| Recent ground-based observations of the jovian environment using new instrumentation and observing techniques reveal a large (about 1000 Jupiter radii-wide) nebula of neutral sodium gas centered on Jupiter. Three years of observations show the nebula to have certain characteristic features. First, the nebula is more extensive east and west of Jupiter than it is to the north and south. Second, the intensity of the nebula to the east and west decreases in inverse proportion to the distance from Jupiter. Third, the emission broadens to the north and south with increasing distance from the planet. Fourth, spectrographic observations of sodium velocities provide direct evidence of escaping gas. Taken together, these features indicate that the nebula is a rapidly-expanding, spiraling disk with increasing thickness away from the center. Furthermore, evidence of variability suggests that more than one process may be acting simultaneously to produce the nebula and that the satellite Io plays a role in modulating the nebula's intensity near Jupiter.;Modeling of the nebula indicates that it is created through interactions between rapidly-flowing plasma and neutral gas populations near Io, resulting in neutral sodium with sufficient energy to reach the great distances where it is observed. Modeling efforts have concentrated on two such interactions for producing the nebula: Resonant charge exchange and the dissociation and/or dissociative recombination of sodium-bearing molecular ions. Both mechanisms produce fast neutral sodium, but a somewhat different nebula results from each. The molecular ion mechanism is a better representation of the data for earlier data sets, whereas the charge exchange model provides better fits for the later data sets. Therefore, the data are consistent with a nebula produced by a time-variable blend of the two mechanisms.;The instruments and observational techniques used may be applied to other neutral and charged gas species in the jovian environment and may have applications elsewhere in the solar system. The lunar sodium atmosphere and structures with similarly large angular extent may be studied, and their interactions with the solar wind and interplanetary magnetic field may also be investigated. |
