PS5.2

Using 30 inbound passes through the Jovian system, we combine measurements from the fields and particles instruments on the Juno spacecraft to survey the properties of Jupiter's plasma disk. Juno's orbit is particularly useful for exploring the variation in plasma conditions with latitude as well as radial distance (from ~10 to ~50 RJ). We present basic plasma properties (composition, density, temperature, velocity, magnetic field strength) to make maps of the plasma environment. Also show that on some of the 53-day orbits the plasma sheet has regular structure (density having roughly Gaussian distribution with latitude and decreasing with distance) but there are also highly irregular orbits with low or erratic density distributions.

How to cite: Bagenal, F., Huscher, E., Wilson, R., Allegrini, F., and Ebert, R.: Survey of Jupiter’s Plasma Disk from Juno Observations, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-989, https://doi.org/10.5194/egusphere-egu21-989, 2021.

We present our latest model of electron radiation belts developed for a large region of Jupiter's magnetosphere (1-50 Rj). For the region inward of Io, electron distributions are computed from a computational code that solves the governing three-dimensional Fokker-Planck equation. This physics-based model accounts for different mechanisms to discuss the energy and spatial distributions of electrons for L values between 1 and 5. The model for the innermost magnetospheric region is expanded to the middle magnetosphere using an empirical approach. In this paper, we first show how our large-scale model of Jupiter's electron radiation belts agrees with data sets from past missions (Pioneer 10 and 11 GTT, Galileo EPD and EPI measurements). We then focus on our effort to combine Juno (JEDI, JADE Electron Ambient Background Counts) and Galileo EPD (> 1.5, 11.5 MeV) datasets to improve our model for both the region beyond Io and the inner edge of the Jovian electron radiation belts. Finally, simulations of Jupiter's synchrotron emission are presented to gauge the contribution of ultra-energetic electrons trapped beyond L ~ 3 at different latitudes to radio emission observed by Juno MWR.

How to cite: Santos-Costa, D., Allegrini, F., Wilson, R., Kollmann, P., Clark, G., Mauk, B., Connerney, J., Jorgensen, J. L., Gulkis, S., Janssen, M. A., Oyafuso, F., Brown, S. T., Levin, S. M., Becker, H. N., and Bolton, S. J.: Reconciling Juno and past mission datasets to improve large-scale models of radiation-belt electron and radio emission distributions at Jupiter, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-6294, https://doi.org/10.5194/egusphere-egu21-6294, 2021.

We have recently developed a new technique that uses the timings of any three consecutive current sheet crossings to determine the instantaneous motion of Jupiter’s current sheet relative to the spacecraft. Using this information on the instantaneous location of Jupiter’s current sheet, we have modeled the external field of the magnetic disc observed by Juno and Galileo spacecraft in terms of a Harris current sheet type equilibrium and obtained a map of the thickness of the Jovian current sheet over all local times and radial distances. Our modeling of Juno and Galileo magnetic field data shows that in all local times the current sheet thickness increases with radial distance. We also find that the Jovian current sheet thickness is highly asymmetric in local time, being at its thinnest in the dawn sector and the thickest in the dusk sector. The current sheet thickness on the dayside is comparable to that in the dusk sector. The nightside current sheet is intermediate in its thickness to the dawn and the dusk sectors.

In this presentation, we use the instantaneous location of the current sheet to model the electron densities measured by the plasma or plasma wave instrument. We show that overall, the scale height of electrons and the current sheet tend to be identical. However, we have encountered many cases where the electrons have a two scale-height structure where a thin plasma sheet is embedded within a thicker current sheet, the cause for which is not known. By using the magnetic field and electron density data, we have computed the plasma content of flux tubes in several local time locations in the magnetosphere. We relate the plasma content of these flux tubes to plasma rotation, plasma density and current sheet thickness. It appears that as flux tubes rotate to the dusk side, they slow down and the plasma scale height increases but the total plasma content remains constant.

How to cite: Khurana, K., Hospodarsky, G., and Paranicas, C.: The Scale Height of Charged Particles in Jupiter’s Magnetosphere, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-3774, https://doi.org/10.5194/egusphere-egu21-3774, 2021.

The Jovian Infrared Auroral Mapper (JIRAM) on board Juno is a spectro-imager which is observing the
atmosphere of Jupiter and its auroral emission using its two imagers in the L (3.3-3.6μm) and M bands (4.5-
5.0μm) and a spectrometer (2-5 μm spectral range).
The highly elliptic orbit of Juno and the unprecedented resolution of the JIRAM imager allowed to retrieve
wealth of details about the morphology of moon-related aurorae. This phenomenon is due to the jovian magnetic
field sweeping past the Galiean moons, which generate Alfven waves travelling towards the ionosphere and set
up field aligned currents. When the associated electrons reach the ionosphere, they interact with the hydrogen
and make it to glow. In particular, the tails of the footprints showed a spot-like substructure consistently, which
were investigated using the L-band of the imager from perijove 4 to perijove 30. This feature was observed close
to the footprints, where the the typical distance between spots lies between 250km and 500km. This distance
decreases to 150km in a group of three observations in the northern emisphere when each moon is close to 250 ◦
west longitude. No correlation with orbital parameters such as the longitude of the moons was found so far,
which suggests that such morphology is almost purely due to ionospheric processes.
Moreover, during PJ 13 a long sequence of images of the Io footprint was shot and it revealed that the
secondary spots appears to corotate with Jupiter. This behaviour is observed also during orbits 14 and 26.
During these sequences JIRAM clearly observed the Io footprint leaving behind a trail of ”footsteps” as bright
spots.
The characteristics of these spots are incompatible with multiple reflection of Alfven waves between the two
emispheres. Instead, we are currently investigating ionospheric processes like the feedback instability (FI) as a
potential candidate to explain the generation of the observed small scale structure. This process relies on local
enhacement of conductivity in the ionosphere, which is affected by electron precipitation. Order of magnitude
estimates from the FI are compatible with the inter-spot distance and the stillness of the spots.

How to cite: Moirano, A., Mura, A., Adriani, A., Sordini, R., Migliorini, A., Zambon, F., Tosi, F., Altieri, F., Dinelli, B. M., Plainaki, C., Cicchetti, A., and Noschese, R.: Small scale structures in the footprint tails of the Galilean moons observed by JIRAM, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-12277, https://doi.org/10.5194/egusphere-egu21-12277, 2021.

Voyager 1 detected the first extra-terrestrial UV auroral emissions when it explored the Jupiter system in 1979 while the planet’s X-ray aurora was discovered later that year by the Einstein Observatory. Electrons are accelerated into Jupiter’s atmosphere near the poles and excite native molecular and atomic hydrogen. These then release UV photons after returning to the ground state. The same population of precipitating electrons can also emit high energy (>2 keV) X-ray photons by bremsstrahlung to produce Jupiter’s hard X-ray aurora. At higher latitudes and within the oval of UV and hard X-ray emissions is where the more diffuse UV and low energy (<2 keV) soft X-ray aurorae are found.  Charge exchange processes between precipitating ions and neutrals in the gas giant planet’s atmosphere are responsible for the soft X-ray emissions.

