First Energetic Neutral Atom images of the Earth’s magnetosphere from the JENI detector on board the JUICE mission during the Lunar and Earth Gravity Assist

Abstract. The Jovian Energetic Neutrals and Ions (JENI), part of the Particle Environment Package (PEP), on the Jupiter Icy Moons Explorer (JUICE) spacecraft is a state-of-the-art detector, capable of analyzing the energy and direction of incident Energetic Neutral Atoms (ENAs) from planetary magnetospheres. Our analysis focuses on 2.86–85.5 keV Hydrogen ENA measurements between 21 to 24 August 2024 when JUICE performed the Lunar and Earth Gravity Assist (LEGA). During the LEGA, JENI had a unique opportunity to obtain continuous, 7.5-minute accumulation time global images of the terrestrial magnetosphere as the spacecraft moved outward from ~17 to 150 Earth Radii. Although there were no indications of substantial geomagnetic storms developing during that time, substorm activity was observed based on eight distinct dips of the SuperMAG AL (SML) index down to ~-250 nT for the four last ones and even down to ~-400 nT for the four first ones. Our results indicate increased ENA emissions associated with the onsets of these events, suggesting direct responses to the increased energetic ion activity during those substorm events. Unlike the periods of intense geomagnetic storms, during which proton intensities build up the ring current evolving from an asymmetric distribution during the main phase to a roughly symmetric distribution during the recovery phase, the images from JENI during this interval show non-uniform ENA emissions in the plasma sheet, suggesting transient processes, such as a series of repeated nightside injections. The nightside ENA intensities are generally brighter than those from the dayside, possibly reflecting magnetotail processes associated with energetic ions being injected.