Small scale structures in the footprint tails of the Galilean moons observed by JIRAM

<p>The Jovian Infrared Auroral Mapper (JIRAM) on board Juno is a spectro-imager which is observing the<br>atmosphere of Jupiter and its auroral emission using its two imagers in the L (3.3-3.6μm) and M bands (4.5-<br>5.0μm) and a spectrometer (2-5 μm spectral range).<br>The highly elliptic orbit of Juno and the unprecedented resolution of the JIRAM imager allowed to retrieve<br>wealth of details about the morphology of moon-related aurorae. This phenomenon is due to the jovian magnetic<br>field sweeping past the Galiean moons, which generate Alfven waves travelling towards the ionosphere and set<br>up field aligned currents. When the associated electrons reach the ionosphere, they interact with the hydrogen<br>and make it to glow. In particular, the tails of the footprints showed a spot-like substructure consistently, which<br>were investigated using the L-band of the imager from perijove 4 to perijove 30. This feature was observed close<br>to the footprints, where the the typical distance between spots lies between 250km and 500km. This distance<br>decreases to 150km in a group of three observations in the northern emisphere when each moon is close to 250 ◦<br>west longitude. No correlation with orbital parameters such as the longitude of the moons was found so far,<br>which suggests that such morphology is almost purely due to ionospheric processes.<br>Moreover, during PJ 13 a long sequence of images of the Io footprint was shot and it revealed that the<br>secondary spots appears to corotate with Jupiter. This behaviour is observed also during orbits 14 and 26.<br>During these sequences JIRAM clearly observed the Io footprint leaving behind a trail of ”footsteps” as bright<br>spots.<br>The characteristics of these spots are incompatible with multiple reflection of Alfven waves between the two<br>emispheres. Instead, we are currently investigating ionospheric processes like the feedback instability (FI) as a<br>potential candidate to explain the generation of the observed small scale structure. This process relies on local<br>enhacement of conductivity in the ionosphere, which is affected by electron precipitation. Order of magnitude<br>estimates from the FI are compatible with the inter-spot distance and the stillness of the spots.</p>