8,368 research outputs found
Comparison of acoustic travel-time measurement of solar meridional circulation from SDO/HMI and SOHO/MDI
Time-distance helioseismology is one of the primary tools for studying the
solar meridional circulation. However, travel-time measurements of the
subsurface meridional flow suffer from a variety of systematic errors, such as
a center-to-limb variation and an offset due to the P-angle uncertainty of
solar images. Here we apply the time-distance technique to contemporaneous
medium-degree Dopplergrams produced by SOHO/MDI and SDO/HMI to obtain the
travel-time difference caused by meridional circulation throughout the solar
convection zone. The P-angle offset in MDI images is measured by
cross-correlating MDI and HMI images. The travel-time measurements in the
south-north and east-west directions are averaged over the same observation
period for the two data sets and then compared to examine the consistency of
MDI and HMI travel times after correcting the systematic errors.
The offsets in the south-north travel-time difference from MDI data induced
by the P-angle error gradually diminish with increasing travel distance.
However, these offsets become noisy for travel distances corresponding to waves
that reach the base of the convection zone. This suggests that a careful
treatment of the P-angle problem is required when studying a deep meridional
flow. After correcting the P-angle and the removal of the center-to-limb
effect, the travel-time measurements from MDI and HMI are consistent within the
error bars for meridional circulation covering the entire convection zone. The
fluctuations observed in both data sets are highly correlated and thus indicate
their solar origin rather than an instrumental origin. Although our results
demonstrate that the ad hoc correction is capable of reducing the wide
discrepancy in the travel-time measurements from MDI and HMI, we cannot exclude
the possibility that there exist other systematic effects acting on the two
data sets in the same way.Comment: accepted for publication in A&
Solar meridional circulation from twenty-one years of SOHO/MDI and SDO/HMI observations: Helioseismic travel times and forward modeling in the ray approximation
The south-north travel-time differences are measured by applying
time-distance helioseismology to the MDI and HMI medium-degree Dopplergrams
covering May 1996-April 2017. Our data analysis corrects for several sources of
systematic effects: P-angle error, surface magnetic field effects, and
center-to-limb variations. An interpretation of the travel-time measurements is
obtained using a forward-modeling approach in the ray approximation. The
travel-time differences are similar in the southern hemisphere for cycles 23
and 24. However, they differ in the northern hemisphere between cycles 23 and
24. Except for cycle 24's northern hemisphere, the measurements favor a
single-cell meridional circulation model where the poleward flows persist down
to 0.8 , accompanied by local inflows toward the activity belts
in the near-surface layers. Cycle 24's northern hemisphere is anomalous:
travel-time differences are significantly smaller when travel distances are
greater than 20. This asymmetry between northern and southern
hemispheres during cycle 24 was not present in previous measurements (e.g.,
Rajaguru & Antia 2015), which assumed a different P-angle error correction
where south-north travel-time differences are shifted to zero at the equator
for all travel distances. In our measurements, the travel-time differences at
the equator are zero for travel distances less than 30, but they
do not vanish for larger travel distances. This equatorial offset for large
travel distances need not be interpreted as a deep cross-equator flow; it could
be due to the presence of asymmetrical local flows at the surface near the end
points of the acoustic ray paths.Comment: accepted for publication in A&
Three-dimensional shelf circulation along an eastern ocean boundary
A linear, three-dimensional, continuously stratified model is used to study wind-driven ocean circulation near an eastern coast in the presence of a continental shelf. A simplifying assumption is that the alongshore flow field is in geostrophic balance. This assumption allows steady solutions to be obtained numerically with a very efficient scheme. As a result, it is possible to find solutions for a wide variety of model parameters and shelf profiles.A band of equatorward wind forces the ocean, and the resulting solutions have many features in common with observations at eastern boundaries. They all have a surface equatorward jet, but do not always have a coastal undercurrent. When the shelf depth is sufficiently shallow or vertical mixing is sufficiently strong, the speed of the undercurrent, if it exists, is usually weak; in that case, only when there is positive wind curl near the coast does its speed reach commonly observed values. Solutions are sensitive to the choice of bottom topographic profile. A general result is that the continental shelf always acts to strengthen the equatorward jet and to weaken or eliminate the undercurrent. The reason is that the shelf induces an equatorward barotropic component to the shelf currents, a component that is not present in flat-bottom solutions
Simulation and analysis of in vitro DNA evolution
We study theoretically the in vitro evolution of a DNA sequence by binding to
a transcription factor. Using a simple model of protein-DNA binding and
available binding constants for the Mnt protein, we perform large-scale,
realistic simulations of evolution starting from a single DNA sequence. We
identify different parameter regimes characterized by distinct evolutionary
behaviors. For each regime we find analytical estimates which agree well with
simulation results. For small population sizes, the DNA evolutional path is a
random walk on a smooth landscape. While for large population sizes, the
evolution dynamics can be well described by a mean-field theory. We also study
how the details of the DNA-protein interaction affect the evolution.Comment: 11 pages, 11 figures. Submitted to PNA
Two-dimensional protein crystallization via metal-ion coordination by naturally occurring surface histidines
A powerful and potentially general approach to the targeting and crystallization of proteins on lipid interfaces through coordination of surface histidine residues to lipid-chelated divalent metal ions is presented. This approach, which should be applicable to the crystallization of a wide range of naturally occurring or engineered proteins, is illustrated here by the crystallization of streptavidin on a monolayer of an iminodiacetate-Cu(II) lipid spread at the air-water interface. This method allows control of the protein orientation at interfaces, which is significant for the facile production of highly ordered protein arrays and for electron density mapping in structural analysis of two-dimensional crystals. Binding of native streptavidin to the iminodiacetate-Cu lipids occurs via His-87, located on the protein surface near the biotin binding pocket. The two-dimensional streptavidin crystals show a previously undescribed microscopic shape that differs from that of crystals formed beneath biotinylated lipids
AUSSAT battery life test program
AUSSAT Pty. Ltd., the Australian National Satellite organization, has contracted with the Hughes Aircraft Company (HAC) for the construction of 3 satellites based on the now familiar HS-376 product line. As part of the AUSSAT contract, HAC is conducting an extensive NiCd battery life test program. The life test program, objectives and test results to date are described. Particular emphasis is given to the evaluation of the FS2117 separator as a future replacement for the Pellon 2505 separator of which only a very limited quantity remains
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