2,915 research outputs found
Distinguishing Solar Flare Types by Differences in Reconnection Regions
Observations show that magnetic reconnection and its slow shocks occur in
solar flares. The basic magnetic structures are similar for long duration event
(LDE) flares and faster compact impulsive (CI) flares, but the former require
less non-thermal electrons than the latter. Slow shocks can produce the
required non-thermal electron spectrum for CI flares by Fermi acceleration if
electrons are injected with large enough energies to resonate with scattering
waves. The dissipation region may provide the injection electrons, so the
overall number of non-thermal electrons reaching the footpoints would depend on
the size of the dissipation region and its distance from the chromosphere. In
this picture, the LDE flares have converging inflows toward a dissipation
region that spans a smaller overall length fraction than for CI flares. Bright
loop-top X-ray spots in some CI flares can be attributed to particle trapping
at fast shocks in the downstream flow, the presence of which is determined by
the angle of the inflow field and velocity to the slow shocks.Comment: 15 pages TeX and 2 .eps figures, accepted to Ap.J.Let
Molecular gyroscopes and biological effects of weak ELF magnetic fields
Extremely-low-frequency magnetic fields are known to affect biological
systems. In many cases, biological effects display `windows' in biologically
effective parameters of the magnetic fields: most dramatic is the fact that
relatively intense magnetic fields sometimes do not cause appreciable effect,
while smaller fields of the order of 10--100 T do. Linear resonant
physical processes do not explain frequency windows in this case. Amplitude
window phenomena suggest a nonlinear physical mechanism. Such a nonlinear
mechanism has been proposed recently to explain those `windows'. It considers
quantum-interference effects on protein-bound substrate ions. Magnetic fields
cause an interference of ion quantum states and change the probability of
ion-protein dissociation. This ion-interference mechanism predicts specific
magnetic-field frequency and amplitude windows within which biological effects
occur. It agrees with a lot of experiments. However, according to the
mechanism, the lifetime of ion quantum states within a protein
cavity should be of unrealistic value, more than 0.01 s for frequency band
10--100 Hz. In this paper, a biophysical mechanism has been proposed that (i)
retains the attractive features of the ion interference mechanism and (ii) uses
the principles of gyroscopic motion and removes the necessity to postulate
large lifetimes. The mechanism considers dynamics of the density matrix of the
molecular groups, which are attached to the walls of protein cavities by two
covalent bonds, i.e., molecular gyroscopes. Numerical computations have shown
almost free rotations of the molecular gyros. The relaxation time due to van
der Waals forces was about 0.01 s for the cavity size of 28 angstr\"{o}ms.Comment: 10 pages, 7 figure
Cross-Industry Performance Modeling: Toward Cooperative Analysis
One of the current unsolved problems in human factors is the difficulty in acquiring information from lessons learned and data collected among human performance analysts in different domains. There are several common concerns and generally accepted issues of importance for human factors, psychology and industry analysts of performance and safety. Among these are the need to incorporate lessons learned in design, to carefully consider implementation of new designs and automation, and the need to reduce human performance-based contributions to risk. In spite of shared concerns, there are several road blocks to widespread sharing of data and lessons learned from operating experience and simulation, including the fact that very few publicly accessible data bases exist(Gertman & Blackman, 1994, and Kirwan, 1997). There is a need to draw together analysts and analytic methodologies to comprise a centralized source of data with sufficient detail to be meaningful while ensuring source anonymity. We propose that a generic source of performance data and a multi-domain data store may provide the first steps toward cooperative performance modeling and analysis across industries
Accretion Disks and Dynamos: Toward a Unified Mean Field Theory
Conversion of gravitational energy into radiation in accretion discs and the
origin of large scale magnetic fields in astrophysical rotators have often been
distinct topics of research. In semi-analytic work on both problems it has been
useful to presume large scale symmetries, necessarily resulting in mean field
theories. MHD turbulence makes the underlying systems locally asymmetric and
nonlinear. Synergy between theory and simulations should aim for the
development of practical mean field models that capture essential physics and
can be used for observational modeling. Mean field dynamo (MFD) theory and
alpha-viscosity accretion theory exemplify such ongoing pursuits. 21st century
MFD theory has more nonlinear predictive power compared to 20th century MFD
theory, whereas accretion theory is still in a 20th century state. In fact,
insights from MFD theory are applicable to accretion theory and the two are
artificially separated pieces of what should be a single theory. I discuss
pieces of progress that provide clues toward a unified theory. A key concept is
that large scale magnetic fields can be sustained via local or global magnetic
helicity fluxes or via relaxation of small scale magnetic fluctuations, without
the kinetic helicity driver of 20th century textbooks. These concepts may help
explain the formation of large scale fields that supply non-local angular
momentum transport via coronae and jets in a unified theory of accretion and
dynamos. In diagnosing the role of helicities and helicity fluxes in disk
simulations, each disk hemisphere should be studied separately to avoid being
misled by cancelation that occurs as a result of reflection asymmetry. The
fraction of helical field energy in disks is expected to be small compared to
the total field in each hemisphere as a result of shear, but can still be
essential for large scale dynamo action.Comment: For the Proceedings of the Third International Conference and
Advanced School "Turbulent Mixing and Beyond," TMB-2011 held on 21 - 28
August 2011 at the Abdus Salam International Centre for Theoretical Physics,
Trieste, http://users.ictp.it/~tmb/index2011.html Italy, To Appear in Physica
Scripta (corrected small items to match version in print
Electron-Positron Jets from a Critically Magnetized Black Hole
The curved spacetime surrounding a rotating black hole dramatically alters
the structure of nearby electromagnetic fields. The Wald field which is an
asymptotically uniform magnetic field aligned with the angular momentum of the
hole provides a convenient starting point to analyze the effects of radiative
corrections on electrodynamics in curved spacetime. Since the curvature of the
spacetime is small on the scale of the electron's Compton wavelength, the tools
of quantum field theory in flat spacetime are reliable and show that a rotating
black hole immersed in a magnetic field approaching the quantum critical value
of ~G cm is unstable. Specifically, a maximally rotating
three-solar-mass black hole immersed in a magnetic field of ~G would be a copious producer of electron-positron pairs with a
luminosity of erg s.Comment: 10 pages, 6 figures, submitted to Phys. Rev.
