343 research outputs found
Scaling laws for convection and jet speeds in the giant planets
Three-dimensional studies of convection in deep spherical shells have been
used to test the hypothesis that the strong jet streams on Jupiter, Saturn,
Uranus, and Neptune result from convection throughout the molecular envelopes.
Due to computational limitations, these simulations must adopt viscosities and
heat fluxes many orders of magnitude larger than the planetary values. Several
numerical investigations have identified trends for how the mean jet speed
varies with heat flux and viscosity, but no previous theories have been
advanced to explain these trends. Here, we show using simple arguments that if
convective release of potential energy pumps the jets and viscosity damps them,
the mean jet speeds split into two regimes. When the convection is weakly
nonlinear, the equilibrated jet speeds should scale approximately with F/nu,
where F is the convective heat flux and nu is the viscosity. When the
convection is strongly nonlinear, the jet speeds are faster and should scale
approximately as (F/nu)^{1/2}. We demonstrate how this regime shift can
naturally result from a shift in the behavior of the jet-pumping efficiency
with heat flux and viscosity. Moreover, the simulations hint at a third regime
where, at sufficiently small viscosities, the jet speed becomes independent of
the viscosity. We show based on mixing-length estimates that if such a regime
exists, mean jet speeds should scale as heat flux to the 1/4 power. Our
scalings provide a good match to the mean jet speeds obtained in previous
Boussinesq and anelastic, three-dimensional simulations of convection within
giant planets over a broad range of parameters. When extrapolated to the real
heat fluxes, these scalings suggest that the mass-weighted jet speeds in the
molecular envelopes of the giant planets are much weaker--by an order of
magnitude or more--than the speeds measured at cloud level.Comment: 23 pages, 10 figures, in press at Icaru
Simple processors of star tracker commands for stabilizing an inertially oriented satellite
Simple processors of star tracker commands for stabilizing inertially oriented satellit
Three-dimensional atmospheric circulation of hot Jupiters on highly eccentric orbits
Of the over 800 exoplanets detected to date, over half are on non-circular
orbits, with eccentricities as high as 0.93. Such orbits lead to time-variable
stellar heating, which has implications for the planet's atmospheric dynamical
regime. However, little is known about this dynamical regime, and how it may
influence observations. Therefore, we present a systematic study of hot
Jupiters on highly eccentric orbits using the SPARC/MITgcm, a model which
couples a three-dimensional general circulation model with a plane-parallel,
two-stream, non-grey radiative transfer model. In our study, we vary the
eccentricity and orbit-average stellar flux over a wide range. We demonstrate
that the eccentric hot Jupiter regime is qualitatively similar to that of
planets on circular orbits; the planets possess a superrotating equatorial jet
and exhibit large day-night temperature variations. We show that these
day-night heating variations induce momentum fluxes equatorward to maintain the
superrotating jet throughout its orbit. As the eccentricity and/or stellar flux
is increased, the superrotating jet strengthens and narrows, due to a smaller
Rossby deformation radius. For a select number of model integrations, we
generate full-orbit lightcurves and find that the timing of transit and
secondary eclipse viewed from Earth with respect to periapse and apoapse can
greatly affect what we see in infrared (IR) lightcurves; the peak in IR flux
can lead or lag secondary eclipse depending on the geometry. For those planets
that have large day-night temperature variations and rapid rotation rates, we
find that the lightcurves exhibit "ringing" as the planet's hottest region
rotates in and out of view from Earth. These results can be used to explain
future observations of eccentric transiting exoplanets.Comment: 20 pages, 18 figures, 2 tables; Accepted to Ap
Meteorology of Jupiter's Equatorial Hot Spots and Plumes from Cassini
We present an updated analysis of Jupiter's equatorial meteorology from
Cassini observations. For two months preceding the spacecraft's closest
approach, the Imaging Science Subsystem (ISS) onboard regularly imaged the
atmosphere. We created time-lapse movies from this period in order to analyze
the dynamics of equatorial hot spots and their interactions with adjacent
latitudes. Hot spots are quasi-stable, rectangular dark areas on
visible-wavelength images, with defined eastern edges that sharply contrast
with surrounding clouds, but diffuse western edges serving as nebulous
boundaries with adjacent equatorial plumes. Hot spots exhibit significant
variations in size and shape over timescales of days and weeks. Some of these
changes correspond with passing vortex systems from adjacent latitudes
interacting with hot spots. Strong anticyclonic gyres present to the south and
southeast of the dark areas appear to circulate into hot spots. Impressive,
bright white plumes occupy spaces in between hot spots. Compact cirrus-like
'scooter' clouds flow rapidly through the plumes before disappearing within the
dark areas. These clouds travel at 150-200 m/s, much faster than the 100 m/s
hot spot and plume drift speed. This raises the possibility that the scooter
clouds may be more illustrative of the actual jet stream speed at these
latitudes. Most previously published zonal wind profiles represent the drift
speed of the hot spots at their latitude from pattern matching of the entire
longitudinal image strip. If a downward branch of an equatorially-trapped
Rossby waves controls the overall appearance of hot spots, however, the
westward phase velocity of the wave leads to underestimates of the true jet
stream speed.Comment: 33 pages, 11 figures; accepted for publication in Icarus; for
supplementary movies, please contact autho
Effects of Temperature on Seta Elongation in Atrichum Undulatum
