We constrain Europa's tenuous atmosphere on the subsolar hemisphere by combining two sets of observations: oxygen emissions at 1,304 and 1,356 Å from Hubble Space Telescope (HST) spectral images and ...Galileo magnetic field measurements from its closest encounter, the E12 flyby. We describe Europa's atmosphere with three neutral gas species: global molecular (O2) and atomic oxygen (O), and localized water (H2O) present as a near‐equatorial plume and as a stable distribution concentrated around the subsolar point on the moon's trailing hemisphere. Our combined modeling based on the ratio of OI 1,356 to OI 1,304 Å emissions from Roth (2021; https://doi.org/10.1029/2021gl094289) and on magnetic field data allows us to derive constraints on the density and location of O2 and H2O in Europa's atmosphere. We demonstrate that 50% of the O2 and between 50% and 75% of the H2O abundances from Roth (2021; https://doi.org/10.1029/2021gl094289) are required to jointly explain the HST and Galileo measurements. These values are conditioned on a column density of O close to the upper limit of 6 × 1016 m−2 derived by Roth (2021; https://doi.org/10.1029/2021gl094289), and on a strongly confined stable H2O atmosphere around the subsolar point. Our analysis yields column densities of 1.2 × 1018 m−2 for O2, and 1.5 × 1019 to 2.2 × 1019 m−2 at the subsolar point for H2O. Both column densities, however, still lie within the uncertainties of Roth (2021; https://doi.org/10.1029/2021gl094289). Our results provide additional evidence for the existence of a stable H2O atmosphere at Europa.
Key Points
We combine Hubble Space Telescope spectral images and Galileo magnetometer data to constrain the density and location of water vapor in Europa's atmosphere
We simulate the plasma interaction for the Galileo E12 flyby with a three‐component atmosphere: global O2, stable confined H2O, and a plume
Using 50% of O2 and from 50% to 75% of H2O column densities from Roth (2021) yields magnetic field signatures consistent with both observations
Context. Electromagnetic coupling of planetary moons with their host planets is well observed in our solar system. Similar couplings of extrasolar planets with their central stars have been studied ...observationally on an individual as well as on a statistical basis. Aims. We aim to model and to better understand the energetics of planet star and moon planet interactions on an individual and as well as on a statistical basis. Methods. We derived analytic expressions for the Poynting flux communicating magnetic field energy from the planetary obstacle to the central body for sub-Alfvénic interaction. We additionally present simplified, readily useable approximations for the total Poynting flux for small Alfvén Mach numbers. These energy fluxes were calculated near the obstacles and thus likely present upper limits for the fluxes arriving at the central body. We applied these expressions to satellites of our solar system and to HD 179949 b. We also performed a statistical analysis for 850 extrasolar planets. Results. Our derived Poynting fluxes compare well with the energetics and luminosities of the satellites’ footprints observed at Jupiter and Saturn. We find that 295 of 850 extrasolar planets are possibly subject to sub-Alfvénic plasma interactions with their stellar winds, but only 258 can magnetically connect to their central stars due to the orientations of the associated Alfvén wings. The total energy fluxes in the magnetic coupling of extrasolar planets vary by many orders of magnitude and can reach values larger than 1019 W. Our calculated energy fluxes generated at HD 179949 b can only explain the observed energy fluxes for exotic planetary and stellar magnetic field properties. In this case, additional energy sources triggered by the Alfvén wave energy launched at the extrasolar planet might be necessary. We provide a list of extrasolar planets where we expect planet star coupling to exhibit the largest energy fluxes. As supplementary information we also attach a table of the modeled stellar wind plasma properties and possible Poynting fluxes near all 850 extrasolar planets included in our study. Conclusions. The orders of magnitude variations in the values for the total Poynting fluxes even for close-in extrasolar planets provide a natural explanation why planet star coupling might have been only observable on an individual basis but not on a statistical basis.
