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Ganymede is the only Solar System moon known to generate a permanent magnetic field. Jovian plasma motions around Ganymede create an upstream magnetopause, where energy flows are thought to be driven by magnetic reconnection. Simulations indicate Ganymedean reconnection events may be transient, but the nature of magnetopause reconnection at Ganymede remains poorly understood, requiring an assessment of reconnection onset theory. We present an analytical model of steady-state conditions at Ganymede's magnetopause, from which the first Ganymedean reconnection onset assessment is conducted. We find that reconnection may occur wherever Ganymede's closed magnetic field encounters Jupiter's ambient magnetic field, regardless of variations in magnetopause conditions. Unrestricted reconnection onset highlights possibilities for multiple X lines or widespread transient reconnection at Ganymede. The reconnection rate is controlled by the ambient Jovian field orientation and hence driven by Jupiter's rotation. Future progress on this topic is highly relevant for the JUpiter ICy moon Explorer mission.

Plain Language Summary Ganymede is the largest moon of Jupiter and the only Solar System moon that produces its own magnetic field. Ganymede's magnetic field is surrounded by Jupiter's much larger magnetic field, which flows around the moon like a river flowing around a rock. The boundary where Jupiter's magnetic field first encounters Ganymede's is called the magnetopause. At this boundary, energy and mass can move between the two magnetic fields through a process called magnetic reconnection. Our paper introduces a simple model of Ganymede's magnetopause and uses this model to show where reconnection can occur on the boundary. We find that reconnection can occur anywhere on the magnetopause for any plausible environmental conditions around Ganymede, so the locations where these energy-releasing events occur may be particularly unpredictable. The rate of energy released by reconnection meanwhile depends on near-Ganymede conditions, which change significantly as Jupiter rotates. These results will help inform the planning of the JUpiter ICy moon Explorer mission to Ganymede.