C. Paranicas

Energetic neutral atoms from a trans-Europa gas torus at Jupiter

The space environments—or magnetospheres—of magnetized planets emit copious quantities of energetic neutral atoms (ENAs) at energies between tens of electron volts to hundreds of kiloelectron volts (keV). These energetic atoms result from charge exchange between magnetically trapped energetic ions and cold neutral atoms, and they carry significant amounts of energy and mass from the magnetospheres. Imaging their distribution allows us to investigate the structure of planetary magnetospheres. Here we report the analysis of 50–80 keV ENA images of Jupiters magnetosphere, where two distinct emission regions dominate: the upper atmosphere of Jupiter itself, and a torus of emission residing just outside the orbit of Jupiters satellite Europa. The trans-Europa component shows that, unexpectedly, Europa generates a gas cloud comparable in gas content to that associated with the volcanic moon Io. The quantity of gas found indicates that Europa has a much greater impact than hitherto believed on the structure of, and the energy flow within, Jupiters magnetosphere.

Transient aurora on Jupiter from injections of magnetospheric electrons

Energetic electrons and ions that are trapped in Earths magnetosphere can suddenly be accelerated towards the planet. Some dynamic features of Earths aurora (the northern and southern lights) are created by the fraction of these injected particles that travels along magnetic field lines and hits the upper atmosphere. Jupiters aurora appears similar to Earths in some respects; both appear as large ovals circling the poles and both show transient events. But the magnetospheres of Jupiter and Earth are so different—particularly in the way they are powered—that it is not known whether the magnetospheric drivers of Earths aurora also cause them on Jupiter. Here we show a direct relationship between Earth-like injections of electrons in Jupiters magnetosphere and a transient auroral feature in Jupiters polar region. This relationship is remarkably similar to what happens at Earth, and therefore suggests that despite the large differences between planetary magnetospheres, some processes that generate aurorae are the same throughout the Solar System.

Electron bombardment of Europa

Energetic electrons trapped in Jupiters magnetic field are a principal source of energy for driving chemistry on the surface of Europa. Here we show that energetic (10 keV-10 MeV) electrons precipitate primarily on the trailing hemisphere, and we give the spatial distribution of the dose-rate deposited in Europas surface. Based on this we propose that energetic electrons are a primary agent in determining the hemispherical differences in Europas albedo, which have been attributed to differences in the incident ion flux. We compare the spatial distribution in the dose-rate deposited by the energetic electrons to the spatial distribution in the hydrate (likely frozen, hydrated sulfuric acid) determined by Galileos near-infrared mapping spectrometer. This comparison supports the idea that radiolysis by the energetic electrons contributes significantly to producing the hydrate.

The ion environment near Europa and its role in surface energetics

This paper gives the composition, energy spectra, and time variability of energetic ions measured just upstream of Europa. From 100 keV to 100 MeV, ion intensities vary by less than a factor of ∼5 among Europa passes considered between 1997 and 2000. We use the data to estimate the radiation dose rate into Europas surface for depths 0.01 mm – 1 m. We find that in a critical fraction of the upper layer on Europas trailing hemisphere, energetic electrons are the principal agent for radiolysis, and their bremsstrahlung photon products, not included in previous studies, dominate the dose below about 1 m. Because ion bombardment is more uniform across Europas surface, the radiation dose on the leading hemisphere is dominated by the proton flux. Differences exist between this calculation and published doses based on the E4 wake pass. For instance, proton doses presented here are much greater below 1 mm.

Jupiter’s magnetosphere and aurorae observed by the Juno spacecraft during its first polar orbits

Juno swoops around giant Jupiter Jupiter is the largest and most massive planet in our solar system. NASAs Juno spacecraft arrived at Jupiter on 4 July 2016 and made its first close pass on 27 August 2016. Bolton et al. present results from Junos flight just above the cloud tops, including images of weather in the polar regions and measurements of the magnetic and gravitational fields. Juno also used microwaves to peer below the visible surface, spotting gas welling up from the deep interior. Connerney et al. measured Jupiters aurorae and plasma environment, both as Juno approached the planet and during its first close orbit. Science, this issue p. 821, p. 826 Juno investigates Jupiter’s magnetosphere and the processes that drive aurorae on the giant planet. The Juno spacecraft acquired direct observations of the jovian magnetosphere and auroral emissions from a vantage point above the poles. Juno’s capture orbit spanned the jovian magnetosphere from bow shock to the planet, providing magnetic field, charged particle, and wave phenomena context for Juno’s passage over the poles and traverse of Jupiter’s hazardous inner radiation belts. Juno’s energetic particle and plasma detectors measured electrons precipitating in the polar regions, exciting intense aurorae, observed simultaneously by the ultraviolet and infrared imaging spectrographs. Juno transited beneath the most intense parts of the radiation belts, passed about 4000 kilometers above the cloud tops at closest approach, well inside the jovian rings, and recorded the electrical signatures of high-velocity impacts with small particles as it traversed the equator.