The electromagnetic interaction between Io, Europa, and Ganymede and the rotating plasma that surrounds Jupiter has a signature in the aurora of the planet. This signature, called the satellite ...footprint, takes the form of a series of spots located slightly downstream of the feet of the field lines passing through the moon under consideration. In the case of Io, these spots are also followed by an extended tail in the downstream direction relative to the plasma flow encountering the moon. A few examples of a tail for the Europa footprint have also been reported in the northern hemisphere. Here we present a simplified Alfvénic model for footprint tails and simulations of vertical brightness profiles for various electron distributions, which favor such a model over quasi‐static models. We also report here additional cases of Europa footprint tails, in both hemispheres, even though such detections are rare and difficult. Furthermore, we show that the Ganymede footprint can also be followed by a similar tail. Finally, we present a case of a 320° long Io footprint tail, while other cases in similar configurations do not display such a length.
Key Points
The length of the footprint tail is not a reliable parameter to differentiate quasi‐static and Afvénic tail generation models
Monte Carlo simulations favor the Alfvénic electron acceleration scenario over the quasi‐static electric field scenario for the tail
The Europa and Ganymede footprints also have a tail, in both hemispheres
Jupiter's ultraviolet (UV) aurorae, the most powerful and intense in the solar system, are caused by energetic electrons precipitating from the magnetosphere into the atmosphere where they excite the ...molecular hydrogen. Previous studies focused on case analyses and/or greater than 30‐keV energy electrons. Here for the first time we provide a comprehensive evaluation of Jovian auroral electron characteristics over the entire relevant range of energies (~100 eV to ~1 MeV). The focus is on the first eight perijoves providing a coarse but complete System III view of the northern and southern auroral regions with corresponding UV observations. The latest magnetic field model JRM09 with a current sheet model is used to map Juno's magnetic foot point onto the UV images and relate the electron measurements to the UV features. We find a recurring pattern where the 3‐ to 30‐keV electron energy flux peaks in a region just equatorward of the main emission. The region corresponds to a minimum of the electron characteristic energy (<10 keV). Its polarward edge corresponds to the equatorward edge of the main oval, which is mapped at M shells of ~51. A refined current sheet model will likely bring this boundary closer to the expected 20–30 RJ. Outside that region, the >100‐keV electrons contribute to most (>~70–80%) of the total downward energy flux and the characteristic energy is usually around 100 keV or higher. We examine the UV brightness per incident energy flux as a function of characteristic energy and compare it to expectations from a model.
Plain Language Summary
Aurorae, also commonly called Northern or Southern Lights, are among the most spectacular displays of nature. They are observed not only at Earth but at other planets too, such as Mars, Jupiter, and Saturn. In fact, Jupiter has the brightest aurora in the solar system. The aurora is created when electrons and/or ions in space precipitate into the atmosphere and excite the ambient gas. At Jupiter, they mostly shine in the ultraviolet which is invisible to our eyes but can be seen with suitable instrumentation. The faster the electrons, the deeper they go into the atmosphere, but also the more energy they carry, which eventually can be converted to create more light. This study is about characterizing the electrons that create Jupiter's aurora using many instruments from the National Aeronautics and Space Administration's Juno Mission. We find that different ultraviolet emissions correspond to different electron characteristics. Knowing the differences will help us to understand the bigger picture to explain the processes that create the aurora.
Key Points
We present a survey of Jovian auroral electrons characteristics from 50 eV to 1000 keV by Juno
We present a metric to identify main oval crossings in electron data using 3‐30 keV electrons energy flux
We estimate the UV brightness per incident electron energy flux as a function of characteristic energy
The influence of the solar wind on Jupiter's magnetosphere is studied via three‐dimensional global MHD simulations. We especially examine how solar wind density variations affect the main auroral ...emission. Our simulations show that a density increase in the solar wind has strong effects on the Jovian magnetosphere: the size of the magnetosphere decreases, the field lines are compressed on the dayside and elongated on the nightside (this effect can be seen even deep inside the magnetosphere), and dawn‐dusk asymmetries are enhanced. Our results also show that the main oval becomes brighter when the solar wind is denser. But the precise response of the main oval to such a density enhancement in the solar wind depends on the local time: on the nightside the main oval becomes brighter, while on the dayside it first turns slightly darker for a few hours and then also becomes brighter. Once the density increase in the solar wind reaches the magnetosphere, the magnetopause moves inward, and in less than 5 h, a new approximate equilibrium position is obtained. But the magnetosphere as a whole needs much longer to adapt to the new solar wind conditions. For instance, the total electrical current closing in the ionosphere slowly increases during the simulation and it takes about 60 h to reach a new equilibrium. By then the currents have increased by as much as 45%.
