509 research outputs found
The roles of charge exchange and dissociation in spreading Saturn's neutral clouds
Neutrals sourced directly from Enceladus's plumes are initially confined to a
dense neutral torus in Enceladus's orbit around Saturn. This neutral torus is
redistributed by charge exchange, impact/photodissociation, and neutral-neutral
collisions to produce Saturn's neutral clouds. Here we consider the former
processes in greater detail than in previous studies. In the case of
dissociation, models have assumed that OH is produced with a single speed of 1
km/s, whereas laboratory measurements suggest a range of speeds between 1 and
1.6 km/s. We show that the high-speed case increases dissociation's range of
influence from 9 to 15 Rs. For charge exchange, we present a new modeling
approach, where the ions are followed within a neutral background, whereas
neutral cloud models are conventionally constructed from the neutrals' point of
view. This approach allows us to comment on the significance of the ions'
gyrophase at the moment charge exchange occurs. Accounting for gyrophase: (1)
has no consequence on the H2O cloud; (2) doubles the local density of OH at the
orbit of Enceladus; and (3) decreases the oxygen densities at Enceladus's orbit
by less than 10%. Finally, we consider velocity-dependent, as well as
species-dependent cross sections and find that the oxygen cloud produced from
charge exchange is spread out more than H2O, whereas the OH cloud is the most
confined.Comment: Accepted to the Journal of Geophysical Research, 49 pages, 10 figure
The crucial role of HST during the NASA Juno mission: a "Juno initiative"
In 2016, the NASA Juno spacecraft will initiate its one-year mission around
Jupiter and become the first probe to explore the polar regions of Jupiter. The
HST UV instruments (STIS and ACS) can greatly contribute to the success of the
Juno mission by providing key complementary views of Jupiter's UV aurora from
Earth orbit. Juno carries an ultraviolet Spectrograph (UVS) and an infrared
spectral mapper (JIRAM) that will obtain high-resolution spectral images
providing the auroral counterpart to Juno's in situ particles and fields
measurements with the plasma JADE and JEDI particle detectors. The Juno mission
will be the first opportunity to measure simultaneously the energetic particles
at high latitude and the auroral emissions they produce. Following programmatic
and technical limitations, the amount of UVS data transmitted to Earth will be
severely restricted. Therefore, it is of extreme importance that HST captures
as much additional information as possible on Jupiter's UV aurora during the
one-year life of the Juno mission. This white paper is a plea for a "Juno
initiative" that will ensure that a sufficient number of orbits is allocated to
this unique solar system mission.Comment: Paper submitted to the Space Telescope Science Institute in response
to the call for HST White Papers for Hubble's 2020 Visio
Periodic plasma escape from the mass‐loaded Kronian magnetosphere
Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/94967/1/jgra20364.pd
Energetic charged particle fluxes relevant to Ganymede's polar region
The JEDI instrument made measurements of energetic charged particles near Ganymede during a close encounter with that moon. Here we find ion flux levels are similar close to Ganymede itself but outside its magnetosphere and on near wake and open field lines. But energetic electron flux levels are more than a factor of 2 lower on polar and near-wake field lines than on nearby Jovian field lines at all energies reported here. Flux levels are relevant to the weathering of the surface, particularly processes that affect the distribution of ice, since surface brightness has been linked to the open-closed field line boundary. For this reason, we estimate the sputtering rates expected in the polar regions due to energetic heavy ions. Other rates, such as those related to radiolysis by plasma and particles that can reach the surface, need to be added to complete the picture of charged particle weathering
Cube law, condition factor and weight-length relationships: history, meta-analysis and recommendations
