329 research outputs found
Quantum cavitation in liquid He: dissipation effects
We have investigated the effect that dissipation may have on the cavitation
process in normal liquid He. Our results indicate that a rather small
dissipation decreases sizeably the quantum-to-thermal crossover temperature
for cavitation in normal liquid He. This is a possible explanation
why recent experiments have not yet found clear evidence of quantum cavitation
at temperatures below the predicted by calculations which neglect
dissipation.Comment: To be published in Physical Review B6
Thermally assisted quantum cavitation in solutions of 3He in 4He
We have investigated the quantum-to-thermal crossover temperature T* for
cavitation in liquid helium mixtures up to 0.05 3He concentrations. With
respect to the pure 4He case, T* is sizeably reduced, to a value below 50 mK
for 3He concentrations above 0.02. As in pure 4He, the homogeneous cavitation
pressure is systematically found close to the spinodal pressure.Comment: Typeset using Revtex, 9 pages and 4 figure
Stability of vortex lines in liquid 3He-4He mixtures at zero temperature
At low temperatures and 3He concentrations below 6.6 %, there is experimental
evidence about the existence in liquid helium mixtures, of stable vortices with
3He-rich cores. When the system is either supersaturated or submitted to a
tensile strength, vortices lose stability becoming metastable and eventually
completely unstable, so that their cores freely expand. Within a density
functional approach, we have determined the pressure-3He concentration curve
along which this instability appears at zero temperature.Comment: Typeset using Revtex, 9 pages and 5 Postscript figure
Generating vortex rings in Bose-Einstein condensates in the line-source approximation
We present a numerical method for generating vortex rings in Bose-Einstein
condensates confined in axially symmetric traps. The vortex ring is generated
using the line-source approximation for the vorticity, i.e., the rotational of
the superfluid velocity field is different from zero only on a circumference of
given radius located on a plane perpendicular to the symmetry axis and coaxial
with it. The particle density is obtained by solving a modified
Gross-Pitaevskii equation that incorporates the effect of the velocity field.
We discuss the appearance of density profiles, the vortex core structure and
the vortex nucleation energy, i.e., the energy difference between vortical and
ground-state configurations. This is used to present a qualitative description
of the vortex dynamics.Comment: Accepted for publication in Phys. Rev.
Cavitation of Electrons Bubbles in Liquid Helium Below saturation Pressure
We have used a Hartree-type electron-helium potential together with a density
functional description of liquid He and He to study the explosion of
electron bubbles submitted to a negative pressure. The critical pressure at
which bubbles explode has been determined as a function of temperature. It has
been found that this critical pressure is very close to the pressure at which
liquid helium becomes globally unstable in the presence of electrons. It is
shown that at high temperatures the capillary model overestimates the critical
pressures. We have checked that a commonly used and rather simple
electron-helium interaction yields results very similar to those obtained using
the more accurate Hartree-type interaction. We have estimated that the
crossover temperature for thermal to quantum nucleation of electron bubbles is
very low, of the order of 6 mK for He.Comment: 22 pages, 9 figure
Derived Backscatter Values from JERS-1 Digital Number Distributions Over Ross Island, Antarctica (McMurdo Station).
This report examines the derived backscatter values for different physical properties from one JERS-1 image over Hut Point Peninsula on Ross Island in the McMurdo Station area. The results discussed in this report will be used to determine a suitable location for transponder testing for the Radarsat Antarctic Mapping Project. Histograms of digital number (DN) values (0-255) were created to determine the DN distribution and approximate backscatter value for five locations on a single JERS-1 image. Two locations were chosen over first year sea ice, and one location each over glacial ice, exposed rock, and the McMurdo Station. From published backscatter values for first year ice of -15dB and a DN of 40, estimated from histograms, an approximation of the calibration constant K was calculated and subsequent backscatter coefficients were computed for the remaining locations. Digital numbers peaked at 255 for both glacial ice and McMurdo Station indicating that an area outside of McMurdo Station would be
preferable for transponder testing
Antarctic Mapping Mission Planning Aids
On November 4, 1995, the Canadian RADARSAT was carried aloft by a NASA rocket launched from Vandenburg Air Force Base. Radarsat is equipped with a C-band Synthetic Aperture Radar (SAR) capable of acquiring high resolution (25 m) images of Earth's surface day or night and under all weather conditions. Along with the attributes familiar to researchers working with SAR data from the European Space Agency's Earth Remote Sensing Satellite and the Japanese Earth Resources Satellite, RADARSAT will have enhanced flexibility to collect data using a variety of swath widths, incidence angles and resolutions. Most importantly, for scientists interested in Antarctica, the agreement for a U.S. launch of RADARSAT includes a provision for rotating in orbit the normally right-looking SAR to a left-looking mode. This 'Antarctic Mode' will provide for the first time a nearly instantaneous, high resolution view of the entirety of Antarctica on each of two proposed mappings separated by 2 years. This is an unprecedented opportunity to finish mapping one of the few remaining uncharted regions of the Earth. The completed maps will also provide two important benchmarks for gauging