8,330 research outputs found
Linear Sensitivity of Helioseismic Travel Times to Local Flows
Time-distance helioseismology is a technique for measuring the time for waves
to travel from one point on the solar surface to another. These wave travel
times are affected by advection by subsurface flows. Inferences of plasma flows
based on observed travel times depend critically on the ability to accurately
model the effects of subsurface flows on time-distance measurements. We present
a Born approximation based computation of the sensitivity of time distance
travel times to weak, steady, inhomogeneous subsurface flows. Three sensitivity
functions are obtained, one for each component of the 3D vector flow. We show
that the depth sensitivity of travel times to horizontally uniform flows is
given approximately by the kinetic energy density of the oscillation modes
which contribute to the travel times. For flows with strong depth dependence,
the Born approximation can give substantially different results than the ray
approximation.Comment: 6 pages, 6 figure
Spatially resolved vertical vorticity in solar supergranulation using helioseismology and local correlation tracking
Flow vorticity is a fundamental property of turbulent convection in rotating
systems. Solar supergranules exhibit a preferred sense of rotation, which
depends on the hemisphere. This is due to the Coriolis force acting on the
diverging horizontal flows. We aim to spatially resolve the vertical flow
vorticity of the average supergranule at different latitudes, both for outflow
and inflow regions. To measure the vertical vorticity, we use two independent
techniques: time-distance helioseismology (TD) and local correlation tracking
of granules in intensity images (LCT) using data from the Helioseismic and
Magnetic Imager (HMI) onboard the Solar Dynamics Observatory (SDO). Both maps
are corrected for center-to-limb systematic errors. We find that 8-h TD and LCT
maps of vertical vorticity are highly correlated at large spatial scales.
Associated with the average supergranule outflow, we find tangential (vortical)
flows that reach about 10 m/s in the clockwise direction at 40{\deg} latitude.
In average inflow regions, the tangential flow reaches the same magnitude, but
in the anti-clockwise direction. These tangential velocities are much smaller
than the radial (diverging) flow component (300 m/s for the average outflow and
200 m/s for the average inflow). The results for TD and LCT as measured from
HMI are in excellent agreement for latitudes between 60{\deg} and 60{\deg}.
From HMI LCT, we measure the vorticity peak of the average supergranule to have
a full width at half maximum of about 13 Mm for outflows and 8 Mm for inflows.
This is larger than the spatial resolution of the LCT measurements (about 3
Mm). On the other hand, the vorticity peak in outflows is about half the value
measured at inflows (e.g. 4/(10^6 s) clockwise compared to 8/(10^6 s)
anti-clockwise at 40{\deg} latitude). Results from MDI/SOHO obtained in 2010
are biased compared to the HMI/SDO results for the same period.Comment: 12 pages, 13 figures (plus appendix), accepted for publication in A&
Equine digital tendons show breedâspecific differences in their mechanical properties that may relate to athletic ability and predisposition to injury
Background Throughout the ages, human subjects have selected horse breeds for their locomotor capacities. Concurrently, tissue properties may have diversified because of specific requirements of different disciplines. Objectives The aim of this study was to compare the biomechanical properties of tendons with different functions between equine breeds traditionally selected for racing or sport. Study design This study used ex vivo tendons and compared the mechanical properties of the common digital extensor tendon (CDET) and superficial digital flexor tendon (SDFT) between racehorses (Thoroughbred [TB]) and sports horses (Friesian Horse [FH], Warmblood [WB]). Methods The SDFT and CDET of FH (n = 12), WBs (n = 12) and TBs (n = 8) aged 3-12 years were harvested. The cross sectional area (cm(2)), maximal load (N), ultimate strain (%), ultimate stress (MPa) and elastic modulus (MPa) were determined and tested for significant differences between the breeds (P<0.05). Results The SDFT from WB horses had a significantly lower elastic modulus than TB horses and failed at a higher strain and load than both FHs and TBs. The mechanical properties of the CDET did not differ between breeds. In agreement with previous studies, the CDET failed at a higher stress and had a higher elastic modulus than the SDFT and, for the WB group of horses only, failed at a significantly lower strain. Interestingly, the mode of failure differed between breeds, particularly with respect to the FHs. Main limitations The exercise history of horses used in this study was unknown and the age-range was relatively large; both these factors may have influenced the absolute properties reported in this study. Conclusions This study shows for the first time that mechanical properties of the SDFT differ between breeds. These properties are likely to be related to selection for high-speed vs. an extravagant elastic gait and may be an important indicator of performance ability. The is available in Spanish - see Supporting Informatio
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Possible immunoenhancement of persistent viremia by feline leukemia virus envelope glycoprotein vaccines in challenge-exposure situations where whole inactivated virus vaccines were protective.
