We report the first in situ observations of electron measurements at a Europa footprint tail (FPT) crossing in the auroral region. During its 12th science perijove pass, Juno crossed magnetic field ...lines connected to Europa's FPT. We find that electrons in the range ~0.4 to ~25 keV, with a characteristic energy of 3.6 ± 0.5 keV, precipitate into Jupiter's atmosphere to create the footprint aurora. The energy flux peaks at ~36 mW/m2, while the peak ultraviolet (UV) brightness is estimated at 37 kR. We estimate the peak electron density and temperature to be 17.3 cm−3 and 1.8 ± 0.1 keV, respectively. Using magnetic flux shell mapping, we estimate that the radial width of the interaction at Europa's orbit spans roughly 3.6 ± 1.0 Europa radii. In contrast to typical Io FPT crossings, the instrument background caused by penetrating energetic radiation (> ~5–10 MeV electrons) increased during the Europa FPT crossing.
Plain Language Summary
Jupiter's moons interact with Jupiter's space environment, or magnetosphere, and create auroral spots and tails in Jupiter's ionosphere. Io's aurora footprint on Jupiter is the strongest and most persistent of all moons, but Ganymede, Callisto, and Europa's auroral footprints are also routinely observed by remote platforms. NASA's Juno mission and its instrument suite occasionally fly through regions that are connected to the moon‐magnetosphere interactions. During these crossings, Juno samples the electrons and ions that create the aurora. This paper is the first report of electron measurements taken during a Juno crossing of Europa's tail. These measurements confirm previous results based on remote observations. Most importantly, they provide a sample of the conditions in the regions associated with Europa's footprint aurora in Jupiter's magnetosphere.
Key Points
This is the first report of in situ electron measurements of a Europa footprint tail crossing
Precipitating electron energies range from ~0.4 to ~25 keV with a characteristic energy of 3.6 keV, consistent with a low color ratio of the auroral emissions
The instrument background caused by > ~5–10 MeV penetrating electrons increased during the crossing, opposite to what is observed at Io
Juno Plasma Wave Observations at Europa Kurth, W. S.; Wilkinson, D. R.; Hospodarsky, G. B. ...
Geophysical research letters,
28 December 2023, Letnik:
50, Številka:
24
Journal Article
Recenzirano
Odprti dostop
Juno passed by Europa at an altitude of 355 km on 29 September, day 272, 2022. As one of Juno's in situ science instruments, the Waves instrument obtained observations of plasma waves that are ...essential contributors to Europa's interaction with its environment. Juno observed chorus, a band at the upper hybrid frequency providing the local plasma density, and electrostatic solitary structures in the wake. In addition, impulses due to micron‐sized dust impacts on Juno were recorded with a local maximum very close to Europa. The peak electron density near Europa was ∼330 cm−3 while the surrounding magnetospheric density was in the range of 50–150 cm−3. There was a significant separation between the Europa flyby and Juno's crossing of Jupiter's magnetic equator, enabling a unique identification of effects associated with the moon as opposed to magnetospheric phenomena normally occurring at the magnetic equator near 10 Jovian radii.
Plain Language Summary
Plasma waves are electromagnetic fields occurring in a plasma due to motions of the charged particles comprising the plasma. These waves can arise at various locations and at a range of frequencies depending on many factors, such as the number density of charged particles and the strength of the magnetic field. Here we discuss plasma waves observed by Juno during its 355‐km flyby of Europa on 29 September 2022. Some waves, called upper hybrid resonance emissions can provide information on the plasma density. Other waves, called electrostatic solitary waves are indicative of electron beams in the plasma. And yet other waves, called whistler‐mode chorus, are important in the interchange of energy between electrons and the waves, resulting in the acceleration of the electrons. Each of these types of waves were observed near Europa by the Juno plasma wave instrument and they are diagnostic of Europa's interaction with the Jovian magnetosphere. The Waves instrument also detects electrical impulses due to the collision of the spacecraft with dust grains moving at over 23 km/s that allow a determination of the concentration of dust near Europa.
Key Points
Two chorus bands, electrostatic solitary waves and upper hybrid emissions are observed at Europa
Plasma densities near Europa derived from the upper hybrid resonance frequency peak near the wake axis at about 330 cm−3
Micron‐sized dust impacts peak near closest approach to Europa
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
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
Jupiter's satellite auroral footprints are a manifestation of the satellite‐magnetosphere interaction of the Galilean moons. Juno's polar elliptical orbit enables crossing the magnetic flux tubes ...connecting each Galilean moon with their associated auroral emission. Its payload allows measuring the fields and particle population in the flux tubes while remotely sensing their associated auroral emissions. During its thirtieth perijove, Juno crossed the flux tube directly connected to Ganymede's leading footprint spot, a unique event in the entire Juno prime mission. Juno revealed a highly‐structured precipitating electron flux, up to 316 mW/m2, while measuring both a small perturbation in the magnetic field azimuthal component and small Poynting flux with an estimated total downward current of 4.2 ± 1.2 kA. Based on the evolution of the footprint morphology and the field and particle measurements, Juno transited for the first time through a region connected to the transhemispheric electron beam of the Ganymede footprint.