Simultaneous observations of Jupiter’s UV and X-ray aurorae were carried out by the Hubble Space Telescope (HST), Hisaki satellite and XMM-Newton in September 2019 to support Juno’s 22^nd perijove. Images of the northern far UV aurora by HST showed internally driven dawn storms and injection events occurring at least twice during the observation period. These features are thought to be caused by magnetic reconnection happening in the middle magnetosphere. This subsequently leads to the dipolarization of the field lines which injects hot magnetospheric plasma from the middle to the inner magnetosphere. Hisaki saw an impulsive brightening in the Io plasma torus on the day of the second event showing that there was indeed a large-scale injection that penetrated the central torus in the inner magnetosphere. At this time, the northern aurora brightened in both extreme UV and hard X-ray, which suggests that there was an increase in electron precipitation.  There was no response from the soft X-ray aurora, and no quasi-periodic pulsations, often observed in the auroral emissions, were detected during either of the events. X-ray spectral analysis reveals that the precipitating ions were iogenic. We conclude that we have witnessed two cases of mass injection in the Jovian inner magnetosphere due to Io mass loading events.

How to cite: Wibisono, A., Branduardi-Raymont, G., Dunn, W., Kimura, T., Coates, A., Grodent, D., Yao, Z., Kita, H., Rodriguez, P., Gladstone, R., Bonfond, B., and Haythornthwaite, R.: Jupiter's X-ray aurora during a mass injection and Io mass loading events observed by XMM-Newton, Hubble, and Hisaki, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-7657, https://doi.org/10.5194/egusphere-egu21-7657, 2021.

Charged particles impacting Jupiter’s atmosphere represent a major energy input, generating the most powerful auroral emissions in the Solar System. Most auroral features have now been explained as the result of impacting particles accelerated by quasi-static electric fields and/or wave-particle interactions in the surrounding space environment. However, the reason for Jupiter’s bright and dynamic polar regions remains a long-standing mystery. Recent spacecraft observations above these regions of “swirl” auroras have shown that high-energy electrons are regularly beamed away from the planet, which is inconsistent with traditional auroral drivers. The unknown downward-electron-acceleration mechanism operating close to Jupiter represents a gap in our fundamental understanding of planetary auroras. Here we propose a possible explanation for both the swirl auroras and the upward electron beams. We show that the perturbations of Jupiter’s strong magnetic field above the swirl regions that are driven by dynamics of the distant space environment can cause magnetic reconnection events at altitudes as low as ~0.2 Jupiter radii, rapidly releasing energy and potentially producing both the required downward and observed upward beams of electrons. Such an auroral driver has never before been postulated, resembling physics at work in the solar corona.

How to cite: Masters, A., Dunn, W., Stallard, T., Manners, H., and Stawarz, J.: Low-altitude magnetic reconnection events as possible drivers of Jupiter’s polar auroras, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-14719, https://doi.org/10.5194/egusphere-egu21-14719, 2021.

Juno's highly eccentric polar orbit was designed to provide the first measurements at low altitudes over the poles to explore Jupiter’s polar magnetosphere and auroras.  Orbit precession moves the initially equatorial perijove to higher northern latitudes at a rate of about one degree per orbit.  One result of the precession is that Juno crosses the equator at decreasing radial distances during the inbound portion of the orbit. Recently, Juno has crossed the magnetic equator at distances of 10 Jovian radii (R_J) and less.  Voyager and Galileo observations have shown the magnetic equator inside of 10 R_J to be the site of numerous plasma wave phenomena including whistler-mode hiss, chorus, electron cyclotron harmonics and upper hybrid bands.  In addition, this is the location of the plasma sheet at the outer edge of the Io and Europa torii.  The Juno orbit, with its near-polar inclination carries the spacecraft through this intriguing region to higher latitudes.  This paper examines the evolution of whistler-mode chorus and hiss as well as electron cyclotron waves from the magnetic equator to higher latitudes.  While there are now statistical studies of electromagnetic waves at intermediate latitudes based on Galileo and Juno observations, this paper is designed to show details of these wave phenomena utilizing the Juno Waves instrument’s burst mode for high resolution.  Each of these wave phenomena has the potential to interact with the electrons in the inner magnetosphere and cause pitch-angle scattering and/or acceleration, so they are important in the flow of mass and energy through the Jovian system.

How to cite: Kurth, W., Hospodarsky, G., Sulaiman, A., Elliott, S., Menietti, J. D., Faden, J., Piker, C., Wilkinson, D., Connerney, J. E. C., Bolton, S., Allegrini, F., and Mauk, B.: Plasma waves in the inner Jovian magnetosphere at low to mid-latitudes, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-8101, https://doi.org/10.5194/egusphere-egu21-8101, 2021.

Because of its large size, fast rotation and multiple atmospheric jets, Jupiter’s atmosphere holds a large variety of vortices. A large anticyclone at 19ºN planetographic latitude persists since at least 2006 after a complex dynamic history. This North Tropical Oval (NTrO) is located in the transition region between the North Equatorial Band (NEBn) and North Tropical Zone (NTrZ) and it is one of the longest-lived anticyclonic oval in the planet, following the Great Red Spot and oval BA. The region where it is located has a strong latitudinal shear, which allows the formation of dark cyclones and usually white anticyclones that stay stable in latitude. The NTrO has survived for years after mergers and disturbances: in February 2013, it merged with another oval and some months later, in September 2013, its color changed from white to red and then, in December 2014, back to white with an external red ring. The oval also survived the North Temperate Belt Disturbance (October 2016) which fully covered the oval, leaving it unobservable for a short time. It reappeared at its expected longitude as a white large oval keeping the same color and morphology from 2017 to 2020. Using JunoCam, Hubble Space Telescope (HST) and PlanetCam-UPV/EHU multi-wavelength observations, we describe the historic evolution of this oval’s properties. We used JunoCam and HST images to measure its size and its internal rotation obtaining a mean value of (10,500±1,000) x (5,800±600) km for the size and a mean relative vorticity of -(2±1)·10^-5s^-1. Contrarily to GRS and BA, which have higher vorticity values than their surroundings, the NTrO’s vorticity is nearly the same as the ambient vorticity of the area, which suggests that this oval is probably sustained by the zonal jets confining it. We also used HST and PlanetCam observations to characterize its color changes. The color and the altitude-opacity indices show that the oval is higher and has redder clouds than its environment but has lower cloud tops than other large ovals like the GRS, and it is less red than the GRS and oval BA. Despite the changes, mergers and disturbances experienced by the oval, its main characteristics remain unaltered and this suggests a vertically extended vortex with properties that could be related with the atmospheric dynamics below the observable cloud deck.

How to cite: Barrado-Izagirre, N., Legarreta, J., Sánchez-Lavega, A., Pérez-Hoyos, S., Hueso, R., Iñurrigarro, P., Rojas, J. F., Mendikoa, I., and Ordoñez-Etxeberria, I.: Color changes and dynamics of the third largest oval on Jupiter, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-7842, https://doi.org/10.5194/egusphere-egu21-7842, 2021.

