Magnetic reconnection has been commonly reported between the solar wind IMF and the magnetic field of Earth and other planets. This article is the first report of a reconnection event in the tail region of Ganymede, the only known moon with an intrinsic magnetic field. We show strong evidence of Juno passing very close to an X-line at Ganymede's tail. We report the observation of distinctive electron Bernstein mode waves with unique characteristics particular to a reconnection site. We detect a clear reversal of a magnetic field component. Electron densities and pitch angle distributions also support the findings. Finally, from the time sequence of the observations by the different instruments on Juno, we reconstruct a likely trajectory of Juno around the reconnection site.

The Juno Waves instrument can be used to accurately determine the electron density inside Io’s orbit, the inner Io torus. These observations have revealed a local peak in the electron density just inside M=5 and at centrifugal latitudes above about 10º that is likely the ’cold torus’ as identified in Earth-based observations of S+ emissions. This peak or ’finger’ is separated from the more dense Io torus by a local minimum or ’trough’ at M ≥ 5. The electron densities are inferred by identifying characteristic frequencies of the plasma such as the low-frequency cutoff of Z-mode radiation at fL=0 and the low-frequency cutoff of ordinary mode radiation at fpe that depend on the electron density. The ’finger’ density ranges from about 0.2 to 65 cm-3 and decreases with increasing centrifugal latitude. The ’trough’ densities range from 0.05 to ~10 cm-3. This pattern of a density ’trough’ followed by the ’finger’ closer to Jupiter is found on repeated passes through the inner Io torus over a range of centrifugal latitudes. Using a simple model for the electron densities measured above about 10º centrifugal latitude, we’ve estimated the scale height of the ’finger’ densities as about 1.17 RJ with respect to the centrifugal equator, which is somewhat surprising given the expected cold temperature of the cold torus. The larger scale height suggests a population of light ions, such as protons, are elevated off the centrifugal equator. This is confirmed by a multi-species diffusive equilibrium model.

A document by Ali H. Sulaiman. Click on the document to view its contents.

At Jupiter, part of the auroral radio emissions are induced by the Galilean moons Io, Europa and Ganymede. Until now, except for Ganymede, they have been only 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 range of magnetic flux tubes which sustain the various Jupiter-satellite interactions, and in turn to sample in situ the associated radio emission regions. In this study, we focus on the detection and the characterization of radio sources associated with Io, Europa and Ganymede. Using electric wave measurements or radio observations (Juno/Waves), in situ electron measurements (Juno/JADE-E), and magnetic field measurements (Juno/MAG) we demonstrate that the Cyclotron Maser Instability (CMI) driven by a loss-cone electron distribution function is responsible for the encountered radio sources. We confirmed that radio emissions are associated with Main (MAW) or Reflected Alfvén Wing (RAW), but also show that for Europa and Ganymede, induced radio emissions are associated with Transhemispheric Electron Beam (TEB). For each traversed radio source, we determine the latitudinal extension, the CMI-resonant electron energy, and the bandwidth of the emission. We show that the presence of Alfvén perturbations and downward field aligned currents are necessary for the radio emissions to be amplified.

Juno’s highly eccentric polar orbit takes it to perijove distances of ∼ 1.06 RJ on each orbit. For the first perijove, this occurred just north of the jovigraphic equator, but has precessed north by about a degree per orbit over the mission. Minimum altitudes vary from ∼3200 to 8000 km through the mission. The Waves instrument observes a number of plasma wave modes in and near the non-auroral ionosphere that provide information on the local electron number density, including electron plasma oscillations that occur at the electron plasma frequency fpe and whistler-mode hiss which has an upper frequency limit of fpe in Jupiter’s strongly magnetized inner magnetosphere. The electron plasma frequency provides the electron number density. Over the ∼59 perijoves analyzed to date, peak densities range from ∼100 to 80,000 cm-3. More recent perijoves reveal topside ionospheric peaks at latitudes greater than about 40°. The density profiles can be highly variable from one perijove to the next. And, there can be deviations from simple smooth increases and decreases with altitude within individual ionospheric passes. Spatial variations may be responsible for some of the variability, perhaps related to Jupiter’s complex, higher order magnetic field. We show the variation in ionospheric density profiles and the distribution of peak densities as a function of latitude and System III longitude as well as other geometric parameters. In addition to the complex magnetic field, possible factors affecting ionospheric density variations investigated here are ionospheric dynamos analogous to those at Earth and precipitation of energetic particles.

