UV Aurora Observations Constrain Electron-Impact Ionization on Ganymede

This study presents a novel approach to quantify electron-impact ionization rates on Ganymede through OI 1356 Å emission brightness observations. The analysis of measured cross-sections reveals that the ionization-to-excitation ratio is limited to 10-60 over all electron energies, reducing the uncertainty of estimating ionization rates. Applying this method to Juno UVS observations, the study finds that the OI 1356 Å brightness of the auroral ovals is well described by 3-5° latitude wide Gaussian distributions centered on the open-closed field line boundary, with an average peak of 120 R. From these observations, a global map of electron-impact ionization rates is derived, showing that these rates are at least an order of magnitude higher than photoionization rates. The estimated total global ionization rate is 1.3-7.6×10^26 s^-1, with average column rates of ~5×10^9 cm^-2s^-1 in the ovals and ~3×10^8 cm^-2s^-1 in the background regions. Comparison of radio occultation measurements with predicted electron densities indicates that transport processes are the dominant loss mechanism in Ganymede's ionosphere. The rate of ionospheric outflow of O2+ is 0.1-2×10^26 s^-1 or 0.5-11 kg s^-1, indicating 0.03-0.5 cm Myr^-1 erosion of Ganymede's surface ice. This research provides valuable insights into the ionization processes in Ganymede's atmosphere and their impact on the moon's ionosphere and surface.

_Context: This intelligence report was compiled by the DailyOrbitalWire Strategy Engine. Verified for Art. 50 Compliance._