Key Points
The solar wind ram pressure and the brightness of the main oval are positively correlated at Jupiter
Local time asymmetries in the magnetosphere are enhanced when the solar wind ram pressure increases
The main oval becomes broader at noon local time when the solar wind ram pressure is high
We analyze precipitating electron fluxes connected to 18 crossings of Io's footprint tail aurora, over altitudes of 0.15 to 1.1 Jovian radii (RJ). The strength of precipitating electron fluxes is ...dominantly organized by “Io‐Alfvén tail distance,” the angle along Io's orbit between Io and an Alfvén wave trajectory connected to the tail aurora. These fluxes best fit an exponential as a function of down‐tail extent with an e‐folding distance of 21°. The acceleration region altitude likely increases down‐tail, and the majority of parallel electron acceleration sustaining the tail aurora occurs above 1 RJ in altitude. We do not find a correlation between the tail fluxes and the power of the initial Alfvén wave launched from Io. Finally, Juno has likely transited Io's Main Alfvén Wing fluxtube, observing a characteristically distinct signature with precipitating electron fluxes ~600 mW/m2 and an acceleration region extending as low as 0.4 RJ in altitude.
Plain Language Summary
The Juno spacecraft crossed magnetic field lines connected to Io's auroral signature in Jupiter's atmosphere. By measuring the electrons sustaining this auroral feature, we find that the region these electrons are accelerated is typically more than one Jovian radius away from Jupiter's atmosphere. For one of the 18 transits, we find Juno has most likely directly transited above the main auroral spot in Io's auroral signature.
Key Points
Electron fluxes are best organized by the “Io‐Alfvén tail distance,” following an exponential with e‐folding distance of 21°
Juno has likely directly crossed the Main Alfvén Wing spot, observing precipitating electron fluxes ~600 mW/m2
The majority of parallel electron acceleration sustaining the Io footprint tail occurs above 1 RJ altitude
Recent observations by the Juno spacecraft have shown that electrons contributing to Jupiter's main auroral emission appear to be frequently characterized by broadband electron distributions, but ...also less often mono‐energetic electron distributions are observed as well. In this work, we quantitatively derive the occurrence rates of the various electron distributions contributing to Jupiter's aurora. We perform a statistical analysis of electrons measured by the JEDI‐instrument within 30–1,200 keV from Juno's first 20 orbits. We determine the electron distributions, either pancake, field‐aligned, mono‐energetic, or broadband, through energy and pitch angles to associate various acceleration mechanisms. The statistical analysis shows that field‐aligned accelerated electrons at magnetic latitudes greater than 76° are observed in 87.6% ± 7.2% of the intervals time averaged over the dipole L‐shells according the main oval. Pancake distributions, indicating diffuse aurora, are prominent at smaller magnetic latitudes (<76°) with an occurrence rate of 86.2% ± 9.6%. Within the field‐aligned electron distributions, we see broadband distributions 93.0% ± 3.8% of the time and a small fraction of isolated mono‐energetic distribution structures 7.0% ± 3.8% of the time. Furthermore, these occurrence statistics coincide with the findings from our energy flux statistics regarding the electron distributions. Occurrence rates thus also characterize the overall energetics of the different distribution types. This study indicates that stochastic acceleration is dominating the auroral processes in contrast to Earth where the discrete aurora is dominating.
Plain Language Summary
With the Juno spacecraft arriving in the magnetosphere of Jupiter, first flyby particle measurements have changed the knowledge about the developing process of Jupiter's intense aurora. The observations of auroral particles show a stochastic behavior rather than a preference for specific energy. Our statistical analysis of the first 20 flybys at Jupiter compares the occurrence of different particle distributions and highlights the importance of different generation theories for Jupiter's aurora. A generation via stochastic rather than mono‐energetic behavior is deduced and supports previous observations.
