8,751 research outputs found
Diurnal variations in optical depth at Mars: Observations and interpretations
Viking lander camera images of the Sun were used to compute atmospheric optical depth at two sites over a period of 1 to 1/3 martian years. The complete set of 1044 optical depth determinations is presented in graphical and tabular form. Error estimates are presented in detail. Optical depths in the morning (AM) are generally larger than in the afternoon (PM). The AM-PM differences are ascribed to condensation of water vapor into atmospheric ice aerosols at night and their evaporation in midday. A smoothed time series of these differences shows several seasonal peaks. These are simulated using a one-dimensional radiative convective model which predicts martial atmospheric temperature profiles. A calculation combining these profiles with water vapor measurements from the Mars Atmospheric Water Detector is used to predict when the diurnal variations of water condensation should occur. The model reproduces a majority of the observed peaks and shows the factors influencing the process. Diurnal variation of condensation is shown to peak when the latitude and season combine to warm the atmosphere to the optimum temperature, cool enough to condense vapor at night and warm enough to cause evaporation at midday
Demonstration of the Zero-Crossing Phasemeter with a LISA Test-bed Interferometer
The Laser Interferometer Space Antenna (LISA) is being designed to detect and
study in detail gravitational waves from sources throughout the Universe such
as massive black hole binaries. The conceptual formulation of the LISA
space-borne gravitational wave detector is now well developed. The
interferometric measurements between the sciencecraft remain one of the most
important technological and scientific design areas for the mission.
Our work has concentrated on developing the interferometric technologies to
create a LISA-like optical signal and to measure the phase of that signal using
commercially available instruments. One of the most important goals of this
research is to demonstrate the LISA phase timing and phase reconstruction for a
LISA-like fringe signal, in the case of a high fringe rate and a low signal
level. We present current results of a test-bed interferometer designed to
produce an optical LISA-like fringe signal previously discussed in the
literature.Comment: find minor corrections in the CQG versio
Proceedings of the MECA Workshop on The Evoluation of the Martian Atmosphere
Topics addressed include: Mars' volatile budget; climatic implications of martian channels; bulk composition of Mars; accreted water inventory; evolution of CO2; dust storms; nonlinear frost albedo feedback on Mars; martian atmospheric evolution; effects of asteroidal and cometary impacts; and water exchange between the regolith and the atmosphere/cap system over obliquity timescales
A Demonstration of LISA Laser Communication
Over the past few years questions have been raised concerning the use of
laser communications links between sciencecraft to transmit phase information
crucial to the reduction of laser frequency noise in the LISA science
measurement. The concern is that applying medium frequency phase modulations to
the laser carrier could compromise the phase stability of the LISA fringe
signal. We have modified the table-top interferometer presented in a previous
article by applying phase modulations to the laser beams in order to evaluate
the effects of such modulations on the LISA science fringe signal. We have
demonstrated that the phase resolution of the science signal is not degraded by
the presence of medium frequency phase modulations.Comment: minor corrections found in the CQG versio
VLBI Polarimetry of 177 Sources from the Caltech-Jodrell Bank Flat-spectrum Survey
We present VLBA observations and a statistical analysis of 5 GHz VLBI
polarimetry data from 177 sources in the Caltech-Jodrell Bank flat-spectrum
(CJF) survey. The CJF survey, a complete, flux-density-limited sample of 293
extragalactic radio sources, gives us the unique opportunity to compare a broad
range of source properties for quasars, galaxies and BL Lacertae objects. We
focus primarily on jet properties, specifically the correlation between the jet
axis angle and the polarization angle in the core and jet. A strong correlation
is found for the electric vector polarization angle in the cores of quasars to
be perpendicular to the jet axis. Contrary to previous claims, no correlation
is found between the jet polarization angle and the jet axis in either quasars
or BL Lac objects. With this large, homogeneous sample we are also able to
investigate cosmological issues and AGN evolution.Comment: Accepted to the Astrophysical Journal: 37 pages, 14 figure
Charge Management for Gravitational Wave Observatories using UV LEDs
Accumulation of electrical charge on the end mirrors of gravitational wave
observatories, such as the space-based LISA mission and ground-based LIGO
detectors, can become a source of noise limiting the sensitivity of such
detectors through electronic couplings to nearby surfaces. Torsion balances
provide an ideal means for testing gravitational wave technologies due to their
high sensitivity to small forces. Our torsion pendulum apparatus consists of a
movable Au-coated Cu plate brought near a Au-coated Si plate pendulum suspended
from a non-conducting quartz fiber. A UV LED located near the pendulum
photoejects electrons from the surface, and a UV LED driven electron gun
directs photoelectrons towards the pendulum surface. We have demonstrated both
charging and discharging of the pendulum with equivalent charging rates of
, as well as spectral measurements of the pendulum
charge resulting in a white noise level equivalent to .Comment: 5 pages, submitted to PR
Extreme tunability of interactions in a Li Bose-Einstein condensate
We use a Feshbach resonance to tune the scattering length a of a
Bose-Einstein condensate of 7Li in the |F = 1, m_F = 1> state. Using the
spatial extent of the trapped condensate we extract a over a range spanning 7
decades from small attractive interactions to extremely strong repulsive
interactions. The shallow zero-crossing in the wing of the Feshbach resonance
enables the determination of a as small as 0.01 Bohr radii. In this regime,
evidence of the weak anisotropic magnetic dipole interaction is obtained by
comparison with different trap geometries
Gluing Initial Data Sets for General Relativity
We establish an optimal gluing construction for general relativistic initial
data sets. The construction is optimal in two distinct ways. First, it applies
to generic initial data sets and the required (generically satisfied)
hypotheses are geometrically and physically natural. Secondly, the construction
is completely local in the sense that the initial data is left unaltered on the
complement of arbitrarily small neighborhoods of the points about which the
gluing takes place. Using this construction we establish the existence of
cosmological, maximal globally hyperbolic, vacuum space-times with no constant
mean curvature spacelike Cauchy surfaces.Comment: Final published version - PRL, 4 page
Collective excitation of a Bose-Einstein condensate by modulation of the atomic scattering length
We excite the lowest-lying quadrupole mode of a Bose-Einstein condensate by
modulating the atomic scattering length via a Feshbach resonance. Excitation
occurs at various modulation frequencies, and resonances located at the natural
quadrupole frequency of the condensate and at the first harmonic are observed.
We also investigate the amplitude of the excited mode as a function of
modulation depth. Numerical simulations based on a variational calculation
agree with our experimental results and provide insight into the observed
behavior.Comment: Submitted to PR
Dissipative Transport of a Bose-Einstein Condensate
We investigate the effects of impurities, either correlated disorder or a
single Gaussian defect, on the collective dipole motion of a Bose-Einstein
condensate of Li in an optical trap. We find that this motion is damped at
a rate dependent on the impurity strength, condensate center-of-mass velocity,
and interatomic interactions. Damping in the Thomas-Fermi regime depends
universally on the disordered potential strength scaled to the condensate
chemical potential and the condensate velocity scaled to the peak speed of
sound. The damping rate is comparatively small in the weakly interacting
regime, and the damping in this case is accompanied by strong condensate
fragmentation. \textit{In situ} and time-of-flight images of the atomic cloud
provide evidence that this fragmentation is driven by dark soliton formation.Comment: 14 pages, 20 figure
- …