Energetic ion composition and acceleration mechanisms in the magnetosphere of Jupiter

Based on the first 15 orbits of the Galileo spacecraft the composition of the energetic ion population of the Jovian magnetosphere has been studied for the first time on a global scale. More specific, three different types of ions are investigated: helium as the tracer of the solar wind, sulfur and oxygen to a large extent as tracers of the internal source Io and protons with a mixed origin from the solar wind, Jovian atmosphere/ionosphere and the Europa gas torus. The ion energy spectrum fundamental for the study of the ion composition is investigated and a characteristic shape is established as a distinct feature of the Jovian magnetosphere. Based on the observed ion energy spectra the relative ion abundance ratios of S/O, S/He, O/He and p/He at a specific energy/nucleon are derived and global maps are constructed. The global coverage of the Galileo trajectories enables a quantitative comparison with results of previous flyby missions for the same radial distance and local time. The large discrepancies derived from the comparison with the Voyager 2 results are attributed to temporal variations and to a strong energy dependence of the ion abundance ratios associated with the energy spectral shapes ...thesi

Energetische Ionenzusammensetzung und Beschleunigungsmechanismen in der Magnetosphäre von Jupiter

Based on the first 15 orbits of the Galileo spacecraft the composition of the energetic ion population of the Jovian magnetosphere has been studied for the first time on a global scale. More specific, three different types of ions are investigated: helium as the tracer of the solar wind, sulfur and oxygen to a large extent as tracers of the internal source Io and protons with a mixed origin from the solar wind, Jovian atmosphere/ionosphere and the Europa gas torus. The ion energy spectrum fundamental for the study of the ion composition is investigated and a characteristic shape is established as a distinct feature of the Jovian magnetosphere. Based on the observed ion energy spectra the relative ion abundance ratios of S/O, S/He, O/He and p/He at a specific energy/nucleon are derived and global maps are constructed. The global coverage of the Galileo trajectories enables a quantitative comparison with results of previous flyby missions for the same radial distance and local time. The large discrepancies derived from the comparison with the Voyager 2 results are attributed to temporal variations and to a strong energy dependence of the ion abundance ratios associated with the energy spectral shapes. The ion energy spectra are further discussed in terms of ion stochastic acceleration by Alfvén waves. Temporal variations of the ion abundance ratios not only on time scales of several years but also of several weeks, only implied by previous missions are now established. They are attributed to the different ion sources but also to time-varying acceleration processes. A comparative study of the ion composition for different local time sectors is for first the time investigated. It is shown that pronounced composition changes along the predawn sector are associated with reconfiguration processes that occur in the magnetotail. The composition in this region is studied in terms of two acceleration mechanisms: nonadiabatic ion interaction with the current sheet where ions are accelerated by the timestationary dusk to dawn convection electric field and ion acceleration due to small-scale variations of the south-north component of the magnetic field during the reconfiguration events, where particles are accelerated by the induced electric field. It is concluded that the latter one is responsible for the observed ion composition.Auf den ersten 15 Umlaufbahnen des Raumfahrzeugs Galileo basierend, ist die Zusammensetzung des Bestandes an energiegeladenen Ionen in der Jupitermagnetosphäre zum ersten Mal in einer umfassenden Grössenordnung studiert worden. Drei unterschiedliche Arten von Ionen werden untersucht: Helium als der Indikator des Solarwinds; Schwefel und Sauerstoff in hohem Mae als Indikatoren der internen Quelle Io; und Protonen, die ihren Ursprung teilweise im Solarwind, in der Jupiteratmosphäre/-ionosphere und im Gastorus des Mondes Europa haben. Das für die Untersuchung des Ionenzusammensetzung grundlegende Ionenenergiespektrum wird erforscht und eine charakteristische Form wird als bestimmende Eigenschaft der Jupitermagnetosph äre ausgemacht. Basierend auf den beobachteten Ionenenergiespektren werden die relativen Verhältnisse der Ionenhäufigkeit von S/O, S/He, O/He und p/He für ein spezifisches Verhältnis Energie/Nukleon bestimmt und globale Karten erstellt. Die globale Abdeckung der Galileo-Flugbahnen ermöglicht einen quantitativen Vergleich mit Resultaten früherer Flyby Missionen für den gleichen Radialabstand und die gleiche Lokalzeit. Die grossen Umstimmigkeiten im Vergleich mit den Resultaten von Voyager 2 werden zeitlichen Veränderungen und einer starken Energieabhängigkeit der Verhältnisse der Ionenhäufigkeit, die mit den spektralen Formen der Energie verbunden sind, zugeschrieben. Die Ionenenergiespektren werden darüber hinaus in Hinblick auf stochastischer Ionenbeschleunigung durch Alfvén-Wellen untersucht. Zeitliche Veränderungen der Verhältnisse der Ionenhäufigkeit nicht nur auf Zeitskalen einiger Jahre, aber auch einiger Wochen, die durch vorhergehende Missionen nur angedeutet wurden, werden jetzt bestätigt. Sie werden den unterschiedlichen Ionenquellen aber auch zeitabhängig variierenden Beschleunigungsprozessen zugeschrieben. Eine Vergleichsstudie des Ionenaufbaus für unterschiedliche Lokalzeit Sektoren wird erstmalig untersucht. Es wird gezeigt, dass ausgeprägte Veränderungen in der Zusammensetzung entlang des predawn (01:30-04:30 Lokalzeit) Sektors mit Neukonfigurationsprozessen, verbunden sind, die im Magnetschweif auftreten. Die Zusammensetzung dieser Region wird in Hinblick auf zwei Beschleunigungsmechanismen untersucht: nicht-adiabatische Ionenwechselwirkungen mit der Stromschicht, in der Ionen durch das zeitlich stationäre dusk-dawn (Abend-Morgen) Konvektion elektrische Feld beschleunigt werden, und Ionenbeschleunigung aufgrund von kleinen Veränderungen der Süd-Nord Komponente des magnetischen Feldes während des Rekonfigurationsprozesses, in dem Partikel durch das induzierte elektrische Feld beschleunigt werden. Es wird geschlussfolgert, dass das letztere verantwortlich ist für den beobachteten Ionenzusammensetzung

