Io’s Optical Aurorae in Jupiter’s Shadow

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 hours at egress. Only ion chemistry can produce such a disparity; Io’s molecular ionosphere is much weaker at egress due to rapid recombination. Auroral emission is also evident from potassium, confirming K as the major source of far red emissions seen in situ. In all cases, direct electron impact on atomic gas is sufficient to explain the brightness without invoking significant dissociative excitation of molecules. The non-response of O and rapid depletion of Na during Io’s eclipse phase is surprisingly inverted from the eclipse phase behavior of the SO2 and NaCl parent molecules.

Io’s [O I] 6300 Å line emission amidst Jupiter scattered light on 24 April 2019 with LBT/PEPSI. The red-to-blue color scheme shows elapsed time in shadow. Scattered light levels increase as Io approaches the jovian limb and dotted lines show the fitting of a background spectrum to this component. Lower panel shows the residual after scattered light subtraction. Gaussians are fit to Io’s emission line, which Doppler shifts slightly in time. The observing geometry for the LBT/PEPSI observations is shown on the left panel.

Read more: Schmidt et al. 2022, PSJ, in press (AAS Planetary Science Journal)