Nicholas Achilleos

Supersonic winds in Jupiter's aurorae

Jupiter has a giant magnetosphere that is coupled to the planets upper atmosphere; as the planet rotates, its magnetic field drags a dense ionized equatorial sheet of plasma, which must interact with the upper atmosphere. Jupiters aurorae are much more powerful, than the Earths, and cause significant local heating of the upper atmosphere. Auroral electrojets—ion winds that race around Jupiters auroral ovals—play a key role in theoretical models of how Jupiters rotational energy is transferred to the plasma sheet, and how winds may transport energy from auroral heating to lower latitudes. But there has hitherto been no direct observational evidence for the existence of such electrojets. Here we report observations of electrojets that have winds approaching or in excess of the local speed of sound. The energy produced by these electrojets could heat the whole upper atmosphere, if the auroral regions couple efficiently with the rest of the planet.

Complex structure within Saturn's infrared aurora

The majority of planetary aurorae are produced by electrical currents flowing between the ionosphere and the magnetosphere which accelerate energetic charged particles that hit the upper atmosphere. At Saturn, these processes collisionally excite hydrogen, causing ultraviolet emission, and ionize the hydrogen, leading to H3+ infrared emission. Although the morphology of these aurorae is affected by changes in the solar wind, the source of the currents which produce them is a matter of debate. Recent models predict only weak emission away from the main auroral oval. Here we report images that show emission both poleward and equatorward of the main oval (separated by a region of low emission). The extensive polar emission is highly variable with time, and disappears when the main oval has a spiral morphology; this suggests that although the polar emission may be associated with minor increases in the dynamic pressure from the solar wind, it is not directly linked to strong magnetospheric compressions. This aurora appears to be unique to Saturn and cannot be explained using our current understanding of Saturn’s magnetosphere. The equatorward arc of emission exists only on the nightside of the planet, and arises from internal magnetospheric processes that are currently unknown.

No sodium in the vapour plumes of Enceladus

The discovery of water vapour and ice particles erupting from Saturn’s moon Enceladus fuelled speculation that an internal ocean was the source. Alternatively, the source might be ice warmed, melted or crushed by tectonic motions. Sodium chloride (that is, salt) is expected to be present in a long-lived ocean in contact with a rocky core. Here we report a ground-based spectroscopic search for atomic sodium near Enceladus that places an upper limit on the mixing ratio in the vapour plumes orders of magnitude below the expected ocean salinity. The low sodium content of escaping vapour, together with the small fraction of salt-bearing particles, argues against a situation in which a near-surface geyser is fuelled by a salty ocean through cracks in the crust. The lack of observable sodium in the vapour is consistent with a wide variety of alternative eruption sources, including a deep ocean, a freshwater reservoir, or ice. The existing data may be insufficient to distinguish between these hypotheses.

Internally driven large-scale changes in the size of Saturn's magnetosphere

Abstract Saturns magnetic field acts as an obstacle to solar wind flow, deflecting plasma around the planet and forming a cavity known as the magnetosphere. The magnetopause defines the boundary between the planetary and solar dominated regimes, and so is strongly influenced by the variable nature of pressure sources both outside and within. Following from Pilkington et al. (2014), crossings of the magnetopause are identified using 7 years of magnetic field and particle data from the Cassini spacecraft and providing unprecedented spatial coverage of the magnetopause boundary. These observations reveal a dynamical interaction where, in addition to the external influence of the solar wind dynamic pressure, internal drivers, and hot plasma dynamics in particular can take almost complete control of the systems dayside shape and size, essentially defying the solar wind conditions. The magnetopause can move by up to 10–15 planetary radii at constant solar wind dynamic pressure, corresponding to relatively “plasma‐loaded” or “plasma‐depleted” states, defined in terms of the internal suprathermal plasma pressure.

