The low‐altitude, high‐velocity trajectory of the Juno spacecraft enables the Jovian Auroral Distributions Experiment to make the first in situ observations of the high‐latitude ionospheric plasma. ...Ions are observed to energies below 1 eV. The high‐latitude ionospheric ions are observed simultaneously with a loss cone in the magnetospheric ions, suggesting precipitating magnetospheric ions contribute to the heating of the upper ionosphere, raising the scale height, and pushing ionospheric ions to altitudes of 0.5 RJ above the planet where they are observed by Jovian Auroral Distributions Experiment. The source of the magnetospheric ions is tied to the Io torus and plasma sheet, indicated by the cutoff seen in both the magnetospheric and ionospheric plasma at the Io M‐shells. Equatorward of the Io M‐shell boundary, the ionospheric ions are not observed, indicating a drop in the scale height of the ionospheric ions at those latitudes.
Plain Language Summary
The Jovian Auroral Distributions Experiment (JADE) ion sensor has made the first in situ observations of the upper, high‐latitude ionosphere of Jupiter. Flown on the Juno spacecraft, JADE observes the ionosphere at altitudes of approximately half a Jovian radii, with the spacecraft traveling at the high speed of ~50 km/s. For comparison, a proton traveling at 50 km/s has an energy of approximately 10 eV. The combination of the low‐altitude and high ram velocity enables JADE to measure ionospheric ions to energies below 1 eV. These observations reveal a cold ionospheric population of protons at high latitudes, seen coincident with precipitating magnetospheric ions. This indicates that the precipitating magnetospheric ions heat the upper ionosphere, raising the height where these protons can be observed. The ionospheric protons are seen in bands in the northern and southern latitudes, bounded on the equator edge by the field lines that connect to Io, and inside the auroral oval to the poleward side.
Key Points
The high‐latitude ionosphere is observed between the magnetic latitudes bounded by the auroral oval and Io's magnetic flux shell
Two populations are observed at high latitudes: (1) magnetospheric ions consisting of H, S, and O ions and (2) cold ionospheric H+ ions
Observation of a loss cone suggests precipitating magnetospheric ions heat the upper ionosphere to heights ~0.5 RJ above the clouds
At Jupiter, part of the auroral radio emissions are induced by the Galilean moons Io, Europa, and Ganymede. Until now, they have been remotely detected, using ground‐based radio telescopes or ...electric antennas aboard spacecraft. The polar trajectory of the Juno orbiter allows the spacecraft to cross the magnetic flux tubes connected to these moons, or their tail, and gives a direct measure of the characteristics of these decametric moon‐induced radio emissions. In this study, we focus on the detection of a radio emission during the crossing of magnetic field lines connected to Ganymede's tail. Using electromagnetic waves (Juno/Waves) and in situ electron measurements (Juno/JADE‐E), we estimate the radio source size of ∼250 km, a radio emission growth rate >3 × 10−4, a resonant electron population of energy
E=4–15 keV and an emission beaming angle of θ = 76–83°, at a frequency ∼1.005–1.021 × fce. We also confirmed that radio emission is associated with Ganymede's downtail far ultraviolet emission.
Plain Language Summary
The Juno spacecraft crossed magnetic field lines connected to Ganymede's auroral signature in Jupiter's atmosphere. At the same time, Juno also crossed a decametric radio source. By measuring the electrons during this radio source crossing, we determine that this emission is produced by the cyclotron maser instability driven by upgoing electrons, at a frequency 0.5% to 2.1% above the cyclotron electronic frequency with electrons of energy 4–15 keV.
Key Points
This study is the first detailed wave/particle investigation of a Ganymede‐induced radio source using Juno/Waves and Juno/JADE instruments
Ganymede‐DAM emission is produced by a loss cone driven cyclotron maser instability, sustained by an Alfvénic acceleration process
Ganymede‐induced radio emission is produced by electrons of ∼4–15 keV, at a beaming angle 76–83°, and a frequency 1.005–1.021 × fce
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.
Key Points
First 100 eV to 100 keV electron measurements in the auroral regions of Jupiter
Upward and downward electron beams observed in the polar regions and on field lines connected to the middle plasma sheet
Upward loss cone on the field lines connected to the inner plasma sheet suggesting a diffuse aurora process
NASA's Interstellar Boundary Explorer (IBEX) mission has operated in space for a full solar activity cycle (Solar Cycle 24), and IBEX observations have exposed the global three-dimensional structure ...of the heliosphere and its interaction with the very local interstellar medium for the first time. Here, we extend the prior IBEX observations of energetic neutral atoms (ENAs) by adding a comprehensive analysis of four additional years (2016 through 2019). We document several improvements and rerelease the entire 11 yr, IBEX-Hi data set. The new observations track the continuing expansion of the outer heliosphere's response to the large solar wind pressure increase in late 2014. We find that the intensification of ENAs from the heliosheath continued to expand progressively over time to directions farther from the initial, closest direction to the heliospheric boundaries, ∼20° south of the upwind direction. This expansion extended beyond the south pole in 2018 and the north pole in 2019, demonstrating that the termination shock and heliopause are closer in the south. The heliotail has not yet responded, indicating that the boundaries are significantly farther away in the downwind direction. Finally, the slow solar wind (∼1 keV) ENAs just started to intensify from the closest regions of the IBEX Ribbon. This is about two and a half years after the initial response from heliosheath ENAs and about four and a half years after the increase in solar wind output, both clearly implicating a "secondary ENA" source in the draped interstellar magnetic field, just beyond the heliopause.