Finite-Size Scaling in the Energy-Entropy Plane for the 2D +- J Ising Spin Glass
For square lattices with the 2D Ising spin glass with
+1 and -1 bonds is found to have a strong correlation between the energy and
the entropy of its ground states. A fit to the data gives the result that each
additional broken bond in the ground state of a particular sample of random
bonds increases the ground state degeneracy by approximately a factor of 10/3.
For (where is the fraction of negative bonds), over this range of
, the characteristic entropy defined by the energy-entropy correlation
scales with size as . Anomalous scaling is not found for the
characteristic energy, which essentially scales as . When , a
crossover to scaling of the entropy is seen near . The results
found here suggest a natural mechanism for the unusual behavior of the low
temperature specific heat of this model, and illustrate the dangers of
extrapolating from small .Comment: 9 pages, two-column format; to appear in J. Statistical Physic
On the Stability and the Approximation of Branching Distribution Flows, with Applications to Nonlinear Multiple Target Filtering
We analyse the exponential stability properties of a class of measure-valued
equations arising in nonlinear multi-target filtering problems. We also prove
the uniform convergence properties w.r.t. the time parameter of a rather
general class of stochastic filtering algorithms, including sequential Monte
Carlo type models and mean eld particle interpretation models. We illustrate
these results in the context of the Bernoulli and the Probability Hypothesis
Density filter, yielding what seems to be the first results of this kind in
this subject
Absence of a metallic phase in random-bond Ising models in two dimensions: applications to disordered superconductors and paired quantum Hall states
When the two-dimensional random-bond Ising model is represented as a
noninteracting fermion problem, it has the same symmetries as an ensemble of
random matrices known as class D. A nonlinear sigma model analysis of the
latter in two dimensions has previously led to the prediction of a metallic
phase, in which the fermion eigenstates at zero energy are extended. In this
paper we argue that such behavior cannot occur in the random-bond Ising model,
by showing that the Ising spin correlations in the metallic phase violate the
bound on such correlations that results from the reality of the Ising
couplings. Some types of disorder in spinless or spin-polarized p-wave
superconductors and paired fractional quantum Hall states allow a mapping onto
an Ising model with real but correlated bonds, and hence a metallic phase is
not possible there either. It is further argued that vortex disorder, which is
generic in the fractional quantum Hall applications, destroys the ordered or
weak-pairing phase, in which nonabelian statistics is obtained in the pure
case.Comment: 13 pages; largely independent of cond-mat/0007254; V. 2: as publishe
Regional variation in primary care improvement strategies and policy: case studies that consider qualitative contextual data for performance measurement in three Canadian provinces
This is the final version. Available on open access from BMJ Publishing Group via the DOI in this recordOBJECTIVE: To explore regional primary care improvement strategies that are potentially determinants of primary care performance. DESIGN: Multiple comparative embedded case study. SETTING: Three regions in Canada: Fraser East, British Columbia; Eastern Ontario Health Unit, Ontario; Central Zone, Nova Scotia. DATA SOURCES: (1) In-depth interviews with purposively selected key informants (eg, primary care decision-makers, physician leads, regulatory agencies) and focus groups with patients and clinicians (n=68 participants) and (2) published and grey literature (n=205 documents). OUTCOME MEASURES: Variations in spread and uptake of primary care improvement strategies across the three study regions. NVivo (V.11) was used to manage data and perform content analysis to identify categories within and across cases. The coding structure was developed by researchers through iterative collaboration, using inductive and deductive processes. RESULTS: Six overarching primary care improvement strategies, differing in focus and spread, were implemented across the three study regions: interprofessional team-based approaches, provider skill mix expansion, physician groups and networks, information systems, remuneration and performance measurement and reporting infrastructure. CONCLUSION: The addition of information on regional improvement strategies to primary care performance reports could add important contextual insights into primary care performance results. This could help identify possible drivers of reported performance outcomes and levers for change in practice, regional and system-level settings.Canadian Institutes of Health ResearchMichael Smith Foundation for Health Researc
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