JunoCam, the visible imager on the Juno mission’s payload that was designed primarily for public-outreach purposes, continues to produce images of Jupiter that provide unexpected scientific benefits.  Juno’s polar orbits enable observing regions of the planet that have not previously been detected at such high resolution by any previous spacecraft. JunoCam has a single CCD detector with an integral color-strip filter that enables the instrument to image in four color bands—blue, green, red and an 889-nm methane band.  JunoCam maps a field of view of 58° across the width of the detector, perpendicular to the spacecraft scan direction. We will describe characteristics and likely origins of bright white compact (~50 km) clouds, informally dubbed “pop-up” clouds by the JunoCam team.  We used the length of shadows of these and other features to determine the relative heights of clouds and assigned a provisional chemical classification based on relative altitudes from equilibrium-chemistry predictions. We tracked the continued interactions of small anticyclonic ovals with Jupiter’s Great Red Spot (GRS) that drew off high-altitude reddish haze into strips (commonly called “flakes”) on its western edge.  A lightning flash was detected in one of the compact circumpolar cyclones in late December. Observations of the south-polar circumpolar cyclones showed that the original unequally sided pentagon becoming a hexagon – with a cyclone filling in an open area, then a pentagon again over the course of 110 days.  In a collaboration with amateur astronomer Clyde Foster (S. Africa), we observed the morphology of an unexpected upwelling in late May of 2020, now known as “Clyde’s Spot”, and tracked its evolution in concert with several ground-based observations.  We also measured ~40-50 m/s winds around the sinuous jet bounding the South Polar Hood, an upper-level haze generated by auroral-related chemistry.  Lightly processed and raw JunoCam data continue to be posted on the JunoCam webpage at https://missionjuno.swri.edu/junocam/processing.   Citizen scientists download these images and upload their processed contributions.

How to cite: Orton, G., Hansen, C., Momary, T., Caplinger, M., Ravine, M., Rogers, J., Eichstaedt, G., Breushaber, S., Wong, M. H., Guillot, T., and Ingersoll, A.: Recent JunoCam Revelations About Discrete Features in Jupiter’s Atmosphere, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-3362, https://doi.org/10.5194/egusphere-egu21-3362, 2021.

The NASA Juno spacecraft is studying Jupiter’s atmosphere in depth since August 2016. The Jupiter Infrared Auroral Mapper (JIRAM) experiment (Adriani et al. 2014), one of the scientific instruments on board Juno, is composed of two broad-band imagers and an infrared spectrometer, dedicated to the observation of the auroral and chemical composition of the Jupiter’s atmosphere. Images and spectral observations in limb view geometry have been acquired since orbit 17 (December 2018) onwards, providing a wealth of details of the atmosphere at mid to equatorial latitudes, with a spatial resolution of the order of hundreds of meters per pixel. CH₄ and H₃⁺ emissions around the 3-μm region show two well separated layers at 200 km and at about 500-600 km above the 1-bar level. The CH₄ emission is quite unexpected and shows a maximum of emission close to the equator. In this work we present the distribution of CH₄ and H₃⁺ as observed at limb from December 2018 to September 2020 with the imaging subsystem of JIRAM. Their vertical distribution, obtained from simultaneous spectral measurements, is also shown. Temperature and volume mixing ratio (VMR) of the two species, retrieved using the spectral region between 3 and 4 μm (Dinelli et al. 2017, 2019) are discussed and compared with previous measurements.

Acknowledgments

The project JIRAM is funded by the Italian Space Agency.

References

Adriani A. Filacchione G., Di Iorio T., et al. (2014). JIRAM, the Jovian infrared Auroral mapper. Space Sci. Rev. 213, 393, https://doi.org/10.1007/s11214-014-0094-y.

Dinelli, B.M., et al. (2017), Preliminary Results from the JIRAM Auroral Observations taken during the first Juno orbit: 1 - Methodology and Analysis Applied to the Jovian Northern Polar Region, Geophys. Res. Lett., doi:10.1002/2017GL072929.

Dinelli B.M., Adriani A., Mura A., Altieri F., Migliorini A., Moriconi M.L., (2019). JUNO/JIRAM’s view of Jupiter’s H₃⁺ emissions, Phil. Trans. R. Soc.

How to cite: Migliorini, A., Dinelli, B. M., Moriconi, M., Altieri, F., Castagnoli, C., Mura, A., Adriani, A., Sordini, R., Atreya, S., Tosi, F., Bolton, S., Piccioni, G., Grassi, D., Moirano, A., Noschese, R., Cicchetti, A., Sindoni, G., Plainaki, C., and Olivieri, A.: Properties of H3+ and CH4 at mid and equatorial latitudes in the Jovian atmosphere, observed with JIRAM on Juno, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-8184, https://doi.org/10.5194/egusphere-egu21-8184, 2021.

The upper tropospheric zonal winds have been measured since decades using cloud tracking with maximum winds speeds of ∼ 100 m/s in the tropical region (Ingersoll et al. 1979). Juno measurements have shown that these winds extend in the deep layers of the planet (Kaspi et al. 2018). In the ionosphere, jets have been detected in the auroral zone with velocities of 1-2 km/s (Rego et al. 1999). In-between these atmospheric regions, in the stratosphere, there are no such tracers as clouds. Even if zonal winds can in principle be indirectly derived from temperature field by assuming the thermal wind balance (e.g. Flasar et al. 2004), this technique relies on a boundary condition often taken as the cloud-top structure which is located at levels that are separated from where the stratospheric temperature field is constrained. Also, this technique breaks down at equatorial latitudes.

Using the Atacama Large Millimeter/submillimeter Array, we mapped Jupiter’s stratospheric HCN emission in March 2017 to directly measure wind-induced Doppler shifts on the spectral lines. We imaged the HCN limb emission with an angular resolution of 1” and a very high spectral resolution. After subtracting the rapid rotation of the planet from the Doppler shifts measured on the spectral lines, we derived the wind speeds as a function of latitude on both limbs.

We find strong tropical jets at 1 mbar with velocities of 100-200 m/s lying atop the layers where the Quasi-Quadrennial Oscillation occurs. Most surprisingly, we find strong non-zonal winds in Jupiter’s polar regions at 0.1 mbar with counter-rotation velocities of 300-400 m/s. Their position coincides with the location of the main auroral oval.

In this paper, we will present our observations and results. We will also discuss their implications on the dynamics and chemistry of Jupiter’s stratosphere.

How to cite: Cavalié, T., Benmahi, B., Hue, V., Moreno, R., Lellouch, E., Fouchet, T., Hartogh, P., Rezac, L., Greathouse, T., Gladstone, R., Sinclair, J., Dobrijevic, M., Billebaud, F., and Jarchow, C.: Direct detection of auroral and equatorial jets in the stratosphere of Jupiter with ALMA, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-8726, https://doi.org/10.5194/egusphere-egu21-8726, 2021.

How to cite: Gavriel, N., Duer, K., Galanti, E., and Kaspi, Y. and the Juno MWR team: The relation between the zonal jets and ammonia anomalies in Jupiter, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-15705, https://doi.org/10.5194/egusphere-egu21-15705, 2021.

