Alternating North‐South Brightness Ratio of Ganymede's Auroral Ovals: Hubble Space Telescope Observations Around the Juno PJ34 Flyby

peer reviewedWe report results of Hubble Space Telescope observations from Ganymede's orbitally trailing side which were taken around the flyby of the Juno spacecraft on 7 June 2021. We find that Ganymede's northern and southern auroral ovals alternate in brightness such that the oval facing Jupiter's magnetospheric plasma sheet is brighter than the other one. This suggests that the generator that powers Ganymede's aurora is the momentum of the Jovian plasma sheet north and south of Ganymede's magnetosphere. Magnetic coupling of Ganymede to the plasma sheet above and below the moon causes asymmetric magnetic stresses and electromagnetic energy fluxes ultimately powering the auroral acceleration process. No clear statistically significant timevariability of the auroral emission on short time scales of 100s could be resolved. We show that electron energy fluxes of several tens of mW m−2 are required for its OI 1,356 Å emission making Ganymede a very poor auroral emitter

In situ observation of decametric Jovian radio sources with Juno: comparison with simultaneous radio, electron and UV data

Since the discovery of the jovian auroral radio emissions, the position of the different radio source components (broadband kilometric (bKOM), hectometric (HOM) and decametric (DAM)) and their association with far ultraviolet (FUV) auroral emissions have been discussed extensively. We recently surveyed Juno’s first fifteen perijoves to track local radio sources from Juno/Waves in situ measurements (50 Hz–40 MHz). We conclude that the bKOM, HOM and DAM radio sources are located at different altitudes on the same magnetic field lines, with M-shell ranging from 10 to 62. Comparisons with the Jovian FUV auroral images simultaneously acquired by the Hubble Space Telescope (HST) reveals a partial spatial colocation between the FUV emission of the main oval and the radio sources studied here. These work focuses on Juno crossings of DAM sources for which FUV observations (HST/STIS, Juno/UVS), radio (Juno/Waves) and electron (Juno/JADE) measurements weer simultaneously acquired. These allow us to (i) better constrain the source locations, (ii) better understand the link between UV and DAM radio emissions, (iii) obtain the electron distribution function and the electron energy flux involved in the DAM radio emissions and (iv) compare with the theoretical radio emission process known as the Cyclotron Maser Instability

Combined Juno observations and modeling of th e Jovian auroral electron interaction with the Jovian upper atmosphere

The Juno mission provides a unique opportunity during each perijove pass to sample the downward electron flux at spacecraft altitude while observing far ultraviolet H2 and infrared H3+ emissions at Juno’s magnetic footprint. In addition, the ratio of the H2 spectral band absorbed by hydrocarbons to the unabsorbed portion of the spectrum (FUV color ratio) is often used as a proxy for the depth of the penetration of energetic electrons (relative to the hydrocarbon homopause). The relationship between the color ratio and the electron penetration has been simulated with a Monte Carlo model solving the Boltzmann transport equation. Analysis of concurrent FUV and IR images obtained during the first perijove (PJ1) suggests that the ratio of H3+ radiance to H2 unabsorbed emission is maximal in regions with low FUV color ratio. This result suggests that part of the H3+ column is lost in reactions with methane which converts H3+ into heavier ions. We also examine the observed relationship between the detailed morphology of the ultraviolet structures and of the associated UV color ratio, the total downward electron energy flux and its spectral characteristics

Braving the Dawn Storm: February 7th 2018 in situ and remote observations from Juno

Thanks to its unique polar orbit around Jupiter, Juno offered us the first complete views of the aurora at Jupiter. By doing so, the spacecraft allowed us to track some of the most spectacular auroral events, the dawn storms, from their initiation to their decay. Its prowess does not stop there: on February 7th 2018, Juno flew right through the magnetic field lines connected to a dawn storm at a distance of 5 Jovian radii. The JADE and JEDI particle instruments both recorded a sudden increase of the particle fluxes. This increase was also observed with the radiation monitors, indicating that even relativistic particles are involved in these events. This is consistent with the observation of particularly high color ratios in the UV auroral emissions. The intensity of the Alfvén waves also sharply increased at the same time as the particle fluxes increased, probably being the source of the particle acceleration. Moreover, Juno traversed several sources of bKOM radio emissions during this time interval. Finally, observations of strong and opposite field aligned currents on each side of the dawn storm supports the idea that dawn storms are associated with a current wedge, further strengthening the similitude between dawn storms at Jupiter and substorms at Earth

Magnetosphere-Ionosphere-Thermosphere Coupling study at Jupiter Based on Juno First 30 Orbits and Modeling Tools

