Juno observations of large‐scale compressions of Jupiter's dawnside magnetopause

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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~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.

“…Geophysical Research Letters ~0.01 cm À3 , with modest increases close to the magnetopause associated with the magnetopause boundary layer Gershman et al, 2017).…”

Section: /2018gl079118mentioning

confidence: 99%

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

Gershman

DiBraccio

Connerney

et al. 2018

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“…Before Juno's first primary science orbit, JADE measured solar wind ions and magnetospheric ions and electrons with key structures identified in McComas et al [2017], evidence for magnetic reconnection in Ebert et al [2017], a very large hot flow anomaly in the solar wind [Valek et al 2017]. JADE also contributed to a study on magnetopause crossings [Gershman et al, 2017]. Szalay et al [2017] mapped the different regions sampled over the auroral regions to the equatorial magnetosphere for the first science orbit.…”

Section: Introductionmentioning

confidence: 99%

Electron beams and loss cones in the auroral regions of Jupiter

Allegrini

Bagenal

Bolton

et al. 2017

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We report on the first observations of 100 eV to 100 keV electrons over the auroral regions of Jupiter by the Jovian Auroral Distributions Experiment (JADE) on board the Juno mission. The focus is on the regions that were magnetically connected to the main auroral oval. Amongst the most remarkable features, JADE observed electron beams, mostly upward going but also some downward going in the south, at latitudes from ~69° to 72° and ~ −66° to −70° corresponding to M shells (“M” for magnetic) from ~18 to 54 and ~28 to 61, respectively. The beams were replaced by upward loss cones at lower latitudes. There was no evidence of strongly accelerated downward electrons analogous to the auroral “inverted Vs” at Earth. Rather, the presence of upward loss cones suggests a diffuse aurora process. The energy spectra resemble tails of distributions or power laws (suggestive of a stochastic acceleration process) but can also have some clear enhancements or even peaks generally between 1 and 10 keV. Electron intensities change on timescales of a second or less at times implying that auroral structures can be of the order of a few tens of kilometers.

“…On board, it carries the Jovian Auroral Distributions Experiment (JADE), an instrument suite that consists of a single ion detector, JADE-I (0.01-50 keV/q), and two electron detectors, JADE-E (0.1-100 keV) [McComas et al, 2013]. Prior to Juno's first science perijove pass, JADE measured the inbound solar wind, magnetosheath, and magnetosphere [McComas et al, 2017], observed a hot flow anomaly [Valek et al, 2017], found evidence for magnetic reconnection at the magnetopause [Ebert et al, 2017], observed mass transport across the magnetopause [Gershman et al, 2017], and found evidence for a connection between solar wind inputs and auroral emissions [Nichols et al, 2017].…”

Section: Introductionmentioning

confidence: 99%

Plasma measurements in the Jovian polar region with Juno/JADE

Szalay

Allegrini

Bagenal

et al. 2017

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Jupiter's main auroral oval provides a window into the complex magnetospheric dynamics of the Jovian system. The Juno spacecraft entered orbit about Jupiter on 5 July 2016 and carries on board the Auroral Distributions Experiment (JADE) that can directly sample the auroral plasma structures. Here we identify five distinct regimes in the JADE data based on composition/energy boundaries and magnetic field mappings, which exhibit considerable symmetry between the northern and southern passes. These intervals correspond to periods when Juno was connected to the Io torus, inner plasma sheet, middle plasma sheet, outer plasma sheet, and the polar region. When connected to the torus and inner plasma sheet, the heavy ions are consistent with a corotating pickup population. For Juno's first perijove, we do not find evidence for a broad auroral acceleration region at Jupiter's main auroral oval for energies below 100 keV.