The Great Red Spot (GRS) of Jupiter is a large anticyclonic vortex present in the Jovian atmosphere. First observed in the XVII century, it is almost constantly located at 22°S and it is arguably one of the main atmospheric phenomena in the Solar System. Despite having been widely studied, the nature of the chromophore species that provide its characteristic colour to the GRS’s upper clouds and hazes is still unclear, as well as its creation and destruction mechanisms.

In this work we have analysed images provided by the Hubble Space Telescope’s Wide Field Camera 3 between 2015 and 2019, with a spectral coverage from the ultraviolet to the near infrared, including two methane absorption bands. These images have undergone a photometric process of cross calibration, ensuring a consistent correlation among the images corresponding to different visits and years. From such calibrated images, we have obtained the spectral reflectivity of the GRS and its surroundings, with particular emphasis on a few, dynamically interesting regions.

We used the NEMESIS radiative transfer suite to retrieve the main atmospheric parameters (particle vertical and size distributions, refractive indices…) that are able to explain the observed spectral reflectivity of the selected regions. Here we report the spatial and temporal variations on such parameters and their implications on the GRS overall dynamics.

How to cite: Anguiano-Arteaga, A., Pérez-Hoyos, S., Sánchez-Lavega, A., and Irwin, P. G. J.: Variations in spectral reflectivity and vertical cloud structure of Jupiter’s Great Red Spot, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-11076, https://doi.org/10.5194/egusphere-egu21-11076, 2021.

JIRAM (the Jovian InfraRed Auroral Mapper) is an infrared camera and
spectrometer on board Juno. JIRAM operates in the 2-5 μm spectral
range and is built to observe both Jupiter's infrared aurora and its
atmosphere. Since 2016, JIRAM has performed several observations of
the polar regions of the planet, thanks to the unique orbital design
of the Juno mission.  In the north polar region, Juno discovered, in
2017, the presence of an eight-cyclone structure around a single polar
cyclone; to the south, a polar cyclone is surrounded by five
circumpolar cyclones. The stability of these structures has been
monitored for almost 4 years. Recent observations, made at the end of
2019, showed that the configuration of the South Pole has temporarily
changed: the structure moved in a hexagon for a few months, before
returning to its original pentagonal shape. To the north, there are
significant hints that the octagonal shape may have been lost for a
similar period of time.
We find that all cyclones show a very slow, westward drift as a rigid
ensemble, and, in addition, they oscillate around their rest position
with similar timescales. These oscillations seem to propagate from
cyclone to cyclone. The implications of these transient deviations
from the symmetrical forms, which appear to be an apparent condition
of equilibrium, are discussed.

How to cite: Mura, A., Plainaki, C., Sindoni, G., Adriani, A., Grassi, D., Moriconi, M., Ciarravano, A., Piccioni, G., Migliorini, A., and Sordini, R.: The evolution of Jupiter polar cyclones, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-16226, https://doi.org/10.5194/egusphere-egu21-16226, 2021.

We derive Jupiter's zonal vorticity profile from JunoCam images, with Juno's polar orbit allowing the observation of latitudes that are difficult to observe from Earth or from equatorial flybys.  We identify cyclonic local vorticity maxima near 77.9°, 65.6°, 59.3°, 50.9°, 42.4°, and 34.3°S planetocentric at a resolution of ~1°, based on analyzing selected JunoCam image pairs taken during the 16 Juno perijove flybys 15-30. We identify zonal anticyclonic local vorticity maxima near 80.7°, 73.8°, 62.1°, 56.4°, 46.9°, 38.0°, and 30.7°S.  These results agree with the known zonal wind profile below 64°S, and reveal novel structure further south, including a prominent cyclonic band centered near 66°S. The anticyclonic vorticity maximum near 73.8°S represents a broad and skewed fluctuating anticyclonic band between ~69.0° and ~76.5°S, and is hence poorly defined. This band may even split temporarily into two or three bands.  The cyclonic vorticity maximum near 77.9°S appears to be fairly stable during these flybys, probably representing irregular cyclonic structures in the region. The area between ~82° and 90°S is relatively small and close to the terminator, resulting in poor statistics, but generally shows a strongly cyclonic mean vorticity, representing the well-known circumpolar cyclone cluster.

The latitude range between ~30°S and ~85°S was particularly well observed, allowing observation periods lasting several hours. For each considered perijove we selected a pair of images separated by about 30 - 60 minutes. We derived high-passed and contrast-normalized south polar equidistant azimuthal maps of Jupiter's cloud tops. They were used to derive maps of local rotation at a resolution of ~1° latitude by stereo-corresponding Monte-Carlo-distributed and Gauss-weighted round tiles for each image pair considered. Only the rotation portion of the stereo correspondence between tiles was used to sample the vorticity maps. For each image pair, we rendered ~40 vorticity maps with different Monte-Carlo runs. The standard deviation of the resulting statistics provided a criterion to define a valid area of the mean vorticity map. Averaging vorticities along circles centered on the south pole returned a zonal vorticity profile for each of the perijoves considered. Averaging the resulting zonal vorticity profiles built the basis for a discussion of the mean profile.

JunoCam also images the northern hemisphere, at higher resolution but with coverage restricted to a briefer time span and smaller area due to the nature of Juno's elliptical orbit, which will restrict our ability to obtain zonal vorticity profiles.

How to cite: Eichstädt, G., Rogers, J., Orton, G., and Hansen, C.: Jupiter's Zonal Vorticity Profile Observed by JunoCam, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-6274, https://doi.org/10.5194/egusphere-egu21-6274, 2021.

Convective storms on Jupiter usually develop in the cyclonic side of the jets or inside cyclones (Vasavada and Showman, 2005). On 31 May 2020 a convective storm developed inside a small cyclone (3º in longitudinal extent) in the South Temperate Belt at planetographic latitude 30ºS. The storm outbreak was captured by amateur astronomer Clyde Foster becoming widely known as Clyde’s spot. The storm was observed 2.5 days later by JunoCam with images displaying an apparent cyclonic structure with two main lobes and high-clouds observable in the methane absorption band. Analysis of these observations show the storm in a decaying phase with associated weak winds. Observations over the following months combined with prior observations (2 years) obtained by JunoCam, HST, IRTF and amateur observers show the long-term evolution of the cyclone before and after the convective eruption. The short-lived storm made the cyclone to display large changes in morphology and colour but not in its size or latitude, except for small fluctuations around a mean latitude and mean drift rate. Ground-based infrared observations at 5 μm show the region where the vortex was located characterized by a weakly warm radiance several months after the convective outbreak, indicating a relative clearing of clouds and haze. We have used the Explicit Planetary Isentropic-Coordinate (EPIC) numerical model (Dowling et. al., 1998) to simulate the cyclone and the effects of convective storms of different strengths and durations on it. These simulations were partially guided by our previous study of a similar convective storm in a different type of cyclone: an elongated structure known as the STB Ghost at the same latitude in 2018 (Iñurrigarro et. al., 2020). Both storms and cyclones were different in terms of their size, morphology and later evolution, but our simulations suggest that in both cases the convective eruptions were of similar power but with different lifetimes indicating that the energy source is water moist convection. We compare these storms and simulations with a similar convective storm observed in 1979 by Voyager 2 at 38ºS that quickly evolved into a Folded-Filamentary Region and investigate the outcome of convective storms at different latitudes from these simulations.

