2,824 research outputs found
Vortices and turbulence in trapped atomic condensates
After over a decade of experiments generating and studying the physics of
quantized vortices in atomic gas Bose-Einstein condensates, research is
beginning to focus on the roles of vortices in quantum turbulence, as well as
other measures of quantum turbulence in atomic condensates. Such research
directions have the potential to uncover new insights into quantum turbulence,
vortices and superfluidity, and also explore the similarities and differences
between quantum and classical turbulence in entirely new settings. Here we
present a critical assessment of theoretical and experimental studies in this
emerging field of quantum turbulence in atomic condensates
Topological Change in Mean Convex Mean Curvature Flow
Consider the mean curvature flow of an (n+1)-dimensional, compact, mean
convex region in Euclidean space (or, if n<7, in a Riemannian manifold). We
prove that elements of the m-th homotopy group of the complementary region can
die only if there is a shrinking S^k x R^(n-k) singularity for some k less than
or equal to m. We also prove that for each m from 1 to n, there is a nonempty
open set of compact, mean convex regions K in R^(n+1) with smooth boundary for
which the resulting mean curvature flow has a shrinking S^m x R^(n-m)
singularity.Comment: 19 pages. This version includes a new section proving that certain
kinds of mean curvature flow singularities persist under arbitrary small
perturbations of the initial surface. Newest update (Oct 2013) fixes some
bibliographic reference
Magnetic field-induced soft mode in spin-gapped high-Tc superconductors
We present an explanation of the dynamical in-gap spin mode in LSCO induced
by an applied magnetic field H as recently observed by J. Chang et al. Our
model consists of a phenomenological spin-only Hamiltonian, and the softening
of the spin mode is caused by vortex pinning of dynamical stripe fluctuations
which we model by a local ordering of the exchange interactions. The spin gap
vanishes experimentally around H=7T which in our scenario corresponds to the
field required for overlapping vortex regions.Comment: 4 pages, 3 fig
Expression profiling of snoRNAs in normal hematopoiesis and AML
Key Points
A subset of snoRNAs is expressed in a developmental- and lineage-specific manner during human hematopoiesis. Neither host gene expression nor alternative splicing accounted for the observed differential expression of snoRNAs in a subset of AML.</jats:p
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Humanism in the Age of COVID-19: Renewing Focus on Communication and Compassion
The global COVID-19 pandemic has become one of the largest clinical and operational challenges faced by emergency medicine, and our EDs continue to see increased volumes of infected patients, many of whom are not only ill, but acutely aware and fearful of their circumstances and potential mortality. Given this, there may be no more important time to focus on staff-patient communication and expression of compassion.However, many of the techniques usually employed by emergency clinicians to provide comfort to patients and their families are made more challenging or impossible by the current circumstances. Geriatric ED patients, who are at increased risk of severe disease, are particularly vulnerable to the effects of isolation.Despite many challenges, emergency clinicians have at their disposal a myriad of tools that can still be used to express compassion and empathy to their patients. Placing emphasis on using these techniques to maximize humanism in the care of COVID-19 patients during this crisis has the potential to bring improvements to ED patient care well after this pandemic has passed
NuSTAR detection of X-ray heating events in the quiet Sun
The explanation of the coronal heating problem potentially lies in the existence of nanoflares, numerous small-scale heating events occurring across the whole solar disk. In this Letter, we present the first imaging spectroscopy X-ray observations of three quiet Sun flares during the Nuclear Spectroscopic Telescope ARray (NuSTAR) solar campaigns on 2016 July 26 and 2017 March 21, concurrent with the Solar Dynamics Observatory/Atmospheric Imaging Assembly (SDO/AIA) observations. Two of the three events showed time lags of a few minutes between peak X-ray and extreme ultraviolet emissions. Isothermal fits with rather low temperatures in the range 3.2â4.1 MK and emission measures of (0.6â15) Ă 1044 cmâ3 describe their spectra well, resulting in thermal energies in the range (2â6) Ă 1026 erg. NuSTAR spectra did not show any signs of a nonthermal or higher temperature component. However, as the estimated upper limits of (hidden) nonthermal energy are comparable to the thermal energy estimates, the lack of a nonthermal component in the observed spectra is not a constraining result. The estimated Geostationary Operational Environmental Satellite (GOES) classes from the fitted values of temperature and emission measure fall between 1/1000 and 1/100 A class level, making them eight orders of magnitude fainter in soft X-ray flux than the largest solar flares
Mass Models and Sunyaev-Zeldovich Effect Predictions for a Flux Limited Sample of 22 Nearby X-Ray Clusters
We define a 90% complete, volume-limited sample of 31 z<0.1 x-ray clusters
and present a systematic analysis of public ROSAT PSPC data on 22 of these
objects. Our efforts are undertaken in support of the Penn/OVRO SZE survey, and
to this end we present predictions for the inverse Compton optical depth
towards all 22 of these clusters. We have performed detailed Monte Carlo
simulations to understand the effects of the cluster profile uncertainties on
the SZE predictions given the OVRO 5.5-meter telescope beam and switching
patterns; we find that the profile uncertainties are one of the least
significant components of our error budget for SZE-based distance measurements.