Plain Language Summary
The interaction between Jupiter's corotating plasma torus and the Galilean satellites generates a set of complex magnetospheric processes. One such interaction produces permanent auroral spots around Jupiter's northern and southern poles, known as footprints. For each close flyby, Juno's in situ instruments can measure such interaction. During its thirtieth perijove, Juno crossed the magnetic field lines connecting the interaction region of Ganymede with one of its auroral spots on Jupiter. This study describes and analyzes the set of measurements associated with that unique event.
Key Points
Juno crossed the magnetic flux tube connected to Ganymede auroral footprint and recorded a multi‐instrument set of measurements
Juno measured ∼316 mW/m2 of precipitating electrons while magnetically tied to Ganymede's leading auroral footprint spot
The associated Juno measurements suggest that it transited through a region linked to the transhemispheric electron beam
Jupiter's satellite auroral footprints are a consequence of the interaction between the Jovian magnetic field with co‐rotating iogenic plasma and the Galilean moons. The disturbances created near the ...moons propagate as Alfvén waves along the magnetic field lines. The position of the moons is therefore “Alfvénically” connected to their respective auroral footprint. The angular separation from the instantaneous magnetic footprint can be estimated by the so‐called lead angle. That lead angle varies periodically as a function of orbital longitude, since the time for the Alfvén waves to reach the Jovian ionosphere varies accordingly. Using spectral images of the Main Alfvén Wing auroral spots collected by Juno‐UVS during the first 43 orbits, this work provides the first empirical model of the Io, Europa, and Ganymede equatorial lead angles for the northern and southern hemispheres. Alfvén travel times between the three innermost Galilean moons to Jupiter's northern and southern hemispheres are estimated from the lead angle measurements. We also demonstrate the accuracy of the mapping from the Juno magnetic field reference model (JRM33) at the completion of the prime mission for M‐shells extending to at least 15 RJ. Finally, we shows how the added knowledge of the lead angle can improve the interpretation of the moon‐induced decametric emissions.
Plain Language Summary
The interaction between the Jovian magnetospheric plasma and the Galilean moons gives rise to a complex set of phenomena, including the generation of auroral spots magnetically related to the moons and the generation of radio emissions. The magnetic perturbations local to the moons propagate at a finite speed along the magnetic field lines, and reach the northern and southern Jovian hemispheres where they produce the auroral spots. Studying the position of these auroral spots and how they vary over a complete Jovian rotation provides information about the magnetic mapping, as they map directly to the actual physical positions of the moons. The magnetic field model derived from Juno's prime mission is in good agreement with the observation of the satellite footprints. This paper provides information about how the electromagnetic perturbation resulting from the interaction propagates at a finite speed to create auroral spots, leading to an angular shift between the instantaneously magnetically‐mapped position of the moon and the auroral footprint, a quantity also known as the ”equatorial lead angle”. The present work provides an empirical fit of the equatorial lead angle for Io, Europa, and Ganymede derived from Juno data.
Key Points
Over 1,600 ultraviolet spectral images of the Io, Europa, and Ganymede footprints from Juno are analyzed
Empirical formulae for the Io, Europa, and Ganymede equatorial lead angles derived from Juno data are provided
Alfvén travel time estimates are derived, constraining the Alfvénic interaction at the three innermost Galilean moons
The ionospheric Alfvén resonator (IAR) is a structure formed by the rapid decrease in the plasma density above a planetary ionosphere. This results in a corresponding increase in the Alfvén speed ...that can provide partial reflection of Alfvén waves. At Earth, the IAR on auroral field lines is associated with the broadband acceleration of auroral particles, sometimes termed the Alfvénic aurora. This arises since phase mixing in the IAR reduces the perpendicular wavelength of the Alfvén waves, which enhances the parallel electric field due to electron inertia. This parallel electric field fluctuates at frequencies of 0.1–20.0 Hz, comparable to the electron transit time through the acceleration region, leading to the broadband acceleration. The prevalence of such broadband acceleration at Jupiter suggests that a similar process can occur in the Jovian IAR. A numerical model of Alfvén wave propagation in the Jovian IAR has been developed to investigate these interactions, indicating that the IAR resonant frequencies are in the same range as those at Earth. This model describes the evolution of the electric and magnetic fields in the low‐altitude region close to Jupiter that is sampled during Juno's perijove passes. In particular, the model relates measurement of magnetic fields below the ion cyclotron frequency from the MAG and Waves instruments on Juno and electric fields from Waves to the associated parallel electric fields that can accelerate auroral particles.
Plain Language Summary
Just like at Earth, the polar regions of the planet Jupiter are circled by a luminous aurora (northern and southern lights) that can be seen from telescopes like the Hubble Space Telescope near Earth. The aurora on both planets is produced by electrons impacting the upper atmosphere, causing the atoms and molecules in this region to emit light. At Earth, these electrons are mainly produced by large voltages that cause all the electrons to be accelerated to nearly the same energy. However, recent observations from the Juno satellite at Jupiter shows that these electrons are mainly accelerated over a broad range of energies. This suggests that the voltages accelerating these electrons are fluctuating rapidly in time. Such fluctuations can be caused by the strong increase in the effective wave speed due to a rapid decrease in the number of electrons as the altitude is increased. We have developed a computer model to help understand these interactions.
