Cassini UVIS Observations of the Io Plasma Torus. IV. Modeling Temporal and Azimuthal Variability

In this fourth paper in a series, we present a model of the remarkable temporal and azimuthal variability of the Io plasma torus observed during the Cassini encounter with Jupiter. Over a period of three months, the Cassini Ultraviolet Imaging Spectrograph (UVIS) observed a dramatic variation in the average torus composition. Superimposed on this long-term variation, is a 10.07-hour periodicity caused by an azimuthal variation in plasma composition subcorotating relative to System III longitude. Quite surprisingly, the amplitude of the azimuthal variation appears to be modulated at the beat frequency between the System III period and the observed 10.07-hour period. Previously, we have successfully modeled the months-long compositional change by supposing a factor of three increase in the amount of material supplied to Io's extended neutral clouds. Here, we extend our torus chemistry model to include an azimuthal dimension. We postulate the existence of two azimuthal variations in the number of super-thermal electrons in the torus: a primary variation that subcorotates with a period of 10.07 hours and a secondary variation that remains fixed in System III longitude. Using these two hot electron variations, our model can reproduce the observed temporal and azimuthal variations observed by Cassini UVIS.Comment: Revised 24 August 2007 Accepted by Icarus, 50 pages, 2 Tables, 8 figure

Weakening of Jupiter's main auroral emission during January 2014

In January 2014 Jupiter's FUV main auroral oval decreased its emitted power by 70% and shifted equatorward by ∼1°. Intense, low-latitude features were also detected. The decrease in emitted power is attributed to a decrease in auroral current density rather than electron energy. This could be caused by a decrease in the source electron density, an order of magnitude increase in the source electron thermal energy, or a combination of these. Both can be explained either by expansion of the magnetosphere or by an increase in the inward transport of hot plasma through the middle magnetosphere and its interchange with cold flux tubes moving outward. In the latter case the hot plasma could have increased the electron temperature in the source region and produced the intense, low-latitude features, while the increased cold plasma transport rate produced the shift of the main oval

Weakening of Jupiter's main auroral emission during January 2014

In January 2014 Jupiter's FUV main auroral oval decreased its emitted power by 70% and shifted equatorward by ∼1°. Intense, low-latitude features were also detected. The decrease in emitted power is attributed to a decrease in auroral current density rather than electron energy. This could be caused by a decrease in the source electron density, an order of magnitude increase in the source electron thermal energy, or a combination of these. Both can be explained either by expansion of the magnetosphere or by an increase in the inward transport of hot plasma through the middle magnetosphere and its interchange with cold flux tubes moving outward. In the latter case the hot plasma could have increased the electron temperature in the source region and produced the intense, low-latitude features, while the increased cold plasma transport rate produced the shift of the main oval

The Science of Sungrazers, Sunskirters, and Other Near-Sun Comets

This review addresses our current understanding of comets that venture close to the Sun, and are hence exposed to much more extreme conditions than comets that are typically studied from Earth. The extreme solar heating and plasma environments that these objects encounter change many aspects of their behaviour, thus yielding valuable information on both the comets themselves that complements other data we have on primitive solar system bodies, as well as on the near-solar environment which they traverse. We propose clear definitions for these comets: We use the term near-Sun comets to encompass all objects that pass sunward of the perihelion distance of planet Mercury (0.307 AU). Sunskirters are defined as objects that pass within 33 solar radii of the Sun’s centre, equal to half of Mercury’s perihelion distance, and the commonly-used phrase sungrazers to be objects that reach perihelion within 3.45 solar radii, i.e. the fluid Roche limit. Finally, comets with orbits that intersect the solar photosphere are termed sundivers. We summarize past studies of these objects, as well as the instruments and facilities used to study them, including space-based platforms that have led to a recent revolution in the quantity and quality of relevant observations. Relevant comet populations are described, including the Kreutz, Marsden, Kracht, and Meyer groups, near-Sun asteroids, and a brief discussion of their origins. The importance of light curves and the clues they provide on cometary composition are emphasized, together with what information has been gleaned about nucleus parameters, including the sizes and masses of objects and their families, and their tensile strengths. The physical processes occurring at these objects are considered in some detail, including the disruption of nuclei, sublimation, and ionisation, and we consider the mass, momentum, and energy loss of comets in the corona and those that venture to lower altitudes. The different components of comae and tails are described, including dust, neutral and ionised gases, their chemical reactions, and their contributions to the near-Sun environment. Comet-solar wind interactions are discussed, including the use of comets as probes of solar wind and coronal conditions in their vicinities. We address the relevance of work on comets near the Sun to similar objects orbiting other stars, and conclude with a discussion of future directions for the field and the planned ground- and space-based facilities that will allow us to address those science topics

Far-UV phase dependence and surface characteristics of Comet 67P/Churyumov-Gerasimenko as observed with Rosetta Alice

International audienceThe Alice far-ultraviolet (FUV) spectrograph onboard Rosetta has, for the first time, imaged the surface of a comet, 67P/Churyumov-Gerasimenko (67P), in the FUV. With spatially resolved data, the nucleus properties are characterized in the FUV, including phase dependence, albedo, and spectral slope. Regional measurements across the nucleus are compared to discern any compositional variations.Methods. Hapke theory was utilized to model the phase dependence of the material on the surface of 67P. The phase dependence of 67P was derived from a subset of data acquired at various phase angles in November 2014, within 50 km of the comet such that the nucleus was spatially resolved. The derived photometric correction was then applied to a different subset of spatially resolved data sampling several distinct geographical regions on the nucleus acquired in August−November 2014 under similar viewing geometries.Results. In the FUV, the surface of 67P is dark, blue sloped, has an average geometric albedo of 0.054±0.008 at 1475 Å near the center of the Alice bandpass, and is mostly uniform from region to region, with the exception of the Hatmehit region, which is slightly more reflective. These results are consistent with the suggestion made by the Rosetta OSIRIS and VIRTIS teams that the surface of 67P is covered with a homogeneous layer of material and that surface ice is not ubiquitous in large abundances. The modeled Hapke parameters, specifically the single scattering albedo (w) and the asymmetry factor (ζ), are determined to be 0.031 ± 0.003 and −0.530 ± 0.025 near the center of the Alice bandpass at 1475 Å. These parameters are consistent with measurements of other comet nuclei that have been observed by flyby missions in the visible and the near-infrared regimes