changes of Antarctica's ice cover. The preparation of a digital mosaic of Antarctica is being conducted under a NASA Pathfinder Project awarded to the Byrd Polar Research Center of The Ohio State University. The primary goal of this proposal is to compile digital SAR mosaics of the entire Antarctic continent using a combination of standard and extended beams during the "Antarctic Mode" of the Radarsat Mission. Agreements with the Canadian Space Agency call for the first Antarctic Mapping Manuever to occur in September, 1997. A mission plan to coordinate that complex acquisition and downlinking of Antarctic data has been developed by NASA's Jet Propulsion Laboratory. The Alaska SAR Facility (ASF) will be used as the primary data collection site supported by collections at the Canadian Gatineau and Prince Albert Ground Stations. ASF will process data into images which will be sent to OSU for compositing into map products using state-of-the-art equipment to be designed by Vexcel Corporation of Boulder Colorado. Imaging geometry will be constrained over the Antarctic using active radar transponders constructed by the Environmental Research Institute of Michigan and by corner reflectors deployed by the British Antarctic Survey. Additional ground control is being supplied by the National Imagery and Mapping Agency. Final products will be distributed through the ASF and the National Snow and Ice Data Center which are both NASA Data Archive Centers. The mosaics and ancillary information will be prepared on CDROM and will be made available to the science community through NASA DAACs. Science opportunities envisioned for the program are summarized on the accompanying table. These include studying the dynamics and variability of the Antarctic Ice Sheet including studies of regions like the Wordie Ice Shelf and the Larsen Ice Shelf which have recently experienced unexplained and nearly catastropic retreat. Geologic applications include large scale mapping of faults, volcanic features, and mountain building processes (particularly the Transantarctic Mountains). Finally, there is simply the unprecedented opportunity to use these digital maps in studies of many previously unexplored areas of the Southern Continent.NASACanadian Space Agenc
Spaceborne synthetic aperture radar: Current status and future directions. A report to the Committee on Earth Sciences, Space Studies Board, National Research Council
This report provides a context in which questions put forth by NASA's Office of Mission to Planet Earth (OMPTE) regarding the next steps in spaceborne synthetic aperture radar (SAR) science and technology can be addressed. It summarizes the state-of-the-art in theory, experimental design, technology, data analysis, and utilization of SAR data for studies of the Earth, and describes potential new applications. The report is divided into five science chapters and a technology assessment. The chapters summarize the value of existing SAR data and currently planned SAR systems, and identify gaps in observational capabilities needing to be filled to address the scientific questions. Cases where SAR provides complementary data to other (non-SAR) measurement techniques are also described. The chapter on technology assessment outlines SAR technology development which is critical not only to NASA's providing societally relevant geophysical parameters but to maintaining competitiveness in SAR technology, and promoting economic development
Surface structure and stability of the Larsen C ice shelf, Antarctic Peninsula
A structural glaciological description and analysis of surface morphological features of the Larsen C ice shelf, Antarctic Peninsula, is derived from satellite images spanning the period 1963-2007. The data are evaluated in two time ranges: a comparison of a 1963 satellite image photomosaic with a modern digital mosaic compiled using 2003/04 austral summer data; and an image series since 2003 showing recent evolution of the shelf. We map the ice-shelf edge, rift swarms, crevasses and crevasse traces, and linear longitudinal structures (called 'flow stripes' or 'streak lines'). The latter are observed to be continuous over distances of up to 200km from the grounding line to the ice-shelf edge, with little evidence of changes in pattern over that distance. Integrated velocity measurements along a flowline indicate that the shelf has been stable for similar to 560years in the mid-shelf area. Linear longitudinal features may be grouped into 12 units, each related to one or a small group of outlet feeder glaciers to the shelf. We observe that the boundaries between these flow units often mark rift terminations. The boundary zones originate upstream at capes, islands or other suture areas between outlet glaciers. In agreement with previous work, our findings imply that rift terminations within such suture zones indicate that they contain anomalously soft ice. We thus suggest that suture zones within the Larsen C ice shelf, and perhaps within ice shelves more generally, may act to stabilize them by reducing regional stress intensities and thus rates of rift lengthening
Structure of vortices in two-component Bose-Einstein condensates
We develop a three-dimensional analysis of the phase separation of
two-species Bose-Einstein condensates in the presence of vorticity within the
Thomas-Fermi approximation. We find different segregation features according to
whether the more repulsive component is in a vortex or in a vortex-free state.
An application of this study is aimed at describing systems formed by two
almost immiscible species of rubidium-87 that are commonly used in
Bose-Einstein condensation experiments. In particular, in this work we
calculate the density profiles of condensates for the same conditions as the
states prepared in the experiments performed at JILA [Matthews et al., Phys.
Rev. Lett. 83, 2498 (1999)]Comment: 4 pages, 3 figure
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