A document by Brieuc Collet. Click on the document to view its contents.

A document by William S Kurth. Click on the document to view its contents.

The Juno Waves instrument measured plasma waves associated with Ganymede’s magnetosphere during its flyby on 7 June, day 158, 2021. Three distinct regions were identified including a wake, and nightside and dayside regions in the magnetosphere distinguished by their electron densities and associated variability. The magnetosphere includes electron cyclotron harmonic emissions including a band at the upper hybrid frequency, as well as whistler-mode chorus and hiss. These waves likely interact with energetic electrons in Ganymede’s magnetosphere by pitch angle scattering and/or accelerating the electrons. The wake is accentuated by low-frequency turbulence and electrostatic solitary waves. Radio emissions observed before and after the flyby likely have their source in Ganymede’s magnetosphere.

We present multi-instrument Juno observations on day-of-year 86, 2017 that link particles and fields in Jupiter’s polar magnetosphere to transient UV emissions in Jupiter’s northern auroral region known as dawn storms. Juno ranged from 42ºN - 51ºN in magnetic latitude and 5.8 – 7.8 jovian radii (1 RJ = 71,492 km) during this period. These dawn storm emissions consisted of two separate, elongated structures which extended into the nightside, rotated with the planet, had enhanced brightness (up to at least 1.4 megaRayleigh) and high color ratios. The color ratio is a proxy for the atmospheric penetration depth and therefore the energy of the electrons that produce the UV emissions. Juno observed electrons and ions on magnetic field lines mapping to these emissions. The electrons were primarily field-aligned, bi-directional, and, at times, exhibited sudden intensity decreases below ~10 keV coincident with intensity enhancements up to energies of ~1000 keV, consistent with the high color ratio observations. The more energetic electron distributions had characteristic energies of ~160 – 280 keV and downward energy fluxes (~70 – 135 mW/m2) that were a significant fraction needed to produce the UV emissions for this event. Magnetic field perturbations up to ~0.7% of the local magnetic field showing evidence of upward and downward field-aligned currents, whistler mode waves, and broadband kilometric radio emissions were also observed along Juno’s trajectory during this timeframe. These high latitude observations show similarities to those in the equatorial magnetosphere associated with dynamics processes such as interchange events, plasma injections, and/or tail reconnection.

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 the Ganymede flux tube. Using electromagnetic waves (Juno/Waves) and in-situ electron measurements (Juno/JADE-E), we estimate the flux tube width to be a few 100 km, a growth rate of the radio emission >3x10-4, an electron population of energy E=4-15 keV and an emission beaming angle of θ=76°-83°, at a frequency 1.005-1.021xfce. We also confirmed that radio emission is associated with Ganymede’s down-tail far-ultraviolet emission.

The Juno spacecraft’s polar orbits have enabled direct sampling of Jupiter’s low-altitude auroral field lines. While various datasets have identified unique features over Jupiter’s main aurora, they are yet to be analyzed altogether to determine how they can be reconciled and fit into the bigger picture of Jupiter’s auroral generation mechanisms. Jupiter’s main aurora has been classified into distinct “zones”, based on repeatable signatures found in energetic electron and proton spectra. We combine fields, particles, and plasma wave datasets to analyze Zone-I and Zone-II, which are suggested to carry the upward and downward field-aligned currents, respectively. We find Zone-I to have well-defined boundaries across all datasets. H+ and/or H3+ cyclotron waves are commonly observed in Zone-I in the presence of energetic upward H+ beams and downward energetic electron beams. Zone-II, on the other hand, does not have a clear poleward boundary with the polar cap, and its signatures are more sporadic. Large-amplitude solitary waves, which are reminiscent of those ubiquitous in Earth’s downward current region, are a key feature of Zone-II. Alfvénic fluctuations are most prominent in the diffuse aurora and are repeatedly found to diminish in Zone-I and Zone-II, likely due to dissipation, at higher altitudes, to energize auroral electrons. Finally, we identify sharp and well-defined electron density depletions, by up to two orders of magnitude, in Zone-I, and discuss their important implications for the development of parallel potentials, Alfvénic dissipation, and radio wave generation.