References:

Dowling et al., 1998. The Explicit Planetary Isentropic-Coordinate (EPIC) Atmospheric Model, Icarus, 132, 221-238.

Iñurrigarro et al., 2020. Observations and numerical modelling of a convective disturbance in a large-scale cyclone in Jupiter’s South Temperate Belt, Icarus, 336, 113475.

Vasavada and Showman, 2005. Jovian atmospheric dynamics: an update after Galileo and Cassini, Reports on Progress in Physics, 68, 1935-1996.

How to cite: Iñurrigarro, P., Hueso, R., Sanchez-Lavega, A., Foster, C., Legarreta, J., Rogers, J. H., Orton, G. S., Hansen, C. J., Eichstädt, G., García-Melendo, E., and Ordoñez-Etxeberria, I.: Short-lived storms inside long-lived cyclones: Simulations of the 2020 storm in the South Temperate Belt, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-10741, https://doi.org/10.5194/egusphere-egu21-10741, 2021.

The stratosphere of Jupiter is subject to an equatorial oscillation of its temperature structure with a quasi-period of 4 years (Orton et al. 1991, Leovy et al. 1991) which could result in a complex vertical and horizontal structure of prograde and retrograde jets. Yet, the stratospheric wind structure in Jupiter’s equatorial zone has never been directly measured. It has only been inferred in the tropical region from the thermal wind balance using temperature measurements in the stratosphere and the cloud-top wind speeds as a boundary condition (Flasar et al. 2004). However, the temperatures are not well-constrained between the upper troposphere and the middle stratosphere from the observations.

In this paper, we obtain for the first time an auto-consistent determination of the tropical wind structure using wind and temperature measurements all performed in the stratosphere. The wind speeds have been measured by Cavalié et al. (submitted) at 1 mbar in the stratosphere of Jupiter in the equatorial and tropical zone in March 2017 with ALMA. The stratospheric thermal field was measured five days apart in the low-to-mid latitudes with the IRTF/TEXES instrument (Giles et al. 2020). For the wind derivation, we use the thermal wind equation (Pedlosky, 1979) and equatorial thermal wind equation (Marcus et al. 2019). We will present and discuss our results.

This paper is a follow-up to the EGU21-8726 paper.

How to cite: Benmahi, B., Cavalié, T., Greathouse, T. K., and Hue, V.: The equatorial wind structure in Jupiter's stratosphere from direct wind and temperature measurements with ALMA and IRTF/TEXES, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-8819, https://doi.org/10.5194/egusphere-egu21-8819, 2021.

Juno's observations of Jupiter's gravity field have revealed extremely low values for the gravitational moments that are difficult to reconcile with the high abundance of metals observed in the atmosphere by both Galileo and Juno. Recent studies chose to arbitrarily get rid of one of these two constraints in order to build models of Jupiter.

In this presentation, I will detail our new Jupiter structure models reconciling Juno and Galileo observational constraints. These models confirm the need to separate Jupiter into at least 4 layers: an outer convective shell, a non-convective zone of compositional change, an inner convective shell and a diluted core representing about 60 percent of the planet in radius. Compared to other studies, these models propose a new idea with important consequences: a decrease in the quantity of metals between the outer and inner convective shells. This would imply that the atmospheric composition is not representative of the internal composition of the planet, contrary to what is regularly admitted, and would strongly impact the Jupiter formation scenarios (localization, migration, accretion).

In particular, the presence of an internal non-convective zone prevents mixing between the two convective envelopes. I will detail the physical processes of this semi-convective zone (layered convection or H-He immiscibility) and explain how they may persist during the evolution of the planet.

These models also impose a limit mass on the compact core, which cannot be heavier than 5 Earth masses. Such a mass, lower than the runaway gas accretion minimum mass, needs to be explained in the light of our understanding of the formation and evolution of giant planets.

I will finally detail the application of our work to Saturn, and what we can expect to learn about the interior of the giant planets in the years to come. 

How to cite: Debras, F. and Chabrier, G.: Interior of Jupiter in the context of Juno and Galileo: signature of a decoupling between the atmosphere and the interior, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-8203, https://doi.org/10.5194/egusphere-egu21-8203, 2021.

The amount and distribution of heavy elements in Jupiter’s interior is crucial to understand how the planet was formed and evolved. The results provided by the Juno mission in the last years have fundamentally changed our view of the interior of Jupiter. The remarkably accurate gravity data, including odd gravity harmonics, have allowed us to put constrains on the zonal flows, the extent of differential rotation and lead us to find that Jupiter has most likely a dilute core. In this study we do interior structure calculations using a Bayesian statistical approach and fitting all observational constrains, to show that a non-homogenous envelope is also a constraint set up by the Juno measurements, which is helping us to get closer to unveiling Jupiter’s deep secrets. 

How to cite: Miguel, Y., Bazot, M., Guillot, T., Galanti, E., Kaspi, Y., and Howard, S.: Jupiter's envelope is not homogeneous, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-14297, https://doi.org/10.5194/egusphere-egu21-14297, 2021.

How to cite: Militzer, B., Wahl, S., and Hubbard, W.: Gravity Measurements of the Juno Spacecraft Matched with Jupiter Models that rely on a Dilute Core and Deep Winds, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-9083, https://doi.org/10.5194/egusphere-egu21-9083, 2021.

Constraining Jupiter's internal structure is crucial for understanding its formation and evolution history. Recent interior models of Jupiter that fit Juno's measured gravitational field suggest an inhomogeneous interior and potentially the existence of a diluted core. These models, however, strongly depend on the model assumptions and the equations of state used. A complementary modelling approach is to use empirical structure models. 
These can later be used to reveal new insights on the planetary interior and be compared to standard models. 
Here we present empirical structure models of Jupiter where the density profile is constructed by piecewise-polytropic equations. With these models we investigate the relation between the normalized moment of inertia (MoI) and the gravitational moments J₂ and J₄. 
Given that only the first few gravitational moments of Jupiter are measured with high precision, we show that an accurate and independent measurement of the MoI value could be used to further constrain Jupiter's interior. An independent measurement of the MoI with an accuracy better than ~0.1% could constrain Jupiter's core region and density discontinuities in its envelope. 
We find that models with a density discontinuity at ~1 Mbar, as would produce a presumed hydrogen-helium separation, correspond to a fuzzy core in Jupiter. 
We next test the appropriateness of using polytropes, by comparing them with empirical models based on polynomials. 
We conclude that both representations result in similar density profiles and ranges of values for quantities like core mass and MoI.

How to cite: Neuenschwander, B. A., Helled, R., Movshovitz, N., and Fortney, J. J.: Connecting gravity field, moment of inertia, and core properties in Jupiter through empirical structure models, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-9835, https://doi.org/10.5194/egusphere-egu21-9835, 2021.

