The absent magnetospheric cushion near Jupiter's dawn terminator?

Observations obtained by the Voyager, Ulysses, and Galileo spacecraft in Jupiter's dayside magnetosphere revealed a `cushion' region, where the local magnetic field became increasingly dipolar and the 10-hour periodicity associated with motion of the magnetodisc was no longer visible. This region, unique to Jupter, was thought to be comprised of magnetic flux tubes that were depleted of mass via magnetic reconnection in Jupiter's magnetotail. The Juno spacecraft, which recently arrived at Jupiter, has sampled the magnetosphere along the dawn terminator more extensively than previous missions. Focused observations at this local time provide critical constraints on the formation physics of the cushion region and for the transport of mass and magnetic flux throughout Jupiter's magnetosphere. Using magnetic field, waves, and energetic particle observations obtained over several orbits (Juno spent weeks at large radial distances (>100 Jovian radii) near the dawn terminator),we observe a persistent 10-hour periodicity with only modest distortions of the field near the magnetopause boundary. These data suggest that the dayside cushion is not formed via mass loss associated with magnetic reconnection in the tail but rather may be due to compression of the dawnside magnetic field as its rotates towards local noon.