We draw a comparison between a solar energetic particle event associated with the release of a slow coronal mass ejection close to the Sun, and the energetic particle population produced in high ...current density field-aligned current structures associated with auroral phenomena in planetary magnetospheres. We suggest that this process is common in CME development and lift off in the corona, and may account for the electron populations that generate Type III radio bursts, as well as for the prompt energetic ion and electron populations typically observed in interplanetary space.
Water‐group gas continuously escapes from Jupiter's icy moons to form co‐orbiting populations of particles or neutral toroidal clouds. These clouds provide insights into their source moons as they ...reveal loss processes and compositions of their parent bodies, alter local plasma composition, and act as sources and sinks for magnetospheric particles. We report the first observations of H2+ pickup ions in Jupiter's magnetosphere from 13 to 18 Jovian radii and find a density ratio of H2+/H+ = 8 ± 4%, confirming the presence of a neutral H2 toroidal cloud. Pickup ion densities monotonically decrease radially beyond 13 RJ consistent with an advecting Europa‐genic toroidal cloud source. From these observations, we derive a total H2 neutral loss rate from Europa of 1.2 ± 0.7 kg s−1. This provides the most direct estimate of Europa's H2 neutral loss rate to date and underscores the importance of both ion composition and neutral toroidal clouds in understanding satellite‐magnetosphere interactions.
Plain Language Summary
Jupiter's moons Europa, Ganymede, and Callisto all have icy surfaces which interact with their local environments. From this interaction, water‐group atoms and molecules are released from the icy surfaces and orbit Jupiter as a collection of material in “neutral toroidal clouds.” The material in these toroidal clouds interact with the local charged particle environment, where neutrals in the toroidal clouds can become charged and incorporated into Jupiter's charged particle environment. Here, we highlight observations of Jupiter's charged particle environment and present the first detections of H2+ in this environment. These H2+ ions are shown to be originally produced from H2 lost from Europa and the abundance of detected ions allows us to determine Europa is losing 1.2 ± 0.7 kg s−1 of neutral H2. This provides the most direct estimate of Europa's H2 neutral loss rate to date and underscores the importance of both ion composition and neutral toroidal clouds in understanding how satellites interact with their local charged particle environments.
Key Points
First identification of H2+ in the Jovian magnetosphere from 13 to 18 RJ, with a density ratio of H2+/H+ = 8 ± 4%
H2+ in Jupiter's magnetosphere is predominantly produced by Europa's neutral toroidal cloud
Europa's total H2 neutral loss rate is 1.2 ± 0.7 kg s−1
Electron Beams at Europa Allegrini, F.; Saur, J.; Szalay, J. R. ...
Geophysical research letters,
16 July 2024, Letnik:
51, Številka:
13
Journal Article
Recenzirano
Odprti dostop
Jupiter's moon Europa contains a subsurface ocean whose presence is inferred from magnetic field measurements, the interpretation of which depends on knowledge of Europa's local plasma environment. A ...recent Juno spacecraft flyby returned new observations of plasma electrons with unprecedented resolution. Specifically, powerful magnetic field‐aligned electron beams were discovered near Europa. These beams, with energies from ∼30 to ∼300 eV, locally enhance electron‐impact‐excited emissions and ionization in Europa's atmosphere by more than a factor three over the local space environment, and are associated with large jumps of the magnetic fields. The beams therefore play an essential role in shaping Europa's plasma and magnetic field environment and thus need to be accounted for electromagnetic sounding of Europa's ocean and plume detection by future missions such as JUICE and Europa Clipper.
Plain Language Summary
A recent Juno spacecraft close flyby of Jupiter's moon Europa revealed the presence of powerful electrons beams. Based on previous observations and modeling of electron beams at the moon Io, such beams were not expected to be observed so close to Europa. Overall, the proximity of the beams to Europa indicates that the acceleration of these electrons takes place much closer to Europa than anticipated and that these beams, therefore, stem from a new and previously unknown acceleration mechanism. The beams are predicted to have an outsized influence on the ionization of the constituents of Europa's tenuous atmosphere and are accompanied with large magnetic field perturbations. Hence, these electron beams are an important ionization source that modify the moon's ionosphere, the electric current systems, and the magnetic field environment. In particular, the presence of electron beams will affect plasma conditions that are used to infer the extent of a subsurface ocean via the magnetic induction signal. These beams significantly impact the space plasma environment around Europa which needs to be accounted for by future missions such as ESA's (European Space Agency) JUICE (Jupiter Icy Moons Explorer) and NASA's (National Aeronautics and Space Administration) Europa Clipper mission.