Fundamental Plasma Processes in Saturn's Magnetosphere

In this chapter, we review selected fundamental plasma processes that control the extensive space environment, or magnetosphere, of Saturn (see Chapter 9, for the global context). This writing occurs at a point in time when some measure of maturity has been achieved in our understanding of the operations of Saturns magnetosphere and its relationship to those of Earth and Jupiter. Our understanding of planetary magnetospheres has exploded in the past decade or so partly because of the presence of orbiting spacecraft (Galileo and Cassini) as well as remote sensing assets (e.g., Hubble Space Telescope). This book and chapter are intended to take stock of where we are in our understanding of Saturns magnetosphere following the successful return and analysis of extensive sets of Cassini data. The end of the prime mission provides us with an opportunity to consolidate older and newer work to provide guidance for continuing investigations.

The Dust Halo of Saturn's Largest Icy Moon, Rhea

Saturns moon Rhea had been considered massive enough to retain a thin, externally generated atmosphere capable of locally affecting Saturns magnetosphere. The Cassini spacecrafts in situ observations reveal that energetic electrons are depleted in the moons vicinity. The absence of a substantial exosphere implies that Rheas magnetospheric interaction region, rather than being exclusively induced by sputtered gas and its products, likely contains solid material that can absorb magnetospheric particles. Combined observations from several instruments suggest that this material is in the form of grains and boulders up to several decimetres in size and orbits Rhea as an equatorial debris disk. Within this disk may reside denser, discrete rings or arcs of material.

Energetic particle observations near Ganymede

This paper combines data from the Galileo spacecraft plasma and energetic particle instruments. Data are included from two Ganymede flybys when the spacecraft was at northern Ganymede latitudes. Electron intensities from the two instruments are in very good agreement. Fits to 60 electron energy spectra are presented. We find the power of precipitating 0.5–3.0 keV electrons into both polar caps of Ganymede is ∼3 × 109 W. By assuming the instruments are intercalibrated, we infer that sulfur dominates the ion intensities at least in the tens of keV energy range. Furthermore, fits to the ion data indicate that the intensities are dominated by heavy ions below about 100 keV. Fits to these data are also used to estimate the sputtering rate of Ganymedes icy surface. It is found that ∼2 × 1026 water molecules/s are sputtered from Ganymedes polar caps which, when not redeposited on the surface, give an erosion rate of ∼8 m/Gyr.

Satellite sputtering in Saturn's magnetosphere

The heavy ion plasma and energetic particles continuously sputter the surfaces of the icy satellites embedded in the inner Saturnian magnetosphere. We evaluate satellite sputtering and compare the resulting H2O source distribution with the source distribution expected for the OH cloud recently observed by Hubble Space Telescope. At each satellite we combine, for the 2rst time, the data from the Plasma Science (PLS) and Low Energy Charged-Particle (LECP) instruments from Voyager 1 and 2, unifying them into a single plasma distribution function. Based on the calculated satellite sources, we conclude that sputtering of the satellite surfaces cannot produce the observed OH cloud and that a large additional source in the inner magnetosphere is needed to fully explain the HST observations. c

Juno observations of energetic charged particles over Jupiter's polar regions: Analysis of monodirectional and bidirectional electron beams

Juno obtained unique low-altitude space environment measurements over Jupiters poles on 27 August 2016. Here Jupiter Energetic-particle Detector Instrument observations are presented for electrons (25–800 keV) and protons (10–1500 keV). We analyze magnetic field-aligned electron angular beams over expected auroral regions that were sometimes symmetric (bidirectional) but more often strongly asymmetric. Included are variable but surprisingly persistent upward, monodirectional electron angular beams emerging from what we term the “polar cap,” poleward of the nominal auroral ovals. The energy spectra of all beams were monotonic and hard (not structured in energy), showing power law-like distributions often extending beyond ~800 keV. Given highly variable downward energy fluxes (below 1 RJ altitudes within the loss cone) as high as 280 mW/m2, we suggest that mechanisms generating these beams are among the primary processes generating Jupiters uniquely intense auroral emissions, distinct from what is typically observed at Earth.