How bright is the Io UV footprint?

The electro-magnetic interaction between Io and the Jovian magnetosphere generates a perturbation in the magnetospheric plasma which propagates along the magnetic field lines and creates auroral footprint emissions in both hemispheres. Recent results showed that this footprint is formed of several spots and an extended tail. Each feature is suggested to correspond to a different step in the propagation of the perturbation and in the electron energization processes. The present study focuses on the variations of the spots' brightness at different timescales from minutes to years through the rotation period of Jupiter. It relies on FUV images acquired with the STIS and ACS instruments onboard the Hubble Space Telescope. Since the footprint is composed of several localized features, a good understanding of the emission region geometry is critical to derive the actual vertical brightness and thus the precipitated energy flux. We developed a 3D emission model in order to assess as precisely as possible the respective contribution of each individual feature and to correctly estimate the precipitating energy flux. As far as the brightness variations on timescales of minutes are concerned, we will present results from the high time resolution campaign executed during summer 2009. On timescale of several hours, we will show that the variation of the emitted power as a function of the location of Io in the plasma torus suggests that the Jovian surface magnetic field strength is an important controlling parameter. Finally, the measured precipitated power and particle fluxes will be discussed in comparison with recent simulations considering both Alfvén waves filamentation and electron acceleration when the Alfvén waves become inertial

Expansion of the main auroral oval at Jupiter : evidence for Io’s control over the Jovian magnetosphere

