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

Abstract

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~10 hr. 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.

Survey of Jupiter's Dawn Magnetosheath Using Juno

“…This most likely means that during our observations, Jupiter is north of the solar current sheet more often than south of it (since the solar dipole is pointing northwards during this solar cycle). As mentioned by Gershman et al (2018), Juno's orbital period is close to double the solar rotation period. So from orbit to orbit, Juno will see similar solar wind conditions, like magnetic field direction, while in the magnetosheath.…”

Plasma Sheet Boundary Layer in Jupiter's Magnetodisk as Observed by Juno

“…Due to the flapping motion of the magnetodisk, Juno periodically crosses the equatorial plane (local B minimum) and reaches the plasma sheet edge (local B maximum). This plasma sheet edge (presumably the PSBL) is characterized by (1) weak thermal electron fluxes, (2) field‐aligned pitch‐angle distributions of high‐energy electrons, and (3) intense wave electric field bursts on top of the continuum radiation (tracking the density variations and indicating that Juno is on closed field lines; see, e.g., Hospodarsky et al., 2017; Gershman et al., 2018). In this study, we collect statistics of Juno observations during such crossings of the magnetodisk.…”

Saturn's Open‐Closed Field Line Boundary: A Cassini Electron Survey at Saturn's Magnetosphere

“…Therefore, the return flow of depleted electrons at dawn found in this study may carry energetic plasma not observable by ELS. Further effort and investigations are required to make a more conclusive statement on this topic at Saturn (and it seems, also at Jupiter: Gershman et al, ).…”

“…This region also exhibits dipolar configurations of the magnetospheric field and magnetic nulls, which arise from instabilities that form at the outer edge of the plasma sheet. Most recently, there is evidence that the concept of the cushion region may need to be reassessed because the Juno spacecraft did not observe a persistent cushion region on the dawn flank (Gershman et al, 2018). Went et al (2011) compared observations from Jupiter (with the Ulysses spacecraft) and Saturn (Cassini) and concluded that Saturn lacked a cushion region of quasi-dipolar flux tubes.…”