Author Institution: Department of Botany, The Ohio State University, Columbus, Ohio 43210Field-collected gametophytes of Atrichum undulatum were placed in two growth chambers which were maintained at the same light intensity and light period, but at two different temperature regimes. After sporophyte development, differences in seta lengths were observed. Measurements of cell lengths revealed that attached setae grown in the high-temperature regime (12°-22°C) were longer than those grown in a low-temperature regime (3°-12°C), as a result of both more cell divisions and a larger average cell length. Thus, temperature appeared to influence both cell division and cell elongation in the setae of Atrichum undulatum
Gravitational signature of Jupiter’s internal dynamics
Telescopic observations and space missions to Jupiter have provided vast information about Jupiter's cloud level winds, but the depth to which these winds penetrate has remained an ongoing mystery. Scheduled to be launched in 2011, the Jupiter orbiter Juno will make high-resolution observations of Jupiter's gravity field. In this paper we show that these measurements are sensitive to the depth of the internal winds. We use dynamical models ranging from an idealized thermal wind balance analysis, using the observed cloud-top winds, to a full general circulation model (GCM). We relate the depth of the dynamics to the external gravity spectrum for different internal wind structure scenarios. In particular, we predict that substantial Jovian winds below a depth of 500 km would lead to detectable (milligal-level) gravity anomalies with respect to the expected gravity for a planet in solid body rotation
Investigation of Jupiter's Equatorial Hotspots and Plumes Using Cassini ISS Observations
We present updated analysis of Jupiter's equatorial meteorology from Cassini observations. For two months preceding the spacecraft's closest approach, the ISS onboard regularly imaged the atmosphere. We created time-lapse movies from this period in order to analyze the dynamics of equatorial 5-micron hot spots and their interactions with adjacent latitudes. Hot spots are quasi-stable, rectangular dark areas on visible-wavelength images, with defined eastern edges that sharply contrast with surrounding clouds, but a diffuse western edge serving as a nebulous boundary with adjacent equatorial plumes. Hot spots exhibit significant variations in size and shape over timescales of days and weeks. Some of these changes correspond with passing vortex systems from adjacent latitudes interacting with hot spots. Strong anticyclonic gyres present to the south and southeast of the dark areas appear to circulate into hot spots. Impressive, bright white plumes occupy spaces in between hot spots. Compact cirrus-iike 'scooter' clouds flow rapidly through the plumes before disappearing within the dark areas. This raises the possibility that the plumes and fast-moving clouds are at higher altitudes, because their speed does not match previously published zonal wind profiles. Most profiles represent the drift speed of the hot spots at their latitude from pattern matching of the entire longitudinal image strip. If a downward branch of an equatorially-trapped Rossby waves controls the overall appearance of hot spots, however, the westward phase velocity of the wave leads to underestimates of the true jet stream speed. Instead, our expanded data set demonstrating the rapid flow of these scooter clouds may be more illustrative of the actual jet stream speed at these latitudes. This research was supported by a NASA JDAP grant and the NASA Postdoctoral Program
A Precise Water Abundance Measurement for the Hot Jupiter WASP-43b
The water abundance in a planetary atmosphere provides a key constraint on
the planet's primordial origins because water ice is expected to play an
important role in the core accretion model of planet formation. However, the
water content of the Solar System giant planets is not well known because water
is sequestered in clouds deep in their atmospheres. By contrast, short-period
exoplanets have such high temperatures that their atmospheres have water in the
gas phase, making it possible to measure the water abundance for these objects.
We present a precise determination of the water abundance in the atmosphere of
the 2 short-period exoplanet WASP-43b based on thermal
emission and transmission spectroscopy measurements obtained with the Hubble
Space Telescope. We find the water content is consistent with the value
expected in a solar composition gas at planetary temperatures (0.4-3.5x solar
at 1 confidence). The metallicity of WASP-43b's atmosphere suggested
by this result extends the trend observed in the Solar System of lower metal
enrichment for higher planet masses.Comment: Accepted to ApJL; this version contains three supplemental figures
that are not included in the published paper. See also our companion paper
"Thermal structure of an exoplanet atmosphere from phase-resolved emission
spectroscopy" by Stevenson et a
The Experience of Existing: small family farms in the Northeast United States
The institution of small-scale family farming dominates the agricultural landscape in both Europe and the Unites States. Characteristically, in addition to the small size of these farms, the farm family supplies the bulk of the needed labour. The continued existence of this institution in the face of growing highly capitalised agribusinesses has puzzled many. This study explores this question through the eyes of small family farmers in the northeast of the United States. It identifies three categories of factors that farmers see as why and how they continue to farm in this traditional fashion. The first category encompasses a love for farming and deep sense of place connected to the farm land. The second category encompasses self-sufficiency and the occupational diversity of farm life. The third category encompasses the ability of small farms to exploit small markets unsuited to large businesses and the role of government in preserving these small markets. This study concludes that the most important of the three categories to perpetuating the small family farm is the love of farmers for farming
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