8,982 research outputs found
First-principles quantum simulations of dissociation of molecular condensates: Atom correlations in momentum space
We investigate the quantum many-body dynamics of dissociation of a
Bose-Einstein condensate of molecular dimers into pairs of constituent bosonic
atoms and analyze the resulting atom-atom correlations. The quantum fields of
both the molecules and atoms are simulated from first principles in three
dimensions using the positive-P representation method. This allows us to
provide an exact treatment of the molecular field depletion and s-wave
scattering interactions between the particles, as well as to extend the
analysis to nonuniform systems. In the simplest uniform case, we find that the
major source of atom-atom decorrelation is atom-atom recombination which
produces molecules outside the initially occupied condensate mode. The unwanted
molecules are formed from dissociated atom pairs with non-opposite momenta. The
net effect of this process -- which becomes increasingly significant for
dissociation durations corresponding to more than about 40% conversion -- is to
reduce the atom-atom correlations. In addition, for nonuniform systems we find
that mode-mixing due to inhomogeneity can result in further degradation of the
correlation signal. We characterize the correlation strength via the degree of
squeezing of particle number-difference fluctuations in a certain
momentum-space volume and show that the correlation strength can be increased
if the signals are binned into larger counting volumes.Comment: Final published version, with updated references and minor
modification
Low-Altitude Reconnection Inflow-Outflow Observations during a 2010 November 3 Solar Eruption
For a solar flare occurring on 2010 November 3, we present observations using
several SDO/AIA extreme-ultraviolet (EUV) passbands of an erupting flux rope
followed by inflows sweeping into a current sheet region. The inflows are soon
followed by outflows appearing to originate from near the termination point of
the inflowing motion - an observation in line with standard magnetic
reconnection models. We measure average inflow plane-of-sky speeds to range
from ~150-690 km/s with the initial, high-temperature inflows being the
fastest. Using the inflow speeds and a range of Alfven speeds, we estimate the
Alfvenic Mach number which appears to decrease with time. We also provide
inflow and outflow times with respect to RHESSI count rates and find that the
fast, high-temperature inflows occur simultaneously with a peak in the RHESSI
thermal lightcurve. Five candidate inflow-outflow pairs are identified with no
more than a minute delay between detections. The inflow speeds of these pairs
are measured to be 10^2 km/s with outflow speeds ranging from 10^2-10^3 km/s -
indicating acceleration during the reconnection process. The fastest of these
outflows are in the form of apparently traveling density enhancements along the
legs of the loops rather than the loop apexes themselves. These flows could
either be accelerated plasma, shocks, or waves prompted by reconnection. The
measurements presented here show an order of magnitude difference between the
retraction speeds of the loops and the speed of the density enhancements within
the loops - presumably exiting the reconnection site.Comment: 31 pages, 13 figures, 1 table, Accepted to ApJ (expected publication
~July 2012
High temporal frequency measurements of greenhouse gas emissions from soils
Carbon dioxide (CO<sub>2</sub>), methane (CH<sub>4</sub>), and nitrous oxide (N<sub>2</sub>O)
are the most important anthropogenic greenhouse gases (GHGs). Variation in soil
moisture can be very dynamic, and it is one of the dominant factors
controlling the net exchange of these three GHGs. Although
technologies for high-frequency, precise measurements of CO<sub>2</sub> have been
available for years, methods for measuring soil fluxes of CH<sub>4</sub> and
N<sub>2</sub>O at high temporal frequency have been hampered by lack of appropriate
technology for in situ real-time measurements. A previously developed
automated chamber system for measuring CO<sub>2</sub> flux from soils was
configured to run in line with a new quantum cascade laser (QCLAS) instrument
that measures N<sub>2</sub>O and CH<sub>4</sub>. Here we present data from a forested
wetland in Maine and an agricultural field in North Dakota, which provided
examples of both net uptake and production for N<sub>2</sub>O and CH<sub>4</sub>. The
objective was to provide a range of conditions in which to run the new system
and to compare results to a traditional manual static-chamber method.
<br><br>
The high-precision and more-than-10-times-lower minimum detectable flux of
the QCLAS system, compared to the manual system, provided confidence in
measurements of small N<sub>2</sub>O uptake in the forested wetland. At the
agricultural field, the greatest difference between the automated and manual
sampling systems came from the effect of the relatively infrequent manual
sampling of the high spatial variation, or "hot spots", in GHG fluxes.
