Juno Constraints on the Formation of Jupiter's Magnetospheric Cushion Region

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.