Variability of Europa’s Optical Aurora on Orbital and Multiyear Timescales

Abstract Europa’s auroras result from interactions between its tenuous atmosphere and the exogenous electron population trapped within Jupiter’s magnetosphere. In this study we analyzed spectra of Europa’s optical wavelength aurora taken during eclipse by Jupiter over 10 nights between 2021 and 2025 using Keck I/HIRES. We found that the bright 630.0 nm O i auroral emission correlated strongly with the expected change in ambient magnetospheric electron density owing to Europa’s changing distance from the centrifugal equator of Jupiter’s magnetic field, suggesting that the optical aurora brightnesses can be used as a proxy for measuring electron density variations when assuming a stable atmosphere composition and density. We derived a typical azimuthal heterogeneity in electron densities with System iii longitude of about ±11% and a long-term secular variability in the azimuthal average of about ±20% over the five years of observations, consistent with in situ measurements of plasma density at Europa’s orbit during the Galileo mission. Finally, we determined that any local sublimated water atmosphere present during daytime must collapse to below 2 × 10 18 m −2 within the first few minutes of eclipse ingress.