiter's magnetic field is tilted by 10 degrees relative to its rotation axis, the plasma sheet and the magnetic equator appear to move up and down as seen from Europa, with a period of 11.2 hours (i.e., the synodic period corresponding to the 9.9-hour rotation rate of Jupiter's magnetic field and Europa's 3.6 day rotation rate). This relative motion produces dramatic changes in the charged particle fluxes and the magnetic field experienced by Europa.47
Though several spacecraft have traveled interior to the orbit of Europa, Galileo is the first to have provided in situ field and plasma measurements near Europa. Galileo collected data in Europa's vicinity four times during its primary mission and, ultimately, an additional seven times during the subsequent Galileo Europa Mission (see Box 2.1). Plasma measurements from Galileo show that Europa acts in a manner similar to a cometary source of plasma; while it both absorbs and emits charged particles, it is a net source of plasma emitted into the magnetosphere. Measurements from the E-4 and E-6 orbits show that the plasma density was enhanced by a factor of two or more within a large volume around Europa. Simultaneous measurements from the energetic-particle detectors showed that the radiation environment near Europa is extremely variable, changing by an order of magnitude between orbits. These are the energetic particles that bombard the surface of Europa to produce its transient neutral atmosphere and ionosphere and to cause resurfacing and migration of material on its surface.
When Europa is located above or below the central plasma sheet, the fluxes of charged particles are relatively low and sputtering is at a minimum. During this time, the variations in the background magnetic field that result from the rotation of the tilted jovian dipole produce a response from Europa.48,49 The oscillating jovian field is somewhat ''neutralized," apparently by the presence of a conducting material within Europa that produces an eddy current in its surface. Simple modeling calculations suggest that the induced field is dipolar in nature and that its magnitude at its pole is equal and opposite to that of the applied oscillating field (thereby canceling it out). The electrical conductivities of the ionosphere (< 10-4 S/m) alone, however, or of the ice crust (< 10-6 S/m) are too small to shield out the oscillating field in this manner. A possible explanation for the presence of a more-conductive medium is that there is a global liquid-water ocean and that it contains dissolved salts.50,51 The