The asymmetric gravity field measured by the Juno spacecraft has allowed the estimation of the depth of Jupiter's zonal jets, showing that the winds extend approximately 3,000 km beneath the cloud level. This estimate was based on an analysis using a combination of all measured odd gravity harmonics, J₃, J₅, J₇, and J₉, but the wind profile's dependence on each of them separately has yet to be investigated. Furthermore, these calculations assumed the meridional profile of the cloud‐level wind extends to depth. However, it is possible that the interior jet profile varies somewhat from that of the cloud level. Here we analyze in detail the possible meridional and vertical structure of Jupiter's deep jet streams that can match the gravity measurements. We find that each odd gravity harmonic constrains the flow at a different depth, with J₃ the most dominant at depths below 3,000 km, J₅ the most restrictive overall, whereas J₉ does not add any constraint on the flow if the other odd harmonics are considered. Interior flow profiles constructed from perturbations to the cloud‐level winds allow a more extensive range of vertical wind profiles, yet when the meridional profiles differ substantially from the cloud level, the ability to match the gravity data significantly diminishes. Overall, we find that while interior wind profiles that do not resemble the cloud level are possible, they are statistically unlikely. Finally, inspired by the Juno microwave radiometer measurements, assuming the brightness temperature is dominated by the ammonia abundance, we find that depth‐dependent flow profiles are still compatible with the gravity measurements.

How to cite: Duer, K., Galanti, E., and Kaspi, Y.: The Range of Jupiter's Flow Structures that Fit the Juno Asymmetric Gravity Measurements, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-15384, https://doi.org/10.5194/egusphere-egu21-15384, 2021.

As of April 2021, Juno is close to complete its nominal mission, awaiting to enter its extended mission. Thanks to the extremely accurate Doppler data (having an accuracy as low as 10 micron/s at an integration time of 60 s) acquired during close perijove passes in the last 4 years, Juno provided an unprecedented view of Jupiter’s gravity field, which is crucial to determine its interior structure. In order to recover the gravity field of the planet, the orbits of Juno have to be reconstructed to a very high accuracy. The latest gravity field reconstruction showed hints to a non-static and/or non-axially symmetric field, possibly related to several different phenomena, such as normal modes, localized atmospheric or deeply-rooted dynamics. These tiny phenomena produces a residual signal at a level of few tens of micron/s in Juno Doppler data. To confidently study these tiny unconventional phenomena, the dynamical model of Juno’s spacecraft have been accurately characterized and possible error sources investigated and ruled out.

The focus of this study is Jupiter’s normal modes. Our main goal is to assess whether the residuals signatures can be explained by the gravitational disturbances induced by normal modes inside the planet, assuming reasonable physical constraints. Ground-based observations of Jupiter’ normal modes can be used as a guide.

How to cite: Durante, D. and Iess, L.: A peek into Jupiter’s normal modes from Juno gravity data, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-15398, https://doi.org/10.5194/egusphere-egu21-15398, 2021.

Quasi-static potentials have long thought to be one of the significant drivers of the main ultraviolet emission associated with Jupiter’s auroral oval. The magnetic field lines connecting to the auroral zone extend into Jupiter’s middle magnetosphere, at radii of 20R_J – 50 R_J. Such quasi-static potential structures are capable of accelerating charged particles into the planetary ionosphere and generating aurora, with the Juno JEDI instrument observing inverted-V potential structures on the order of megavolts. However, Juno’s observation of quasi-static potentials has not been as ubiquitous as was initially theorised. Juno has observed more frequent instances of bi-directional electron beams on the same field line, indicating the presence of dynamic processes occurring at different altitudes. In addition, this suggests that quasi-static potentials may not be a significant driver for the main UV emission.

In this paper, we present new results from a 1-D Vlasov model of the high-latitude magnetic field lines in the Jovian mid-magnetosphere. Our model is time-dependent and features a non-uniform mesh close to the ionosphere, allowing us to examine the formation of quasi-static potential structures in the upward current region over the course of a simulation. We will also present simulations showing the collapse and reformation of these potential structures, with the collapse showing the propagation of electron beams in both directions along the modelled field line.

How to cite: Constable, D., Ray, L., Badman, S., Arridge, C., and Gunell, H.: Modelling 1-D quasi-static potential structures at Jupiter, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-16144, https://doi.org/10.5194/egusphere-egu21-16144, 2021.

The dedicated Energetic Neutral Atom (ENA) imager on the Cassini spacecraft provided indispensable measurements of magnetospheric processes at Saturn. At Jupiter, Cassini provided only a few serendipitous ENA images as the spacecraft flew by Jupiter at large radial distances.  The Juno spacecraft, now in a polar orbit around Jupiter, carries no ENA camera, but the energetic particle JEDI instrument is sensitive to ENA’s with energies > 50 keV, provided there are few charged particles in the environment to mask their presence.  Even with limited ENA capabilities, the Juno mission has revealed important differences between Saturn and Jupiter with regard to how charged ions are lost from these magnetospheric systems. Specifically, a major contribution to ENA emissions at Jupiter come from Jupiter’s polar atmosphere. These ENAs likely arise from energetic ions that nearly precipitate in the auroral zone, only to mirror magnetically within the atmosphere where they charge exchange with atoms in Jupiter’s upper atmosphere. Cassini did not observe this precipitating component at Saturn despite the abundance of quality ENA measurements obtained there. We conclude that ion precipitation into Jupiter’s atmosphere is competitive with other loss processes.  In contrast, in the Saturn system, it is likely that losses associated with the dense neutral gas populations near the equator dominate the loss of energetic particles.

How to cite: Mauk, B., Allegrini, F., Bagenal, F., Bolton, S., Clark, G., Connerney, J., Gladstone, R., Haggerty, D., Kollmann, P., Mitchell, D. G., Paranicas, C., Roelof, E., and Rymer, A.: Comparing energetic particle loss processes in the magnetospheres of Jupiter and Saturn using Energetic Neutral Atom (ENA) remote sensing, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-6504, https://doi.org/10.5194/egusphere-egu21-6504, 2021.

The strong zonal flows observed at the cloud-level of the gas giants extend thousands of kilometers deep into the planetary interior, as indicated by the Juno and Cassini gravity measurements. However, the gravity measurements alone, which are by definition an integrative measure of mass, cannot constrain with high certainty the detailed vertical structure of the flow below the cloud-level. Here we show that taking into account the recent magnetic field measurements of Saturn and past secular variations of Jupiter's magnetic field, give an additional physical constraint on the vertical decay profile of the observed zonal flows in these planets. In Saturn, we find that the cloud-level winds extend into the planet with very little decay (barotropically) down to a depth of around 7,000 km, and then decay rapidly, so that within the next 1,000 km their value reduces to about 1% of that at the cloud-level. This optimal deep flow profile structure of Saturn matches simultaneously both the gravity field and the high-order latitudinal variations in the magnetic field discovered by the recent measurements. In the Jupiter case, using the recent findings indicating the flows in the planet semiconducting region are order centimeters per second, we show that with such a constraint, a flow structure similar to the Saturnian one is consistent with the Juno gravity measurements. Here the winds extend unaltered from the cloud-level to a depth of around 2,000 km and then decay rapidly within the next 600 km to values of around 1%. Thus, in both giant planets, we find that the observed winds  extend unaltered (baroctropically) down to the semiconducting region, and then decay abruptly. While it is plausible that the interaction with the magnetic field in the semiconducting region is responsible for winds final decay, it is yet to be understood whether another mechanism is involved in the process, especially in the initial decay form the strong 10s meter per seconds winds.