Key Points
We present a statistical study of Jupiter's auroral electrons within 30–1,200 keV based on Juno's first 20 perijoves
Broadband electron distributions dominates Jupiter's main auroral zone as they are observed in 93% ± 3% of the intervals studied here
Dominance of broadband distributions underlines the importance of a turbulent or stochastic acceleration process
Juno's highly inclined orbits provide opportunities to sample high‐latitude magnetic field lines connected to the orbit of Io, among the other Galilean satellites. Its payload offers both ...remote‐sensing and in‐situ measurements of the Io‐Jupiter interaction. These are at discrete points along Io's footprint tail and at least one event (12th perijove) was confirmed to be on a flux tube Alfvénically connected to Io, allowing for an investigation of how the interaction evolves down‐tail. Here we present Alfvén Poynting fluxes and field‐aligned current densities along field lines connected to Io and its orbit. We explore their dependence as a function of down‐tail distance and show the expected decay as seen in UV brightness and electron energy fluxes. We show that the Alfvén Poynting and electron energy fluxes are highly correlated and related by an efficiency that is fully consistent with acceleration from Alfvén wave filamentation via a turbulent cascade process.
Plain Language Summary
Io and Jupiter are electrodynamically coupled resulting in the Io footprint tail. This is one of the most persistent, stable, and recognizable features of Jupiter's aurora. The Juno spacecraft routinely samples magnetic field lines connected to Io's orbit, allowing for an investigation of this powerful coupling. We use data recorded by Juno to estimate a proxy for the strength of this interaction, that is, electromagnetic energy, and show its dependence downstream of Io and how the interaction decays. We further show that the available electromagnetic energy and electron energy are intimately linked, suggesting a transfer of energy between wave and particles. This is the basis upon which electrons end up precipitating into Jupiter's upper atmosphere and generate some of the brightest auroras.
Key Points
Alfvénic Poynting fluxes and electron energy fluxes are highly correlated on magnetic field lines connected to Io's orbit
The efficiency in the Main Alfvén Wing is ∼10%, fully consistent with Alfvén wave filamentation via a turbulent cascade process
Field‐aligned current densities are quantified and exhibit a decay in magnitude down‐tail of Io
The Juno spacecraft had previously observed intense high frequency wave emission, broadband electron and energetic proton energy distributions within magnetic flux tubes connected to Io, Europa, ...Ganymede, and their wakes. In this work, we report consistent enhancements in <46 keV energy proton fluxes during these satellite flux tube transit intervals. We find enhanced fluxes at discrete energies linearly separated in velocity for proton distributions within Io wake flux tubes, and both proton and electron distributions within Europa and Ganymede wake flux tubes. We propose these discrete enhancements to be a result of resonances between particles' bounce motion with standing Alfvén waves generated by the satellite‐magnetosphere interaction. We corroborate this hypothesis by comparing the bounce and field‐line resonance periods expected at the satellites' orbits. Hence, we find bounce‐resonant acceleration is a fundamental process that can accelerate particles in Jupiter's inner magnetosphere and other astrophysical plasmas.
Plain Language Summary
The passage of the Galilean moons‐ Io, Europa, Ganymede, and Callisto, perturbs the plasma flow in Jupiter's magnetosphere, creating waves that travel from the moon and reflect off Jupiter's ionosphere. These waves have been proposed to accelerate charged particles, and such accelerated particles had been observed by the Juno spacecraft during its passage through magnetic field lines connected to the satellite wakes. In this work, we find instances when this acceleration occurs selectively at specific energies that have constant separation in speed. We propose that this selective acceleration is due to resonance between particle bounce motion and the waves arising from the satellite wake perturbation. Bounce‐resonant acceleration is a promising fundamental process which can accelerate particles in Jupiter's inner magnetosphere and other plasma systems with similar geometries.
Key Points
Proton and electron flux enhancements in satellite and wake flux tubes often occur at discrete energies linearly separated in speed
Broadband proton flux enhancements at <46 keV energies were also observed within satellite flux tube crossings
Particles can be accelerated via resonance between bounce motion and standing Alfvén waves generated by moon‐magnetosphere interactions
PDF
