  Decline and recovery timescales surrounding eclipse are indicative of the
controlling physical processes in Io's atmosphere. Recent studies have
established that the majority of Io's molecular atmosphere, SO2 and SO,
condenses during its passage through Jupiter's shadow. The eclipse response of
Io's atomic atmosphere is less certain, having been characterized solely by
ultraviolet aurorae. Here we explore the response of optical aurorae for the
first time. We find oxygen to be indifferent to the changing illumination, with
[O I] brightness merely tracking the plasma density at Io's position in the
torus. In shadow, line ratios confirm sparse SO2 coverage relative to O, since
their collisions would otherwise quench the emission. Io's sodium aurora mostly
disappears in eclipse and e-folding timescales, for decline and recovery differ
sharply: ~10 minutes at ingress and nearly 2 hr at egress. Only ion chemistry
can produce such a disparity; Io's molecular ionosphere is weaker at egress due
to rapid recombination. Interruption of a NaCl+ photochemical pathway best
explains Na behavior surrounding eclipse, implying that the role of electron
impact ionization is minor relative to photons. Auroral emission is also
evident from potassium, confirming K as the major source of far red emissions
seen with spacecraft imaging at Jupiter. In all cases, direct electron impact
on atomic gas is sufficient to explain the brightness without invoking
significant dissociative excitation of molecules. Surprisingly, the nonresponse
of O and rapid depletion of Na is opposite the temporal behavior of their SO2
and NaCl parent molecules during Io's eclipse phase.