Hot spots greatly influenced the seasonal estimates, particularly for
N<sub>2</sub>O, over one 74-day alfalfa crop cycle. The high temporal frequency of
the automated system clearly characterized the transient response of all
three GHGs to precipitation and demonstrated a clear diel pattern related to
temperature for GHGs. A combination of high-frequency automated and
spatially distributed chambers would be ideal for characterizing hot
spots and "hot moments" of GHG fluxes
Complex Dynamic Flows in Solar Flare Sheet Structures
Observations of high-energy emission from solar flares often reveal the presence of large sheet-like structures, sometimes extending over a space comparable to the Sun's radius. Given that these structures are found between a departing coronal mass ejection and the post-eruption flare arcade, it is natural to associate the structure with a current sheet; though the relationship is unclear. Moreover, recent high-resolution observations have begun to reveal that the motions in this region are highly complex, including reconnection outflows, oscillations, and apparent wakes and eddies. We present a detailed first look at the complicated dynamics within this supra-arcade plasma, and consider implications for the interrelationship between the plasma and its embedded magnetic field
A hydrogen energy carrier. Volume 1: Summary
The production, technology, transportation, and implementation of hydrogen into the energy system are discussed along with the fossil fuel cycle, hydrogen fuel cycle, and the demands for energy. The cost of hydrogen production by coal gasification; electrolysis by nuclear energy, and solar energy are presented. The legal aspects of a hydrogen economy are also discussed
Posteruptive phenomena in coronal mass ejections and substorms: Indicators of a universal process?
[1] We examine phenomena associated with eruptions in the two different regimes of the solar corona and the terrestrial magnetosphere. We find striking similarities between the speeds of shrinking magnetic field lines in the corona and dipolarization fronts traversing the magnetosphere. We also examine the similarities between supra-arcade downflows observed during solar flares and bursty bulk flows seen in the magnetotail and find that these phenomena have remarkably similar speeds, velocity profiles, and size scales. Thus we show manifest similarities in the magnetic reconfiguration in response to the ejection of coronal mass ejections in the corona and the ejection of plasmoids in the magnetotail. The subsequent return of loops to a quasi-potential state in the corona and field dipolarization in the magnetotail are physical analogs and trigger similar phenomena such as downflows, which provides key insights into the underlying drivers of the plasma dynamics
GHRS and ORFEUS-II Observations of the Highly Ionized Interstellar Medium Toward ESO141-055
We present Goddard High Resolution Spectrograph and ORFEUS-II measurements of
Si IV, CIV, N V, and O VI absorption in the interstellar medium of the Galactic
disk and halo toward the nucleus of the Seyfert galaxy ESO141-055. The high
ionization absorption is strong, with line strengths consistent with the
spectral signature expected for hot (log T = 5-6) collisionally ionized gas in
either a ``Galactic fountain'' or an inhomogeneous medium containing a mixture
of conductive interfaces and turbulent mixing layers. The total O VI column
density of log N ~ 15 suggests that the scale height of O VI is large (>3 kpc)
in this direction. Comparison of the high ion column densities with
measurements for other sight lines indicates that the highly ionized gas
distribution is patchy. The amount of O VI perpendicular to the Galactic plane
varies by at least a factor of ~4 among the complete halo sight lines thus far
studied. In addition to the high ion absorption, lines of low ionization
species are also present in the spectra. With the possible exception of Ar I,
which may have a lower than expected abundance resulting from partial
photoionization of gas along the sight line, the absorption strengths are
typical of those expected for the warm, neutral interstellar medium. The sight
line intercepts a cold molecular cloud with log N(H2) ~ 19. The cloud has an
identifiable counterpart in IRAS 100-micron emission maps of this region of the
sky. We detect a Ly-alpha absorber associated with ESO141-055 at z = 0.03492.
This study presents an enticing glimpse into the interstellar and intergalactic
absorption patterns that will be observed at high spectral resolution by the
Far Ultraviolet Spectroscopic Explorer.Comment: 24 pages + 8 figures, uses aaspp4.sty. Accepted for publication in
Ap
Spatial and temporal filtering of a 10-W Nd:YAG laser with a Fabry-Perot ring-cavity premode cleaner
We report on the use of a fixed-spacer Fabry–Perot ring cavity to filter spatially and temporally a 10-W laser-diode-pumped Nd:YAG master-oscillator power amplifier. The spatial filtering leads to a 7.6-W TEMinfinity beam with 0.1% higher-order transverse mode content. The temporal filtering reduces the relative power fluctuations at 10 MHz to 2.8 x 10^-/sqrtHz, which is 1 dB above the shot-noise limit for 50 mA of detected photocurrent
Second Harmonic Coherent Driving of a Spin Qubit in a Si/SiGe Quantum Dot
We demonstrate coherent driving of a single electron spin using second
harmonic excitation in a Si/SiGe quantum dot. Our estimates suggest that the
anharmonic dot confining potential combined with a gradient in the transverse
magnetic field dominates the second harmonic response. As expected, the Rabi
frequency depends quadratically on the driving amplitude and the periodicity
with respect to the phase of the drive is twice that of the fundamental
harmonic. The maximum Rabi frequency observed for the second harmonic is just a
factor of two lower than that achieved for the first harmonic when driving at
the same power. Combined with the lower demands on microwave circuitry when
operating at half the qubit frequency, these observations indicate that second
harmonic driving can be a useful technique for future quantum computation
architectures.Comment: 9 pages, 9 figure
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