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The electromagnetic interaction between Europa and the plasma sheet in the Jovian magnetosphere generates Alfvén waves, ultimately generating auroral footprints in Jupiter's atmosphere. The position of Europa's auroral footprint is a proxy for travel time of the Alfvén waves. We measured Europa's footprint position using the far‐ultraviolet images of Jupiter obtained by the Hubble Space Telescope in two observing campaigns in 2014 and 2022. The measured footprint position indicates a longer Alfvén travel time in the 2022 campaign. We retrieved the plasma sheet parameters at Europa's orbit from the footprint position by tracing the Alfvén waves launched at Europa and found an increase of both mass density and temperature in the plasma sheet in 2022. The Poynting flux generated at Europa is calculated with the retrieved plasma sheet parameters, which suggests the total energy transfer from Europa to its auroral footprint is similar to the case of Io. Plain Language Summary: Europa is an obstacle to the plasma corotating with Jupiter's magnetosphere. Through the interaction between Europa and the magnetospheric plasma flow, Alfvén waves are launched at Europa. The Alfvén waves propagate along the field line and ultimately generate auroral emissions at locations distant from the instantaneous magnetic footprint of Europa. The position of Europa's auroral footprint is a proxy for the travel time of the Alfvén waves. We measured the position of Europa's auroral footprint using the far‐ultraviolet images of Jupiter obtained by the Hubble Space Telescope in two observing campaigns in 2014 and 2022. We found large deviations of the footprint position between the two observing campaigns. By tracing the Alfvén waves launched at Europa, we retrieved plasma mass density and temperature at Europa's orbit from the measured footprint position. It is revealed that time variation in the plasma mass density and temperature caused the deviations in the footprint position. We also calculated the Poynting flux generated at Europa using the retrieved plasma parameters and found that the total energy transfer from Europa to its auroral footprint is similar to the case of Io. Key Points: We measured the equatorial lead angle of Europa's auroral footprint in Jupiter's atmosphere with the HST data taken in 2014 and 2022Plasma conditions at Europa's orbit are retrieved from the measured lead angle by tracing the Europa‐originated Alfvén wavesChanges in the plasma conditions at Europa's orbit can account for the variation of the footprint lead angle [ABSTRACT FROM AUTHOR]

The ionospheres of Jupiter's icy moons have been observed by in situ plasma measurements and radio science. However, their spatial structures have not yet been fully characterized. To address this, we developed a new ray tracing method for modeling the radio occultation of the ionospheres using Jovian auroral radio sources. Applying our method to Jovian auroral radio observations with the Galileo spacecraft, we derived the electron density of the ionosphere of Ganymede and Callisto. For Ganymede's ionosphere, we found that the maximum electron density on the surface was 76.5–288.5 cm−3 in the open magnetic field line regions and 5.0–20.5 cm−3 in the closed magnetic field line region during the Galileo Ganymede 01 flyby. The difference in the electron density distribution was correlated with the accessibility of Jovian magnetospheric plasma to the atmosphere and surface of the moons. These results indicated that electron‐impact ionization of the Ganymede exosphere and sputtering of the surface water ice were effective for the producing Ganymede's ionosphere. For Callisto's ionosphere, we found that the densities were approximately 350 and 12.5 cm−3 on the night side hemisphere during Callisto 09 and 30 flybys, respectively. These results combined with previous observations indicated that atmospheric production through sublimation controlled the ionospheric density of Callisto. This method is also applicable to upcoming Jovian radio observation data from the Jupiter Icy Moon Explorer, JUICE. Key Points: We developed a ray tracing method for Jupiter's icy moon ionospheric occultation of Jovian auroral radio sourcesThe ionospheric density of Ganymede is associated with electron‐impact ionization and atmospheric production via ion sputteringThe Ionospheric density of Callisto is associated with atmospheric production by solar illumination on its surface [ABSTRACT FROM AUTHOR]