Fast sodium atoms, which appear to be associated with the motion of the magnetic "field lines" downstream from Io (Schneider et al. 1991 Sciences 253, 1394-1397), are thought to be produced by dissociation of NaX+ in Io's plasma torus. It was suggested that these molecular ions are formed near Io's exobase by electron-impact ionization and picked up by the corotating field lines (Wilson and Schneider 1994, Icarus 111, 31-44). Because electron cooling processes dominate electron-impact ionization in Io's corona, two reactions are suggested here as sources of NaX+ and, hence, as the ultimate sources of the observed fast Na. First, the charge-exchange cross sections for forming NaX+ are shown to be large for plasma torus ions (O+, S+, O2+, S2+, S3+) colliding with NaXmolecules in Io's corona. Sputtering from the surface (Chrisey et al. 1988,Icarus 75, 233-244) and rapid transport to the exobase is a potential source of NaX, where X is O, S, Na, or a molecular species such as NaS or O2. Second ions of the more abundant molecules (e.g., SO+, SO+2, O+2, S+2) can react efficiently with atomic Na in Io's corona to form NaX+. Here cross sections are estimated for these two processes, as well as for charge exchange in O+ and S+ on Na collisions.
