The distribution of sodium in Io's cloud: Implications

Models for the distribution of sodium in Io's vicinity and in a disk in Io's orbital plane, compared with observational data, support arguments (1) that Io is the source of the sodium, (2) that sodium is ejected from the inside hemisphere and most of the high velocity sodium which is observed is ejected from the leading inside quadrant, (3) that most of the sodium leads Io in Io's vicinity but follows Io at distances of more than 7R_j from Jupiter, (4) that a significant fraction of the sodium flux is ejected at large angles with respect to Io's orbital plane, (5) that the source velocity distribution has a pronounced high-velocity tail, and (6) that impact ionization by electrons is significant at large distances from Io.