167 research outputs found
Cassini UVIS Observations of the Io Plasma Torus. III. Observations of Temporal and Azimuthal Variability
In this third paper in a series presenting observations by the Cassini
Ultraviolet Imaging Spectrometer (UVIS) of the Io plasma torus, we show
remarkable, though subtle, spatio-temporal variations in torus properties. The
Io torus is found to exhibit significant, near-sinusoidal variations in ion
composition as a function of azimuthal position. The azimuthal variation in
composition is such that the mixing ratio of S II is strongly correlated with
the mixing ratio of S III and the equatorial electron density and strongly
anti-correlated with the mixing ratios of both S IV and O II and the equatorial
electron temperature. Surprisingly, the azimuthal variation in ion composition
is observed to have a period of 10.07 hours--1.5% longer than the System III
rotation period of Jupiter, yet 1.3% shorter than the System IV period defined
by Brown (1995). Although the amplitude of the azimuthal variation of S III and
O II remained in the range of 2-5%, the amplitude of the S II and S IV
compositional variation ranged between 5-25% during the UVIS observations.
Furthermore, the amplitude of the azimuthal variations of S II and S IV appears
to be modulated by its location in System III longitude, such that when the
region of maximum S II mixing ratio (minimum S IV mixing ratio) is aligned with
a System III longitude of ~200 +/- 15 degrees, the amplitude is a factor of ~4
greater than when the variation is anti-aligned. This behavior can explain
numerous, often apparently contradictory, observations of variations in the
properties of the Io plasma torus with the System III and System IV coordinate
systems.Comment: 35 pages including 12 figures and 2 table
First Constraints on Rings in the Pluto System
Simple theoretical calculations have suggested that small body impacts onto
Pluto's newly discovered small satellites, Nix and Hydra, are capable of
generating time-variable rings or dust sheets in the Pluto system. Using
HST/ACS data obtained on 2006 February 15 and 2006 March 2, we find no
observational evidence for such a ring system and present the first constraints
on the present-day I/F and optical depth of a putative ring system. At the
1500-km radial resolution of our search, we place a 3-sigma upper limit on the
azimuthally-averaged normal I/F of ring particles of 5.1x10^-7 at a distance of
42,000 km from the Pluto-Charon barycenter, the minimum distance for a
dynamically stable ring (Stern et al., 1994; Nagy et al., 2006); 4.4x10^-7 at
the orbit of Nix; and 2.5x10^-7 at the orbit of Hydra. For an assumed ring
particle albedo of 0.04 (0.38), these I/F limits translate into 3-sigma upper
limits on the normal optical depth of macroscopic ring particles of 1.3x10^-5
(1.4x10^-6), 1.1x10^-5 (1.2x10^-6), 6.4x10^-6 (6.7x10^-7), respectively. Were
the New Horizons spacecraft to fly through a ring system with optical depth of
1.3x10^-5, it would collide with a significant number of potentially damaging
ring particles. We therefore recommend that unless tighter constraints can be
obtained, New Horizons cross the putative ring plane within 42,000 km of the
Pluto-Charon barycenter, where rings are dynamically unstable. We derive a
crude estimate of the lifetime of putative ring paritcles of 900 years.Comment: 14 pages, including 3 figures and 2 table
A Sensitivity Study of the Enceladus Torus
We have developed a homogeneous model of physical chemistry to investigate
the neutral-dominated, water-based Enceladus torus. Electrons are treated as
the summation of two isotropic Maxwellian distributionsa thermal component
and a hot component. The effects of electron impact, electron recombination,
charge exchange, and photochemistry are included. The mass source is neutral
HO, and a rigidly-corotating magnetosphere introduces energy via pickup of
freshly-ionized neutrals. A small fraction of energy is also input by Coulomb
collisions with a small population ( 1%) of supra-thermal electrons. Mass
and energy are lost due to radial diffusion, escaping fast neutrals produced by
charge exchange and recombination, and a small amount of radiative cooling. We
explore a constrained parameter space spanned by water source rate, ion radial
diffusion, hot-electron temperature, and hot-electron density. The key findings
are: (1) radial transport must take longer than 12 days; (2) water is input at
a rate of 100--180 kg s; (3) hot electrons have energies between 100 and
250 eV; (4) neutrals dominate ions by a ratio of 40:1 and continue to dominate
even when thermal electrons have temperatures as high as 5 eV; (5)
hot electrons do not exceed 1% of the total electron population within the
torus; (6) if hot electrons alone drive the observed longitudinal variation in
thermal electron density, then they also drive a significant variation in ion
composition.Comment: 9 pages text, 3 tables, 9 figure
The Nature and Frequency of the Gas Outbursts in Comet 67P/Churyumov-Gerasimenko observed by the Alice Far-ultraviolet Spectrograph on Rosetta
Alice is a far-ultraviolet imaging spectrograph onboard Rosetta that, amongst
multiple objectives, is designed to observe emissions from various atomic and
molecular species from within the coma of comet 67P/Churyumov-Gerasimenko. The
initial observations, made following orbit insertion in August 2014, showed
emissions of atomic hydrogen and oxygen spatially localized close to the
nucleus and attributed to photoelectron impact dissociation of H2O vapor.
Weaker emissions from atomic carbon were subsequently detected and also
attributed to electron impact dissociation, of CO2, the relative H I and C I
line intensities reflecting the variation of CO2 to H2O column abundance along
the line-of-sight through the coma. Beginning in mid-April 2015, Alice
sporadically observed a number of outbursts above the sunward limb
characterized by sudden increases in the atomic emissions, particularly the
semi-forbidden O I 1356 multiplet, over a period of 10-30 minutes, without a
corresponding enhancement in long wavelength solar reflected light
characteristic of dust production. A large increase in the brightness ratio O I
1356/O I 1304 suggests O2 as the principal source of the additional gas. These
outbursts do not correlate with any of the visible images of outbursts taken
with either OSIRIS or the navigation camera. Beginning in June 2015 the nature
of the Alice spectrum changed considerably with CO Fourth Positive band
emission observed continuously, varying with pointing but otherwise fairly
constant in time. However, CO does not appear to be a major driver of any of
the observed outbursts.Comment: 6 pages, 4 figures, accepted for publication in the Astrophysical
Journal Letter
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