In spring 2007, New Horizons' Jupiter fly-by provided a unique opportunity for the largest observation campaign dedicated to the Jovian aurora ever carried out by the Hubble Space Telescope. UV images of the aurora have been acquired on a quasi-daily basis from mid-February to mid-June 2007. Polar projection of the auroral emissions clearly show a continuous long-term expansion of main oval additionally to day by day variations. The main oval moved so much that the Ganymede footprint, which is usually located equatorward of the main emissions, has even been observed inside of it. Simultaneously, the occurrence rate of large equatorward isolated auroral features increased over the season. These emission patches are generally attributed to injections of depleted flux tubes. On 6th June, one of these features exceptionally moved down to the Io footpath. The Io footprint seemed to disappear while the footprint moved through this patch of emission. This disappearance is a unique case among all the UV images of the aurora acquired during the last 12 years. We suggest that all these changes seen in the Jovian aurora are evidence for a major reconfiguration of the magnetosphere induced by increased volcanic activity on Io. Indeed, New Horizons observed particularly intense activity from the Tvashtar volcano in late February 2007. Moreover, sodium cloud brightening caused by volcanic outbursts have also been seen in late May 2007. According to our interpretation, repeated volcanic outbursts beefed up the plasma torus density and its mass outflow rate. This caused the corotation breakdown boundary to migrate closer to Jupiter. Consequently, the main auroral oval moved equatorward. As heavy flux tubes move outward, sparsely filled ones should be injected into the inner magnetosphere in order to conserve the magnetic flux in this region. This phenomenon could explain the large number of injection signatures observed in May-June 2007. Such a cloud of depleted flux tubes probably disrupted the Io-magnetosphere interaction, leading to an abnormally faint Io footprint

Auroral evidence of Io's control over the magnetosphere of Jupiter

Contrary to the case of the Earth, the main auroral oval on Jupiter is related to the breakdown of plasma corotation in the middle magnetosphere. Even if the root causes for the main auroral emissions are Io's volcanism and Jupiter's fast rotation, changes in the aurora could be attributed either to these internal factors or to fluctuations of the solar wind. Here we show multiple lines of evidence from the aurora for a major internally-controlled magnetospheric reconfiguration that took place in Spring 2007. Hubble Space Telescope far-UV images show that the main oval continuously expanded over a few months, engulfing the Ganymede footprint on its way. Simultaneously, there was an increased occurrence rate of large equatorward isolated auroral features attributed to injection of depleted flux tubes. Furthermore, the unique disappearance of the Io footprint on 6 June appears to be related to the exceptional equatorward migration of such a feature. The contemporary observation of the spectacular Tvashtar volcanic plume by the New-Horizons probe as well as direct measurement of increased Io plasma torus emissions suggest that these dramatic changes were triggered by Io's volcanic activity

Jupiter's changing auroral location

[1] We examine the case of significant latitudinal shifts of the Jovian northern auroral emissions appearing in a data set spanning nine years of observations with the Hubble Space Telescope in the far ultraviolet. The extended data set makes it possible to compare the location of the main auroral emission with similar viewing geometries and satellite positions. The main auroral emission is assumed to originate from beyond the orbit of Ganymede (15 Jovian radii). At these distances, near corotation enforcement and transfer of momentum from Jupiter to the magnetospheric plasma is ensured by means of field aligned currents. The field aligned currents away from Jupiter are carried by downward energetic electrons loosing their energy to the polar atmosphere and giving rise to the main auroral emission. Analysis of the polar projected images shows that the latitudinal location of the main emission has changed by up to 3 degrees over long periods of time. It also shows that the footprint of Ganymede follows a similar trend. We have used the VIP4 magnetic field model to map the emission down to the equatorial plane. This mapping suggests that internal variations of the current sheet parameters might be used as an alternative or complementary explanation to the changing solar wind conditions at Jupiter to explain the observed shift of auroral latitudes