How to cite: Galanti, E. and Kaspi, Y.: Combined magnetic and gravity measurements probe the deep zonal flows of the gas giants, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-10553, https://doi.org/10.5194/egusphere-egu21-10553, 2021.

How to cite: Kaspi, Y. and Galanti, E. and the Juno Science Team: Constraints on the latitudinal structure of the deep zonal flows of Jupiter and Saturn, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-15158, https://doi.org/10.5194/egusphere-egu21-15158, 2021.

Titan is a unique body in the solar system in particular because of its earth-like surface features, its putative undersurface liquid water ocean and its large organic content in the atmosphere and on the surface . These chemical species evolve with season, as Titan follows Saturn in its orbit around the Sun with an inclination of about 27°. We performed an analysis of spectra acquired by Cassini/CIRS at high resolution covering the range from 10 to 1500 cm^-1 since the beginning and until the last flyby of Titan in 2017 and describe the temperature and composition variations ([1-3]. By applying our radiative transfer code (ARTT) to the high-resolution CIRS spectra we study the stratospheric evolution over almost two Titan seasons [1,2]. CIRS nadir and limb spectral together show variations in temperature and chemical composition in the stratosphere during the Cassini mission, before and after the Northern Spring Equinox (NSE) and also during one Titan year.

Since the 2010 equinox we have thus reported on monitoring of Titan’s stratosphere near the poles and in particular on the observed strong temperature decrease and compositional enhancement above Titan’s southern polar latitudes since 2012 and until 2014 of several trace species, such as complex hydrocarbons and nitriles, which were previously observed only at high northern latitudes. This effect followed the transition of Titan’s seasons from northern winter in 2002 to northern summer in 2017, while at that latter time the southern hemisphere was entering winter.

Our data show a continued decrease of the abundances which we first reported to have started in 2015. The 2017 data we have acquired and analyzed here are important because they are the only ones recorded since 2014 close to the south pole in the far-infrared nadir mode at high resolution. A large temperature increase in the southern polar stratosphere (by 10-50 K in the 0.5 mbar-0.05 mbar pressure range) is found and a change in the temperature profile’s shape. The 2017 observations also show a related significant decrease in most of the abundances which must have started sometime between 2014 and 2017 [3]. In our work, we show that the equatorial latitudes remain rather constant throughout the Cassini mission.

We have thus shown that the south pole of Titan is now losing its strong enhancement, while the north pole also slowly continues its decrease in gaseous opacities. It would have been interesting to see when this might happen, but the Cassini mission ended in September 2017. Perhaps future ground-based measurements and the Dragonfly mission can pursue this investigation and monitor Titan’s atmosphere to characterize the seasonal events. Our results set constraints on GCM and photochemical models.

References:

 [1] Coustenis et al., 2016, Icarus 270, 409-420; [2] Coustenis et al., 2018, Astroph. J., Lett., 854, no2; [3] Coustenis et al., 2020. Titan’s neutral atmosphere seasonal variations up to the end of the Cassini mission. Icarus 344, 113413. https://doi.org/10.1016/j.icarus.2019.113413.

How to cite: Coustenis, A., Jennings, D., Achterberg, R., Lavvas, P., Nixon, C., Bampasidis, G., and Flasar, F. M.: Evolution of Titan’s stratosphere with Cassini/CIRS, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-6267, https://doi.org/10.5194/egusphere-egu21-6267, 2021.

In this study we present occurences of SLAMS (short large-amplitude magnetic structures) upstream of the quasi-parallel bow shock of Saturn. Five events are analyzed in more detail using the data of the CAPS and MAG instruments of Cassini. Directional and speed analysis of the backstreaming particles related to ULF wave formation (and subsequent SLAMS evolution) in the foreshock region is presented. We also correlate the measured the ULF wave frequencies with the variations of the upstream magnetic field.
With a simple model we estimate the distance of the observed SLAMS from the bow shock front based on the measured plasma pressure. We also 
discuss the spatial characteristics of SLAMS observed near Saturn.

How to cite: Bebesi, Z. and Juhasz, A.: Effects of upstream conditions on ULF waves and SLAMS formation at Saturn, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-5987, https://doi.org/10.5194/egusphere-egu21-5987, 2021.

During the Grand Finale of the Cassini mission, the southern hemisphere of Saturn was shadowed by its rings and the substructures within, whose more intense shadows can be mapped to specific ionospheric altitudes. We successfully connect small-scale variations (dips) in the ionospheric H₂⁺ density below 2500 km, measured by the Ion and Neutral Mass Spectrometer (INMS) during orbits 288 and 292, to the shadows of individual ringlets and plateaus in the C Ring. From the H₂⁺ density signatures we estimate lower limits of the associated ringlet or plateau opacities. These will be compared with results obtained from stellar occultations and potential implications/constraints on the ionospheric dynamics will be discussed. The ringlet and plateau shadows are not associated with obvious dips in the electron density.

How to cite: Dreyer, J. and Vigren, E.: Ionospheric shadowing signatures of ringlets and plateaus in Saturn's C Ring, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-13316, https://doi.org/10.5194/egusphere-egu21-13316, 2021.

Gravity field measurements only weakly constrain the deep interiors of Jupiter and Saturn, stymieing efforts to measure the mass and compactness of these planets' cores, crucial properties for understanding their formation pathways and evolution. However, studies of Saturn's rings by Cassini have revealed waves driven by pulsation modes within Saturn, offering independent seismic probes of Saturn's interior. The observations reveal gravity mode (g mode) pulsations that indicate that a part of Saturn's interior is stably stratified by composition gradients, and the g mode frequencies directly probe the buoyancy frequency within the planet.

We compare structure models with gravity and new seismic measurements from Cassini to show that the data can only be explained by a diffuse, stably stratified core-envelope transition region in Saturn extending to approximately 60% of the planet's radius. This predominantly stable interior imposes significant constraints on Saturn's intrinsic magnetic field generation. The gradual distribution of heavy elements required by the seismology constrains mixing processes at work in Saturn, and it may reflect the planet's primordial structure and accretion history.

How to cite: Mankovich, C. and Fuller, J.: Saturn's Diffuse Core from Ring Seismology, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-13455, https://doi.org/10.5194/egusphere-egu21-13455, 2021.