We also present baryonic masses and baryon mass fractions derived under the
assumption of hydrostatic equilibrium for these 22 clusters. The mean baryonic
mass fraction within R_500 \sim 500 h^-1 kpc is (7.02 \pm 0.28) x 10^-2 h^-3/2,
or (19.8 \pm 0.8) x 10^-2 for h=0.5. We confirm the Allen et al. (1993) claim
of an excess absorbing column density towards Abell 478, but do not find
similar anomalies in the other 21 clusters in our sample. We also find some
evidence for an excess of soft counts in the ROSAT PSPC data.
A measurement of H_o using these models and OVRO SZE determinations will be
presented in a second paper.Comment: 51 pages, 6 figures included in text. Added comparison of different
cosmologies; accepted for publication in Ap
Microflare Heating of a Solar Active Region Observed with NuSTAR, Hinode/XRT, and SDO/AIA
NuSTAR is a highly sensitive focusing hard X-ray (HXR) telescope and has
observed several small microflares in its initial solar pointings. In this
paper, we present the first joint observation of a microflare with NuSTAR and
Hinode/XRT on 2015 April 29 at ~11:29 UT. This microflare shows heating of
material to several million Kelvin, observed in Soft X-rays (SXRs) with
Hinode/XRT, and was faintly visible in Extreme Ultraviolet (EUV) with SDO/AIA.
For three of the four NuSTAR observations of this region (pre-, decay, and post
phases) the spectrum is well fitted by a single thermal model of 3.2-3.5 MK,
but the spectrum during the impulsive phase shows additional emission up to 10
MK, emission equivalent to A0.1 GOES class. We recover the differential
emission measure (DEM) using SDO/AIA, Hinode/XRT, and NuSTAR, giving
unprecedented coverage in temperature. We find the pre-flare DEM peaks at ~3 MK
and falls off sharply by 5 MK; but during the microflare's impulsive phase the
emission above 3 MK is brighter and extends to 10 MK, giving a heating rate of
about erg s. As the NuSTAR spectrum is purely
thermal we determined upper-limits on the possible non-thermal bremsstrahlung
emission. We find that for the accelerated electrons to be the source of the
heating requires a power-law spectrum of with a low energy
cut-off keV. In summary, this first NuSTAR microflare
strongly resembles much more powerful flares.Comment: Accepted for publication in ApJ. 14 pages with 12 figures and 1 tabl
Fast imaging of filaments in the X-point region of Alcator C-Mod
A rich variety of field-aligned fluctuations has been revealed using fast imaging of Dαemission from Alcator C-Mod's lower X-point region. Field-aligned filamentary fluctuations are observed along the inner divertor leg, within the Private-Flux-Zone (PFZ), in the Scrape-Off Layer (SOL) outside the outer divertor leg, and, under some conditions, at or above the X-point. The locations and dynamics of the filaments in these regions are strikingly complex in C-Mod. Changes in the filamentsâ generation appear to be ordered by plasma density and magnetic configuration. Filaments are not observed for plasmas with n/nGreenwaldâČ 0.12 nor are they observed in Upper Single Null configurations. In a Lower Single Null with 0.12 âČ n/nGreenwald âČ 0.45 and BxâB directed down, filaments typically move up the inner divertor leg toward the X-point. Reversing the field direction results in the appearance of filaments outside of the outer divertor leg. With the divertor targets âdetachedâ, filaments inside the LCFS are seen. These studies were motivated by observations of filaments in the X-point and PFZ regions in MAST, and comparisons with those observations are made. Keywords: Alcator C-Mod; Turbulence; Divertor; X-point; Filament
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