Benjamin Palmaerts

Transient small‐scale structure in the main auroral emission at Jupiter

The main auroral emission at Jupiter results from the ionosphere-magnetosphere coupling current system associated with the corotation breakdown of iogenic plasma in the current sheet. The morphology and brightness of the main auroral emission are generally suggested to be stable during time intervals of the order of an hour. Here we reveal a transient small-scale structure in the main emission that is characterized by a localized brightness enhancement close to noon local time. The evolution of this small-scale structure is investigated in both hemispheres on the basis of far UV images obtained with the Hubble Space Telescope between 1997 and 2007. Our observations indicate that the transient feature vary within a few tens of minutes. As one plausible explanation based on Galileo observations, we suggest that the localized enhancement of the field-aligned currents associated with the transient structure is due to the shear induced by intermittent inward plasma flow near noon in the equatorial plane.

A radiation belt of energetic protons located between Saturn and its rings

Cassinis final phase of exploration The Cassini spacecraft spent 13 years orbiting Saturn; as it ran low on fuel, the trajectory was changed to sample regions it had not yet visited. A series of orbits close to the rings was followed by a Grand Finale orbit, which took the spacecraft through the gap between Saturn and its rings before the spacecraft was destroyed when it entered the planets upper atmosphere. Six papers in this issue report results from these final phases of the Cassini mission. Dougherty et al. measured the magnetic field close to Saturn, which implies a complex multilayer dynamo process inside the planet. Roussos et al. detected an additional radiation belt trapped within the rings, sustained by the radioactive decay of free neutrons. Lamy et al. present plasma measurements taken as Cassini flew through regions emitting kilometric radiation, connected to the planets aurorae. Hsu et al. determined the composition of large, solid dust particles falling from the rings into the planet, whereas Mitchell et al. investigated the smaller dust nanograins and show how they interact with the planets upper atmosphere. Finally, Waite et al. identified molecules in the infalling material and directly measured the composition of Saturns atmosphere. Science, this issue p. eaat5434, p. eaat1962, p. eaat2027, p. eaat3185, p. eaat2236, p. eaat2382 INTRODUCTION Most magnetized planets are known to possess radiation belts, where high-energy charged particles are trapped in large numbers. The possibility that a radiation belt could exist also in the confined region between Saturn and its main rings has been proposed on the basis of remote sensing observations and simulations. It was not until the final 5 months of the Cassini mission that in situ measurements were obtained from this region with the Magnetosphere Imaging Instrument (MIMI). This paper provides an overview of these measurements and their interpretation. RATIONALE Saturn’s main rings prevent the inward transport of trapped charged particles in the magnetosphere. Material from the outer radiation belts cannot directly access the low-altitude region within the rings. The isolation of this region allows the study of energetic particle source and loss processes because it is only indirectly coupled to the dynamics of the rest of the magnetosphere. Potential sources include cosmic ray albedo neutron decay (CRAND) and multiple-charge exchange, whereas losses are likely dominated by energy deposition and scattering of trapped particles by dust and atmospheric neutrals. All of these mechanisms involve charged particle interactions with materials in space, meaning that MIMI measurements can provide information to probe the material itself—particularly the tenuous D-ring, the innermost component of Saturn’s main rings, which is difficult to constrain by remote sensing observations. RESULTS We observed an inner radiation belt extending between 1.03 and 1.22 Saturn radii (1 RS = 60,268 km) at the equatorial plane, dominated by protons with energies from 25 MeV up to the giga–electron volt range. This belt is limited by the atmosphere at its inner edge and by the D73 ringlet (at 1.22 RS), a component of the D-ring, at its outer boundary. Another ringlet (D68 at 1.12 RS) splits the trapped particle population in two. The outer sector overlaps with the extended D-ring, and its intensity is reduced compared with that of the inner sector, owing to proton losses on ring dust. The proton angular distributions are highly anisotropic with fluxes that are orders of magnitude higher near the magnetic equator compared with fluxes of particles that can reach high latitudes. No time variability could be discerned in the >25-MeV proton population over the 5-month period of the observations. Trapping of lower-energy (tens of kilo–electron volt) protons was clearly observed in at least one case by imaging the emission of energetic neutral atoms (ENAs) coming from below ~1.06 RS (altitude < 3800 km). Energetic electrons (18 keV to several mega–electron volts) and heavy ions (27 keV per nucleon to hundreds of mega–electron volts per nucleon), if present, have fluxes close to or lower than the detection limit of the MIMI sensors. CONCLUSION The radial profile, the stability of the >25-MeV proton fluxes, and the lack of heavy ions are features consistent with a radiation belt originating from CRAND. The strong anisotropy of the proton distributions is primarily the result of proton losses in collisions with atmospheric neutrals, though an anisotropy in the production of CRAND protons from Saturn’s rings may also contribute. The low-altitude, kilo–electron volt proton population is transient and derives from charge stripping of planetward ENAs, which are generated at the variable magnetospheric ring current. Saturn’s proton radiation belts. Saturn’s permanent proton radiation belt extends outward to the orbit of the moon Tethys but is segmented because of proton absorption by moons and rings. The innermost radiation belt (inset) threads through Saturn’s D-ring and contains protons with energies up to several giga–electron volts, much higher than observed outside the main rings. These protons are among the β-decay products of neutrons, which are released through galactic cosmic ray collisions with Saturn’s rings (CRAND process). Saturn has a sufficiently strong dipole magnetic field to trap high-energy charged particles and form radiation belts, which have been observed outside its rings. Whether stable radiation belts exist near the planet and inward of the rings was previously unknown. The Cassini spacecraft’s Magnetosphere Imaging Instrument obtained measurements of a radiation belt that lies just above Saturn’s dense atmosphere and is decoupled from the rest of the magnetosphere by the planet’s A- to C-rings. The belt extends across the D-ring and comprises protons produced through cosmic ray albedo neutron decay and multiple charge-exchange reactions. These protons are lost to atmospheric neutrals and D-ring dust. Strong proton depletions that map onto features on the D-ring indicate a highly structured and diverse dust environment near Saturn.

