Evidence for Magnetic Reconnection at Ganymede's Upstream Magnetopause During the PJ34 Juno Flyby

Juno made a close flyby of Ganymede and flew through its magnetosphere on 7 June 2021, including an outbound crossing of Ganymede's upstream magnetopause. We present plasma and magnetic field observations near the upstream magnetopause from Juno's Jovian Auroral Distributions Experiment (JADE) and magnetometer. JADE observed enhanced electron fluxes, including field‐aligned electrons accelerated up to 2–3 keV/q, some having bidirectional pitch angle distributions, as Juno crossed Ganymede's magnetopause. Energy enhancements of cold protons and heavy ions originating from Ganymede were also observed on approach to the magnetopause. We interpret the presence of accelerated, field‐aligned electrons as indicating that magnetic reconnection is occurring on magnetic field lines that connect the spacecraft to Ganymede's magnetopause at that time. Counter‐streaming electrons observed on both sides of the magnetopause suggest the presence of multiple reconnection sites, both north and south of the spacecraft. Plain Language Summary Magnetic reconnection is a fundamental plasma physics process whereby magnetic field lines are able to merge and reorient. This process also results in the conversion of stored magnetic energy that can heat and accelerate ions and electrons present in the region. Magnetic reconnection is expected to occur between Jupiter's magnetic field and the magnetic field of its satellite Ganymede, the boundary between these two regimes being the magnetopause. In this study, we interpret the presence of accelerated electrons traveling along the magnetic field at Ganymede's magnetopause as evidence that magnetic reconnection is occurring there during the Juno flyby. These observations further support the notion that magnetic reconnection at Ganymede's magnetopause can be a driver of dynamic processes in the local space environment around the satellite. Key Points Juno observed enhanced electron fluxes and cold protons and heavy ions near its crossing of Ganymede's upstream magnetopause Accelerated, field‐aligned electrons observed near Ganymede's magnetopause provide evidence for magnetic reconnection Counter‐streaming electrons observed across the magnetopause suggest reconnection sites both north and south of the spacecraft