Cassini observations of Saturn’s southern polar cusp

The magnetospheric cusps are important sites of the coupling of a magnetosphere with the solar wind. The combination of both ground- and space-based observations at Earth have enabled considerable progress to be made in understanding the terrestrial cusp and its role in the coupling of the magnetosphere to the solar wind via the polar magnetosphere. Voyager 2 fully explored Neptunes cusp in 1989 but highly inclined orbits of the Cassini spacecraft at Saturn present the most recent opportunity to repeatedly studying the polar magnetosphere of a rapidly rotating planet. In this paper we discuss observations made by Cassini during two passes through Saturns southern polar magnetosphere. Our main findings are that i) Cassini directly encounters the southern polar cusp with evidence for the entry of magnetosheath plasma into the cusp via magnetopause reconnection, ii) magnetopause reconnection and entry of plasma into the cusp can occur over a range of solar wind conditions, and iii) double cusp morphologies are consistent with the position of the cusp oscillating in phase with Saturns global magnetospheric periodicities.

VESPA: A community-driven Virtual Observatory in Planetary Science

The VESPA data access system focuses on applying Virtual Observatory (VO) standards and tools to Planetary Science. Building on a previous EC-funded Europlanet program, it has reached maturity during the first year of a new Europlanet 2020 program (started in 2015 for 4 years). The infrastructure has been upgraded to handle many fields of Solar System studies, with a focus both on users and data providers. This paper describes the broad lines of the current VESPA infrastructure as seen by a potential user, and provides examples of real use cases in several thematic areas. These use cases are also intended to identify hints for future developments and adaptations of VO tools to Planetary Science.

DUSK-BRIGHTENING EVENT IN SATURN'S H+3 : AURORA

We report on a unique dusk-brightening event within Saturns aurorae. Measurements of the H infrared aurora using the CSHELL instrument on NASAs Infrared Telescope Facility (IRTF), made in 2005 December, show an auroral intensity structure unlike anything previously detected. The aurora has a significantly brighter dusk sector over three Earth nights, a period in excess of 5 Saturnian days, suggesting a consistent source for this emission, stable in position within the magnetosphere. However, unlike previously detected dawn-brightening events, the overall auroral brightness remains low and the ion wind structure appears unaffected. Using the location of magnetopause crossings as a proxy for the solar wind pressure, the solar wind appears to be exceptionally rarefied. This leads us to conclude that the dusk-brightening event is strongly linked with the unusual solar wind conditions at the time of the observations.

The impact of comet Shoemaker-Levy 9 on the Jovian ionosphere and aurorae

Abstract We outline the effects of a “typical” fragment of Comet Shoemaker-Levy 9 on the ionosphere and aurorae of Jupiter, viewed chronologically from approach, through impact to longer term effects. The ionosphere is found to have been a sensitive tracer of the impact events. The powerful jovian magnetic field transmitted the effects of the collisions across hemispheres. Electrodynamical processes were particularly important around the time of fragment impact, along with shock heating of the ionosphere. Chemical reactions not usual in the jovian upper atmosphere played a key role in diminishing ionospheric and auroral emission intensities.

Modeling the compressibility of Saturn’s magnetosphere in response to internal and external influences

The location of a planetary magnetopause is principally determined by the balance between solar wind dynamic pressure DP and magnetic and plasma pressures inside the magnetopause boundary. Previous empirical studies assumed that Saturns magnetopause standoff distance varies as math formula and measured a constant compressibility parameter α corresponding to behavior intermediate between a vacuum dipole appropriate for Earth (α≈6) and a more easily compressible case appropriate for Jupiter (α≈4). In this study we employ a 2-D force balance model of Saturns magnetosphere to investigate magnetospheric compressibility in response to changes in DP and global hot plasma content. For hot plasma levels compatible with Saturn observations, we model the magnetosphere at a range of standoff distances and estimate the corresponding DP values by assuming pressure balance across the magnetopause boundary. We find that for “average” hot plasma levels, our estimates of α are not constant with DP but vary from ∼4.8 for high DP conditions, when the magnetosphere is compressed (≤25 RS), to ∼3.5 for low DP conditions. This corresponds to the magnetosphere becoming more easily compressible as it expands. We find that the global hot plasma content influences magnetospheric compressibility even at fixed DP, with α estimates ranging from ∼5.4 to ∼3.3 across the range of our parameterized hot plasma content. We suggest that this behavior is predominantly driven by reconfiguration of the magnetospheric magnetic field into a more disk-like structure under such conditions. In a broader context, the compressibility of the magnetopause reveals information about global stress balance in the magnetosphere.