Key Points
Broadband acceleration of auroral particles at Jupiter, can be achieved by Alfvén waves propagating in the ionospheric Alfvén resonator
Numerical results indicate that electrons could be accelerated to the 10–100 keV range for observed levels of Alfvén wave activity
There is also an Alfvén resonator in the high‐Alfvén speed velocity region between the ionosphere and the dense plasma sheet
Solid lipid nanoparticles (SLNs), the spheroidal-shaped, colloids state lipophilic-natured, innovative nanoscale particulate materials, are being concurrently prepared by the quality-by-design ...approach for cellular and sub-cellular delivery of drugs and other payloads with facilitated physicochemical characteristics for targeted delivery. The delivery of drugs, other pharmaceuticals and biopharmaceutical materials, and genes to the diseased body organs, tissues, and cellular mass have been developed as promising nanocarriers for different high-incidence cancers and other disease therapies, including the Alzheimer’s, Parkinson’s, and tuberculosis. SLNs have evolved as favorable lipid-based formulation, and have served as oral and intravenous carriers that targeted the drug with stable and sterile transport, sustained delivery, controlled drug/payload deloading, and requisite biodistributions. SLNs advantages, shortcomings, and bottlenecks have been discussed with plausible remediation strategies. The laboratory-scale and bulk preparations, use of different lipids in various preparation, surface coatings, physicochemical properties of the final product, and characterization protocols are also encompassed, as are the routes of administrations, specific-sites-targeting, and on-site outreach with biocompatibility, bioavailability, and the absorption, distribution, metabolism, and excretion and pharmacokinetics, and pharmacodynamics inputs with relevance to the therapy. Plausible applications in complex and genetic disorders, and as personalized medicine, also of traditional and alternative medicine prospects, are also discussed.
Upward‐moving energetic electrons with energies of 1 MeV and above were observed over the entire Jovian polar region. The electrons were found to be associated with intense broadband whistler mode ...waves, similar to terrestrial whistler mode auroral hiss. Upward‐propagating whistler mode hiss at Earth is known to be generated by upward‐moving, magnetic field‐aligned electron beams (from electric field‐aligned potentials), by a beam‐plasma instability at the Landau resonance. Assuming this process at Jupiter, we present a linear stability analysis, showing the electron distribution functions (based on inverted‐V observations made by the Juno Jovian Auroral Distributions Experiment, JADE‐E, instrument) are unstable. The polarization of the modeled waves is consistent with whistler mode hiss (right‐hand circularly polarized). From the results of the linear stability analysis, we find that the calculated growth rates are sufficient to produce the observed whistler mode waves.
Key Points
Upgoing electron beams can generate upward‐propagating whistler mode waves over the Jovian polar cap region
Numerical simulations show the electron beams are unstable and capable of producing the observed whistler mode waves
Large growth rates are found for Landau (n = 0) resonance
We characterize the precipitating electrons accelerated in the Europa‐magnetosphere interaction by analyzing in situ measurements and remote sensing observations recorded during 10 crossings of the ...flux tubes connected to Europa's auroral footprint tail by Juno. The electron downward energy flux, ranging from 34 to 0.8 mW/m2, exhibits an exponential decay as a function of downtail distance, with an e‐folding factor of 7.4°. Electrons are accelerated at energies between 0.3 and 25 keV, with a characteristic energy that decreases downtail. The electron distributions form non‐monotonic spectra in the near tail (i.e., within an angular separation of less than 4°) that become broadband in the far tail. The size of the interaction region at the equator is estimated to be 4.2 ± 0.9 Europa radii, consistent with previous estimates based on theory and UV observations.
Plain Language Summary
The space environment close to Jupiter is dominated by the magnetic field of the giant planet in a so‐called magnetosphere. The four Galilean moons, including Europa, orbit deep inside the Jovian magnetosphere and therefore constantly interact with the rapidly rotating plasma flow made of charged particles trapped by the magnetic field of the giant planet. The interaction between moons and plasma generates electromagnetic waves, accelerate particles and produce emissions at various wavelengths, including bright UV auroral spots and tails in the atmosphere of Jupiter. In this work, we present 10 events where the Juno spacecraft observed both in situ and remotely the acceleration of electrons due to the interaction between the icy moon Europa and the magnetospheric environment. We characterize the properties of the accelerated electrons. In particular, we find that acceleration is maximum near the moon itself, and that two distinct families of electron distributions exist.
Key Points
Juno unambiguously observed 10 events of downward electron acceleration from Europa at various downtail separations with the moon
Precipitating energy fluxes decrease exponentially as a function of downtail distance from the moon, with an e‐folding of 7.4°
Two types of electron distributions exist: non‐monotonic in the near tail and broadband in the far tail