International audienceThe dynamics of the Jovian magnetosphere is controlled by the complex interplay of the planet's fast rotation, its solar-wind interaction and its main plasma source at the Io torus, mediated by coupling processes involving its magnetosphere, ionosphere, and thermosphere. At the ionospheric level, these processes can be characterized by a set of key parameters including conductances, field-aligned and horizontal currents, electric fields, transport of charged particles along field lines, including the fluxes of electrons precipitating into the upper atmosphere which trigger auroral emissions, as well as the particle and Joule heating power dissipation rates into the upper atmosphere. Determination of these keys parameters makes it possible to estimate the net transfer of momentum and energy between Jovian upper atmosphere and equatorial magnetosphere. A method based on a combined use of Juno multi-instrument data and three modeling tools was developed by Wang et al. (2021) and applied to an analysis of the first nine orbits to retrieve these key parameters along the Juno magnetic footprint. In this article, we extend this method to the first thirty Juno science orbits and to both north and south auroral crossings. Our results reveal a large variability of these parameters from orbit to orbit and between the two hemispheres, but they also show dominant trends. Southern current systems are consistent with the generation of a region of sub-corotating ionospheric plasma flows, while both super-corotating and sub-corotating plasma flows are found in the north. These results are discussed in the light of the previous space and ground-based observations and currently available models of plasma convection and current systems, and their implications on our understanding of MIT coupling at Jupiter are assessed

Magnetosphere-Ionosphere-Thermosphere Coupling study at Jupiter Based on Juno First 30 Orbits and Modeling Tools

International audienceThe dynamics of the Jovian magnetosphere is controlled by the complex interplay of the planet's fast rotation, its solar-wind interaction and its main plasma source at the Io torus, mediated by coupling processes involving its magnetosphere, ionosphere, and thermosphere. At the ionospheric level, these processes can be characterized by a set of key parameters including conductances, field-aligned and horizontal currents, electric fields, transport of charged particles along field lines, including the fluxes of electrons precipitating into the upper atmosphere which trigger auroral emissions, as well as the particle and Joule heating power dissipation rates into the upper atmosphere. Determination of these keys parameters makes it possible to estimate the net transfer of momentum and energy between Jovian upper atmosphere and equatorial magnetosphere. A method based on a combined use of Juno multi-instrument data and three modeling tools was developed by Wang et al. (2021) and applied to an analysis of the first nine orbits to retrieve these key parameters along the Juno magnetic footprint. In this article, we extend this method to the first thirty Juno science orbits and to both north and south auroral crossings. Our results reveal a large variability of these parameters from orbit to orbit and between the two hemispheres, but they also show dominant trends. Southern current systems are consistent with the generation of a region of sub-corotating ionospheric plasma flows, while both super-corotating and sub-corotating plasma flows are found in the north. These results are discussed in the light of the previous space and ground-based observations and currently available models of plasma convection and current systems, and their implications on our understanding of MIT coupling at Jupiter are assessed

Magnetosphere-Ionosphere-Thermosphere Coupling study at Jupiter Based on Juno First 30 Orbits and Modeling Tools

International audienceThe dynamics of the Jovian magnetosphere is controlled by the complex interplay of the planet's fast rotation, its solar-wind interaction and its main plasma source at the Io torus, mediated by coupling processes involving its magnetosphere, ionosphere, and thermosphere. At the ionospheric level, these processes can be characterized by a set of key parameters including conductances, field-aligned and horizontal currents, electric fields, transport of charged particles along field lines, including the fluxes of electrons precipitating into the upper atmosphere which trigger auroral emissions, as well as the particle and Joule heating power dissipation rates into the upper atmosphere. Determination of these keys parameters makes it possible to estimate the net transfer of momentum and energy between Jovian upper atmosphere and equatorial magnetosphere. A method based on a combined use of Juno multi-instrument data and three modeling tools was developed by Wang et al. (2021) and applied to an analysis of the first nine orbits to retrieve these key parameters along the Juno magnetic footprint. In this article, we extend this method to the first thirty Juno science orbits and to both north and south auroral crossings. Our results reveal a large variability of these parameters from orbit to orbit and between the two hemispheres, but they also show dominant trends. Southern current systems are consistent with the generation of a region of sub-corotating ionospheric plasma flows, while both super-corotating and sub-corotating plasma flows are found in the north. These results are discussed in the light of the previous space and ground-based observations and currently available models of plasma convection and current systems, and their implications on our understanding of MIT coupling at Jupiter are assessed

An Initial Survey of Juno-UVS Auroral Emission Spectra

We present an initial Juno-UVS survey of spectra of the major features of Jupiter’s ultraviolet auroras, which primarily include band emissions of H2 excited by electron impact, and the Lyman series of H arising from electron impact dissociative excitation of H2. The primary difference found in most of the observed spectra is the column of hydrocarbons (mostly methane) overlying the aurora production layer in Jupiter’s atmosphere. This leads to the “color ratio” of the emissions, commonly defined as the ratio of auroral emissions at wavelengths 155-162 nm, where methane is transparent, to those at 125-130 nm, where methane is strongly absorbing. Over the course of the Juno mission, it has been found that the brightness of most auroral features known from previous observations from Earth orbit have a strong dependence on local time. The primary purpose of this survey is to examine if other details in the spectra of these features are likewise correlated with local time, and whether they are also sensitive to other changes, either in the precipitating particles (e.g., the mean electron energy) or in the auroral atmosphere (e.g., the ambient H2 vibrational distribution)