Mechanisms of Saturn's Near‐Noon Transient Aurora: In Situ Evidence From Cassini Measurements

Although auroral emissions at giant planets have been observed for decades, the physical mechanisms of aurorae at giant planets remain unclear. One key reason is the lack of simultaneous measurements in the magnetosphere while remote sensing of the aurora. We report a dynamic auroral event identified with the Cassini Ultraviolet Imaging Spectrograph (UVIS) at Saturn on 13 July 2008 with coordinated measurements of the magnetic field and plasma in the magnetosphere. The auroral intensification was transient, only lasting for ∼30 min. The magnetic field and plasma are perturbed during the auroral intensification period. We suggest that this intensification was caused by wave mode conversion generated field-aligned currents, and we propose two potential mechanisms for the generation of this plasma wave and the transient auroral intensification. A survey of the Cassini UVIS database reveals that this type of transient auroral intensification is very common (10/11 time sequences, and ∼10% of the total images).

Jupiter's Aurora Observed With HST During Juno Orbits 3 to 7

A large set of observations of Jupiters ultraviolet aurora was collected with the Hubble Space Telescope concurrently with the NASA‐Juno mission, during an eight‐month period, from 30 November 2016 to 18 July 2017. These Hubble observations cover Juno orbits 3 to 7 during which Juno in situ and remote sensing instruments, as well as other observatories, obtained a wealth of unprecedented information on Jupiters magnetosphere and the connection with its auroral ionosphere. Jupiters ultraviolet aurora is known to vary rapidly, with timescales ranging from seconds to one Jovian rotation. The main objective of the present study is to provide a simplified description of the global ultraviolet auroral morphology that can be used for comparison with other quantities, such as those obtained with Juno. This represents an entirely new approach from which logical connections between different morphologies may be inferred. For that purpose, we define three auroral subregions in which we evaluate the auroral emitted power as a function of time. In parallel, we define six auroral morphology families that allow us to quantify the variations of the spatial distribution of the auroral emission. These variations are associated with changes in the state of the Jovian magnetosphere, possibly influenced by Io and the Io plasma torus and by the conditions prevailing in the upstream interplanetary medium. This study shows that the auroral morphology evolved differently during the five ~2 week periods bracketing the times of Juno perijove (PJ03 to PJ07), suggesting that during these periods, the Jovian magnetosphere adopted various states.

Heliospheric conditions at Saturn during Cassini's Ring‐Grazing and Proximal Orbits

We surveyed energetic charged particle measurements by Cassini between 2016 and the end of the mission in order to identify transients of solar energetic particles and galactic cosmic rays. Such transients offer hints about the state of the heliosphere and valuable context for interpreting space weather phenomena in Saturns magnetosphere. The period studied includes the missions Ring-Grazing and Proximal orbits, which due to their week-long periods are ideal for capturing short timescale dynamics in the planets magnetosphere, including solar periodicities. We find that Saturn was exposed to corotating interaction regions for nearly all the 21 final months of Cassini and encountered just two interplanetary coronal mass ejections. Several independent studies report solar periodicities and storm-time conditions in the magnetosphere at times, which coincide with the transients that we identify here.