Kittens immunized with purified native FeLV-gp70 or -gp85 envelope proteins developed ELISA, but not virus neutralizing, antibodies in their serum to both whole FeLV and FeLV-gp70. Kittens vaccinated with envelope proteins and infected with feline sarcoma virus (FeSV) developed smaller tumors than nonvaccinates, but a greater incidence of persistent retroviremia. Similarly, FeLV-gp70 and -gp85 vaccinated kittens were more apt to become persistently retroviremic following virulent FeLV challenge exposure than nonvaccinates. Kittens vaccinated with inactivated whole FeLV developed smaller tumors after FeSV inoculation and had a lower incidence of persistent retroviremia than nonvaccinates. The protective effect of inactivated whole FeLV vaccine against persistent retroviremia was also seen with FeLV challenge-exposed cats. Protection afforded by inactivated whole FeLV vaccine was not associated with virus neutralizing antibodies, although ELISA antibodies to both whole FeLV and FeLV-gp70 were induced by vaccination
Time-distance helioseismology: Sensitivity of f-mode travel times to flows
Time-distance helioseismology has shown that f-mode travel times contain
information about horizontal flows in the Sun. The purpose of this study is to
provide a simple interpretation of these travel times. We study the interaction
of surface-gravity waves with horizontal flows in an incompressible,
plane-parallel solar atmosphere. We show that for uniform flows less than
roughly 250 m s, the travel-time shifts are linear in the flow
amplitude. For stronger flows, perturbation theory up to third order is needed
to model waveforms. The case of small-amplitude spatially-varying flows is
treated using the first-order Born approximation. We derive two-dimensional
Fr\'{e}chet kernels that give the sensitivity of travel-time shifts to local
flows. We show that the effect of flows on travel times depends on wave damping
and on the direction from which the observations are made. The main physical
effect is the advection of the waves by the flow rather than the advection of
wave sources or the effect of flows on wave damping. We compare the
two-dimensional sensitivity kernels with simplified three-dimensional kernels
that only account for wave advection and assume a vertical line of sight. We
find that the three-dimensional f-mode kernels approximately separate in the
horizontal and vertical coordinates, with the horizontal variations given by
the simplified two-dimensional kernels. This consistency between quite
different models gives us confidence in the usefulness of these kernels for
interpreting quiet-Sun observations.Comment: 34 pages, accepted to Astrophysical Journa
K-Rational D-Brane Crystals
In this paper the problem of constructing spacetime from string theory is
addressed in the context of D-brane physics. It is suggested that the knowledge
of discrete configurations of D-branes is sufficient to reconstruct the motivic
building blocks of certain Calabi-Yau varieties. The collections of D-branes
involved have algebraic base points, leading to the notion of K-arithmetic
D-crystals for algebraic number fields K. This idea can be tested for D0-branes
in the framework of toroidal compactifications via the conjectures of Birch and
Swinnerton-Dyer. For the special class of D0-crystals of Heegner type these
conjectures can be interpreted as formulae that relate the canonical Neron-Tate
height of the base points of the D-crystals to special values of the motivic
L-function at the central point. In simple cases the knowledge of the
D-crystals of Heegner type suffices to uniquely determine the geometry.Comment: 36 page
Chester Rapkin: Planner, Teacher, Scholar
The seminal thinkers of the profession are now largely historical figures, few \u27heroes\u27 have emerged to replace them, Michael P. Brooks recently wrote (Brooks, 1988). Brooks is unduly alarmist. Significant figures like Daniel Burnham and Rexford Tugwell have their counterparts today. But these contemporary planners are different. They do not espouse exaggerated visions nor call brashly for revolutionary changes. American life also is different. Big cities are no longer novel nor is the economy emerging from a major depression. The country now is dealing with seemingly intransigent issues like the underclass and runaway metropolitan growth and adjusting to major industrial restructuring
The Unsheltered Woman: Definition and Needs
One-third of the nation has a housing problem. Twenty-three millions households are ill-housed. They are a diverse group - the elderly, families with children and single people of all races. Most significantly, they tend to be women. More than 40 percent of the group - or 10 million - are female householders. Females head about 27 percent of all American households today; yet, they are disproportionately represented among those experiencing housing problems. In fact, numerically, they are the largest subgroup of the poorly sheltered population
Design, Process and Institutions
Although many Anglo-American social historians would like to believe that they have invented planning history, their assumption is incorrect. The field has deeper roots. Its earliest practitioners - architects, archaeologists and classicists - engaged in questions of urban design, the origin of cities and urbanization
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