Key Points
Powerful electron beams that significantly shape Europa's space environment are discovered during a Juno flyby
The beams enhance electron‐impact‐excited emissions in Europa's atmosphere and are associated with large jumps of the magnetic fields
The beams' proximity to Europa and their pitch angle distribution constrain the source acceleration to be near or within the plasma disk
Context.
The observation of numerous magnetic switchbacks and associated plasma jets in Parker Solar Probe (PSP) during its first five orbits, particularly near the Sun, has attracted considerable ...attention. Switchbacks have been found to be systematically associated with correlated reversals in the direction of the propagation of Alfvénic fluctuations, as well as similar reversals of the electron strahl.
Aims.
Here we aim to see whether the energetic particles change direction at the magnetic field switchbacks.
Methods.
We use magnetic field data from the MAG suite’s fluxgate magnetometer instrument to identify switchback regions. We examine the radial anisotropy of the energetic particles measured by the EPI-Lo instrument of the IS⊙IS suite.
Results.
We find that energetic particles measured by EPI-Lo generally do not preferentially change their directionality from that of the background magnetic field to that of the switchbacks.
Conclusions.
A reasonable hypothesis is that particles with smaller gyroradii, such as strahl electrons, can reverse direction by following the magnetic field in switchbacks, but that larger gyroradii particles cannot. This provides the possibility of setting a constraint on the radius of the curvature of the magnetic field in switchbacks, a property not otherwise observed by PSP. We expect that particles at higher energies than those detectable by EPI-Lo will also not respond to switchbacks. The observed reversals of radial energetic particle flux are separate phenomena, likely associated with source locations or other propagation effects occurring at greater radial distances.
We compare electron and UV observations mapping to the same location in Jupiter's northern polar region, poleward of the main aurora, during Juno perijove 5. Simultaneous peaks in UV brightness and ...electron energy flux are identified when observations map to the same location at the same time. The downward energy flux during these simultaneous observations was not sufficient to generate the observed UV brightness; the upward energy flux was. We propose that the primary acceleration region is below Juno's altitude, from which the more intense upward electrons originate. For the complete interval, the UV brightness peaked at ~240 kilorayleigh (kR); the downward and upward energy fluxes peaked at 60 and 700 mW/m2, respectively. Increased downward energy fluxes are associated with increased contributions from tens of keV electrons. These observations provide evidence that bidirectional electron beams with broad energy distributions can produce tens to hundreds of kilorayleigh polar UV emissions.
Plain Language Summary
Jupiter's ultraviolet (UV) aurora is produced by electrons that precipitate into the planet's atmosphere and interact with hydrogen molecules. A number of different UV auroral emission regions have been identified such as the main aurora, the aurora associated with Jupiter's satellites, and the polar aurora located poleward of the main aurora. We examine electron and UV observations from Juno in Jupiter's northern polar region to investigate the processes responsible for producing Jupiter's polar aurora. We show electrons and UV emissions having simultaneous enhancements during a time when they map to the same location of Jupiter's upper atmosphere at the same time. We present evidence that electrons with energies between 0.1 and 100 kilo electron volts (keV) are capable of producing the polar UV emissions studied here and that further acceleration of these electrons may be occurring at altitudes below the spacecraft.
Key Points
Simultaneous peaks are observed in electron energy flux and UV brightness when they map to same location in Jupiter's polar region at the same time
Upward greater than downward electron energy fluxes are observed, suggesting that primary acceleration region may be below ~1.5 jovian radii
Downward energy fluxes able to produce tens to hundreds of kilorayleigh polar UV emissions are identified; increases in energy flux due to tens of keV electrons
Juno obtained unique low‐altitude space environment measurements over Jupiter's poles on 27 August 2016. Here Jupiter Energetic‐particle Detector Instrument observations are presented for electrons ...(25–800 keV) and protons (10–1500 keV). We analyze magnetic field‐aligned electron angular beams over expected auroral regions that were sometimes symmetric (bidirectional) but more often strongly asymmetric. Included are variable but surprisingly persistent upward, monodirectional electron angular beams emerging from what we term the “polar cap,” poleward of the nominal auroral ovals. The energy spectra of all beams were monotonic and hard (not structured in energy), showing power law‐like distributions often extending beyond ~800 keV. Given highly variable downward energy fluxes (below 1 RJ altitudes within the loss cone) as high as 280 mW/m2, we suggest that mechanisms generating these beams are among the primary processes generating Jupiter's uniquely intense auroral emissions, distinct from what is typically observed at Earth.
Key Points
Upward, energy‐monotonic energetic electron angular beams are unexpectedly persistent over Jupiter's polar caps
Jupiter's aurora appears not to be associated with monoenergetic electron beams but with other processes
Jupiter's aurora is powered by the downward portion of bidirectional, energy‐monotonic electron angular beams and diffuse precipitation
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