The regions of Saturn’s cloud-covered atmosphere polewards of 60^o latitude are dominated in each hemisphere near the cloud tops by an intense, cyclonic polar vortex surrounded by a strong, high latitude eastward zonal jet. In the north, this high latitude jet takes the form of a remarkably regular zonal wavenumber m=6 hexagonal pattern that has been present at least since the Voyager spacecraft encounters with Saturn in 1980-81, and probably much longer. The origin of this feature, and the absence of a similar feature in the south, has remained poorly understood since its discovery. In this work, we present some new analyses of horizontal wind measurements at Saturn’s cloud tops polewards of 60 degrees in both the northern and southern hemispheres, previously published by Antuñano et al. (2015) using images from the Cassini mission, in which we compute kinetic energy spectra and the transfer rates of kinetic energy (KE) and enstrophy between different scales. 2D KE spectra are consistent with a zonostrophic regime, with a steep (~n^-5) spectrum for the mean zonal flow (n is the total wavenumber) and a shallower Kolmogorov-like KE spectrum (~n^-5/3) for the residual (eddy) flow, much as previously found for Jupiter’s atmosphere (Galperin et al. 2014; Young & Read 2017). Three different methods are used to compute the energy and enstrophy transfers, (a) as latitude-dependent zonal spectral fluxes, (b) as latitude-dependent structure functions and (c) as spatially filtered energy fluxes. The results of all three methods are largely in agreement in indicating a direct (forward) enstrophy cascade across most scales, averaged across the whole domain, an inverse kinetic energy cascade to large scales and a weak direct KE cascade at the smallest scales. The pattern of transfers has a more complex dependence on latitude, however. But it is clear that the m=6 North Polar Hexagon (NPH) wave was transferring KE into its zonal jet at 78^o N (planetographic) at a rate of ∏_E ≈ 1.8 x 10^-4 W kg^-1 at the time the Cassini images were acquired. This implies that the NPH was not maintained by a barotropic instability at this time, but may have been driven via a baroclinic instability or possibly from deep convection. Further implications of these results will be discussed.

References

Antuñano, A., T. del Río-Gaztelurrutia, A. Sánchez-Lavega, and R. Hueso (2015), Dynamics of Saturn’s polar regions, J. Geophys. Res. Planets, 120, 155–176, doi:10.1002/2014JE004709.

Galperin, B., R. M.B. Young, S. Sukoriansky, N. Dikovskaya, P. L. Read, A. J. Lancaster & D. Armstrong (2014) Cassini observations reveal a regime of zonostrophic macroturbulence on Jupiter, Icarus, 229, 295–320.doi: 10.1016/j.icarus.2013.08.030

Young, R. M. B. & Read, P. L. (2017) Forward and inverse kinetic energy cascades in Jupiter’s turbulent weather layer, Nature Phys., 13, 1135-1140. Doi:10.1038/NPHYS4227

How to cite: Read, P. L., Antuñano, A., Cabanes, S., Colyer, G., del Rio-Gaztelurrutia, T., and Sánchez-Lavega, A.: Turbulent kinetic energy spectra and cascades in the polar atmosphere of Saturn, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-14857, https://doi.org/10.5194/egusphere-egu21-14857, 2021.

Introduction: The Saturn's Semi-Annual Oscillation (SSAO) observed by Cassini is a source of debate within the community, because of its similarities (sometimes conflicting) with both the terrestrial Quasi-Biennial Oscillation (QBO) and the terrestrial Semi-Annual Oscillation (SAO). As the QBO, the downward propagation of the SSAO occurs almost to the tropopause (Schinder et al. 2011). In contrast, the half a Saturn year period of the SSAO is advocated for a seasonal forcing and hints the SAO mechanism driving. Moreover, observation of anomalies in warm temperature and high hydrocarbon concentration at winter tropics is interpreted as the downwelling branch of a meridional stratospheric circulation. 
Using DYNAMICO-Saturn Global Climate Model (GCM) -- with an higher vertical discretization (96 vertical levels from 3x10⁵ to 10^-1 Pa) than previous works (Spiga et al. 2020, Bardet et al. 2021)  -- we performed simulations lasting at 13 simulated Saturn years, to study Saturn's stratospheric equatorial oscillation, its inter-hemispheric circulation and the driving mechanism connecting them. 

Results: Firstly, DYNAMICO-Saturn depicts a stratospheric equatorial oscillation of temperature and zonal wind. The new vertical resolution permits to stabilize more the oscillation periodicity and its eastward phase compared to previous study. The period varies between 0.5 and 1 simulated Saturn years. Indeed, because of irregularity in the waves and eddy-to-mean forcings, the downward propagation is carried out by episodes of descent followed by episodes of stagnation at a given level of pressure. The amplitude of the associated temperature oscillation is under-estimated by 10 K compared to the Cassini observations.

Secondly, DYNAMICO-Saturn also models an inter-hemispheric circulation taking place from the summer tropical latitudes to the winter ones, with a strong subsidence between 20 and 40° in the winter hemisphere. The main subsidence branch is located in the same latitude region as temperature and hydrocarbons anomalies observed by Cassini (Guerlet et al. 2009, 2010, Sinclair et al. 2013, Fletcher et al. 2015 and Sylvestre et al. 2015). Furthermore, eddy-to-mean interaction diagnostics show that the phases of Saturn's equatorial oscillation are controlled by the inter-hemispheric circulation. During the solstices, the cross-equatorial drift of the inter-hemispheric circulation, associated to the forcing of the mid-latitude planetary-scale Rossby waves, drive the equatorial zonal wind to westward direction. In contrast, during the equinoctial overturning of the inter-hemispheric circulation, the residual mean circulation is reduced to an unique ascendance at the equator to permit the transport and eastward moment deposition of Kelvin waves from the troposphere.

Perspectives: This present modelling study of the dynamics of Saturn's stratosphere confirms the SAO-like character of the Saturn's equatorial oscillation. However, we will also explore the putative part of the QBO-like character of it. We plan to use this new vertical resolution combine to the subgrid-scale gravity wave parameterization. 

How to cite: Bardet, D., Spiga, A., and Guerlet, S.: Global Climate modelling of Saturn to determine the nature of its equatorial oscillation , EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-5668, https://doi.org/10.5194/egusphere-egu21-5668, 2021.

The rotation rate of the outer planet Saturn is not well constrained by classical measurements of periodic signals [1]. Recent and diverse approaches using a broad spectrum of Cassini and other observational data related to shape, winds, and oscillations are converging toward a value about 6 to 7 minutes faster than the Voyager rotation period.
Here we present our method of using zonal wind data and the even harmonics J₂ to J₁₀ measured during the Cassini Grand Finale tour [2] to infer the deep rotation rate of Saturn. We assume differential rotation on cylinders and generate adiabatic density profiles that match the low-order J₂ and J₄
values. Theory of Figures to 7th order is applied to estimate the differences in the high-order moments J₆ to J₁₀ that may result from the winds and the assumed reference rotation rate. Presented results are preliminary as the method is under construction [3].

[1] Fortney, Helled, Nettelmann et al, in: 'Saturn in the 21st century', Cambridge U Press (2018)
[2] Iess, Militzer, Kaspi, Science 364:2965 (2019)
[3] Nettelmann, AGU Fall Meeting, P066-0007 (2020)

How to cite: Nettelmann, N. and Fortney, J. J.: Toward constraining Saturn's rotation rate by interior modeling, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-6442, https://doi.org/10.5194/egusphere-egu21-6442, 2021.