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Observations by the Pioneer, Voyager, Ulysses, and Galileo spacecraft in Jupiter's dayside magnetosphere revealed a cushion region, where the magnetic field became increasingly dipolar and the 10-hr periodicity associated with rotation of the magnetodisc was no longer visible. Focused observations at the dawn terminator by the Juno spacecraft provide critical constraints on the formation physics of the dayside cushion. We observe a persistent 10-hr periodicity at dawn with only minor distortions of the field near the magnetopause boundary, indicating the absence of a systematic dawn cushion region. These data suggest that the dayside cushion is not formed via mass loss associated with magnetic reconnection along a localized X line but rather may be due to the gradual compression of the dawnside magnetic field as it rotates toward local noon.

Plain Language Summary Jupiter's space environment is strongly influenced by its fast rotation period of similar to 10hr. Dipolar magnetic field lines are stretched out by centrifugal forces out to distances up to 100 times Jupiter's radius. Observations of Jupiter's dayside magnetosphere from several spacecraft revealed a cushion region that was thought to contain an absence of plasma, constraining how mass and magnetic field are circulated around the planet. This cushion was thought to extend to dawn local times, but there have been very few spacecraft observations of this region until Juno. The Juno data presented here demonstrates that no systematic cushion region exists near the dawn terminator such that flux circulation models need to be reassessed in order to account for a cushion region that is confined closer towards noon local times.