This study presents a historical review, a meta-analysis, and recommendations for users about weight–length relationships, condition factors and relative weight equations. The historical review traces the developments of the respective concepts. The meta-analysis explores 3929 weight–length relationships of the type W = aLb for 1773 species of fishes. It shows that 82% of the variance in a plot of log a over b can be explained by allometric versus isometric growth patterns and by different body shapes of the respective species. Across species median b = 3.03 is significantly larger than 3.0, thus indicating a tendency towards slightly positive-allometric growth (increase in relative body thickness or plumpness) in most fishes. The expected range of 2.5 < b < 3.5 is confirmed. Mean estimates of b outside this range are often based on only one or two weight–length relationships per species. However, true cases of strong allometric growth do exist and three examples are given. Within species, a plot of log a vs b can be used to detect outliers in weight–length relationships. An equation to calculate mean condition factors from weight–length relationships is given as Kmean = 100aLb−3. Relative weight Wrm = 100W/(amLbm) can be used for comparing the condition of individuals across populations, where am is the geometric mean of a and bm is the mean of b across all available weight–length relationships for a given species. Twelve recommendations for proper use and presentation of weight–length relationships, condition factors and relative weight are given
Modeling magnetospheric fields in the Jupiter system
The various processes which generate magnetic fields within the Jupiter
system are exemplary for a large class of similar processes occurring at other
planets in the solar system, but also around extrasolar planets. Jupiter's
large internal dynamo magnetic field generates a gigantic magnetosphere, which
is strongly rotational driven and possesses large plasma sources located deeply
within the magnetosphere. The combination of the latter two effects is the
primary reason for Jupiter's main auroral ovals. Jupiter's moon Ganymede is the
only known moon with an intrinsic dynamo magnetic field, which generates a
mini-magnetosphere located within Jupiter's larger magnetosphere including two
auroral ovals. Ganymede's magnetosphere is qualitatively different compared to
the one from Jupiter. It possesses no bow shock but develops Alfv\'en wings
similar to most of the extrasolar planets which orbit their host stars within
0.1 AU. New numerical models of Jupiter's and Ganymede's magnetospheres
presented here provide quantitative insight into the processes that maintain
these magnetospheres. Jupiter's magnetospheric field is approximately
time-periodic at the locations of Jupiter's moons and induces secondary
magnetic fields in electrically conductive layers such as subsurface oceans. In
the case of Ganymede, these secondary magnetic fields influence the oscillation
of the location of its auroral ovals. Based on dedicated Hubble Space Telescope
observations, an analysis of the amplitudes of the auroral oscillations
provides evidence that Ganymede harbors a subsurface ocean. Callisto in
contrast does not possess a mini-magnetosphere, but still shows a perturbed
magnetic field environment. Callisto's ionosphere and atmospheric UV emission
is different compared to the other Galilean satellites as it is primarily been
generated by solar photons compared to magnetospheric electrons.Comment: Chapter for Book: Planetary Magnetis
Interaction of Saturn's magnetosphere and its moons: 1. Interaction between corotating plasma and standard obstacles
Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/95046/1/jgra20170.pd
Report of the President, Bowdoin College 1914-1915
https://digitalcommons.bowdoin.edu/presidents-reports/1023/thumbnail.jp
Jupiter’s auroras during the Juno approach phase as observed by the Hubble Space Telescope
We present movies of the Hubble Space Telescope (HST) observations of Jupiter’s FUV auroras observed during the Juno approach phase and first capture orbit, and compare with Juno observations of the interplanetary medium near Jupiter and inside the magnetosphere. Jupiter’s FUV auroras indicate the nature of the dynamic processes occurring in Jupiter’s magnetosphere, and the approach phase provided a unique opportunity to obtain a full set of interplanetary data near to Jupiter at the time of a program of HST observations, along with the first simultaneous with Juno observations inside the magnetosphere. The overall goal was to determine the nature of the solar wind effect on Jupiter’s magnetosphere. HST observations were obtained with typically 1 orbit per day over three intervals: 16 May – 7 June, 22-30 June and 11-18 July, i.e. while Juno was in the solar wind, around the bow shock and magnetosphere crossings, and in the mid-latitude middle-outer magnetospheres. We show that these intervals are characterised by particularly dynamic polar auroras, and significant variations in the auroral power output caused by e.g. dawn storms, intense main emission and poleward forms. We compare the variation of these features with Juno observations of interplanetary compression regions and the magnetospheric environment during the intervals of these observations
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