Similarity of the Jovian satellite footprints:spots multiplicity and dynamics

In the magnetospheres of Jupiter and Saturn, the intense interaction of the satellites Io, Europa, Ganymede and Enceladus with their surrounding plasma environment leaves a signature in the aurora of the planet. Called satellite footprints, these auroral features appear either as a single spot (Europa and Enceladus) or as multiple spots (Io and Ganymede). Moreover, they can be followed by extended trailing tails in the case of Io and Europa, while no tail has been reported for Ganymede and Enceladus, yet. Here we show that all Jovian footprints can be made of several spots. Furthermore, the footprints all experience brightness variations on timescale of 2–3 min. We also demonstrate that the satellite location relative to the plasma sheet is not the only driver for the footprint brightness, but that the plasma environment and the magnetic field strength also play a role. These new findings demonstrate that the Europa and Ganymede footprints are very similar to the Io footprint. As a consequence, the processes expected to take place at Io, such as the bi-directional electron acceleration by Alfvén waves or the partial reflection of these waves on plasma density gradients, can most likely be extended to the other footprints, suggesting that they are indeed universal processes

Bifurcations of the main auroral ring at Saturn: ionospheric signatures of consecutive reconnection events at the magnetopause

peer reviewedThis work reports for the first time on bifurcations of the main auroral ring at Saturn observed with the UVIS instrument onboard Cassini. The observation sequence starts with an intensification on the main oval, close to noon, which is possibly associated with dayside reconnection. Consecutive bifurcations appear with the onset of dayside reconnection, between 11 and 18 magnetic local time, while the area poleward of the main emission expands to lower latitudes. The bifurcations depart with time from the main ring of emission, which is related to the open-closed field line boundary. The augmentation of the area poleward of the main emission following its expansion is balanced by the area occupied by the bifurcations, suggesting that these auroral features represent the amount of newly open flux and could be related to consecutive reconnection events at the flank of the magnetopause. The observations show that the open flux along the sequence increases when bifurcations appear. Magnetopause reconnection can lead to significant augmentation of the open flux within a couple of days and each reconnection event opens ∼10% of the flux contained within the polar cap. Additionally, the observations imply an overall length of the reconnection line of ∼4 hours of local time and suggest that dayside reconnection at Saturn can occur at several positions on the magnetopause consecutively or simultaneously

Planetary space weather: scientific aspects and future perspectives

International audienceIn this paper, we review the scientific aspects of planetary space weather at different regions of our Solar System, performing a comparative planetology analysis that includes a direct reference to the circum-terrestrial case. Through an interdisciplinary analysis of existing results based both on observational data and theoretical models, we review the nature of the interactions between the environment of a Solar System body other than the Earth and the impinging plasma/radiation, and we offer some considerations related to the planning of future space observations. We highlight the importance of such comparative studies for data interpretations in the context of future space missions (e.g. ESA JUICE; ESA/JAXA BEPI COLOMBO). Moreover, we discuss how the study of planetary space weather can provide feedback for better understanding the traditional circum-terrestrial space weather. Finally, a strategy for future global investigations related to this thematic is proposed

The Io UV footprint: Location, inter-spot distances and tail vertical extent

The Io footprint (IFP) consists of one or several spots observed in both jovian hemispheres and is related to the electromagnetic interaction between Io and the magnetosphere. These spots are followed by an auroral curtain, called the tail, extending more than 90° longitude in the direction of planetary rotation. We use recent Hubble Space Telescope images of Jupiter to analyze the location of the footprint spots and tail as a function of Io's location in the jovian magnetic field. We present here a new IFP reference contour---the locus of all possible IFP positions---with an unprecedented accuracy, especially in previously poorly covered sectors. We also demonstrate that the lead angle - the longitudinal shift between Io and the actual IFP position - is not a reliable quantity for validation of the interaction models. Instead, the evolution of the inter-spot distances appears to be a better diagnosis of the Io-Jupiter interaction. Moreover, we present observations of the tail vertical profiles as seen above the limb. The emission peak altitude is ~900 km and remains relatively constant with the distance from the main spot. The altitudinal extent of the vertical emission profiles is not compatible with precipitation of a mono-energetic electron population. The best fit is obtained for a kappa distribution with a characteristic energy of ~70 eV and a spectral index of 2.3. The broadness of the inferred electron energy spectrum gives insight into the physics of the electron acceleration mechanism at play above the IFP tail