687 research outputs found
Do quasi-regular structures really exist in the solar photosphere? I. Observational evidence
Two series of solar-granulation images -- the La Palma series of 5 June 1993
and the SOHO MDI series of 17--18 January 1997 -- are analysed both
qualitatively and quantitatively. New evidence is presented for the existence
of long-lived, quasi-regular structures (first reported by Getling and Brandt
(2002)), which no longer appear unusual in images averaged over 1--2-h time
intervals. Such structures appear as families of light and dark concentric
rings or families of light and dark parallel strips (``ridges'' and
``trenches'' in the brightness distributions). In some cases, rings are
combined with radial ``spokes'' and can thus form ``web'' patterns. The
characteristic width of a ridge or trench is somewhat larger than the typical
size of granules. Running-average movies constructed from the series of images
are used to seek such structures. An algorithm is developed to obtain, for
automatically selected centres, the radial distributions of the azimuthally
averaged intensity, which highlight the concentric-ring patterns. We also
present a time-averaged granulation image processed with a software package
intended for the detection of geological structures in aerospace images. A
technique of running-average-based correlations between the brightness
variations at various points of the granular field is developed and indications
are found for a dynamical link between the emergence and sinking of hot and
cool parcels of the solar plasma. In particular, such a correlation analysis
confirms our suggestion that granules -- overheated blobs -- may repeatedly
emerge on the solar surface. Based on our study, the critical remarks by Rast
(2002) on the original paper by Getling and Brandt (2002) can be dismissed.Comment: 21 page, 8 figures; accepted by "Solar Physics
Subdiffusive transport in intergranular lanes on the Sun. The Leighton model revisited
In this paper we consider a random motion of magnetic bright points (MBP)
associated with magnetic fields at the solar photosphere. The MBP transport in
the short time range [0-20 minutes] has a subdiffusive character as the
magnetic flux tends to accumulate at sinks of the flow field. Such a behavior
can be rigorously described in the framework of a continuous time random walk
leading to the fractional Fokker-Planck dynamics. This formalism, applied for
the analysis of the solar subdiffusion of magnetic fields, generalizes the
Leighton's model.Comment: 7 page
Logarithmic two-loop corrections to the Lamb shift in hydrogen
Higher order logarithmic corrections to the
hydrogen Lamb shift are calculated. The results obtained show the two-loop
contribution has a very peculiar behavior, and significantly alter the
theoretical predictions for low lying S-states.Comment: 14 pages, including 2 figures, submitted to Phys. Rev. A, updated
with minor change
Can we avoid high coupling?
It is considered good software design practice to organize source code into modules and to favour within-module connections (cohesion) over between-module connections (coupling), leading to the oft-repeated maxim "low coupling/high cohesion". Prior research into network theory and its application to software systems has found evidence that many important properties in real software systems exhibit approximately scale-free structure, including coupling; researchers have claimed that such scale-free structures are ubiquitous. This implies that high coupling must be unavoidable, statistically speaking, apparently contradicting standard ideas about software structure. We present a model that leads to the simple predictions that approximately scale-free structures ought to arise both for between-module connectivity and overall connectivity, and not as the result of poor design or optimization shortcuts. These predictions are borne out by our large-scale empirical study. Hence we conclude that high coupling is not avoidable--and that this is in fact quite reasonable
Generalised quantum weakest preconditions
Generalisation of the quantum weakest precondition result of D'Hondt and
Panangaden is presented. In particular the most general notion of quantum
predicate as positive operator valued measure (POVM) is introduced. The
previously known quantum weakest precondition result has been extended to cover
the case of POVM playing the role of a quantum predicate. Additionally, our
result is valid in infinite dimension case and also holds for a quantum
programs defined as a positive but not necessary completely positive
transformations of a quantum states.Comment: 7 pages, no figures, added references, changed conten
The Effects of Atmospheric Dispersion on High-Resolution Solar Spectroscopy
We investigate the effects of atmospheric dispersion on observations of the
Sun at the ever-higher spatial resolutions afforded by increased apertures and
improved techniques. The problems induced by atmospheric refraction are
particularly significant for solar physics because the Sun is often best
observed at low elevations, and the effect of the image displacement is not
merely a loss of efficiency, but the mixing of information originating from
different points on the solar surface. We calculate the magnitude of the
atmospheric dispersion for the Sun during the year and examine the problems
produced by this dispersion in both spectrographic and filter observations. We
describe an observing technique for scanning spectrograph observations that
minimizes the effects of the atmospheric dispersion while maintaining a regular
scanning geometry. Such an approach could be useful for the new class of
high-resolution solar spectrographs, such as SPINOR, POLIS, TRIPPEL, and ViSP
Power-law distributions and Levy-stable intermittent fluctuations in stochastic systems of many autocatalytic elements
A generic model of stochastic autocatalytic dynamics with many degrees of
freedom is studied using computer simulations. The time
evolution of the 's combines a random multiplicative dynamics at the individual level with a global coupling through a
constraint which does not allow the 's to fall below a lower cutoff given
by , where is their momentary average and is a
constant. The dynamic variables are found to exhibit a power-law
distribution of the form . The exponent
is quite insensitive to the distribution of the random factor
, but it is non-universal, and increases monotonically as a function
of . The "thermodynamic" limit, N goes to infty and the limit of decoupled
free multiplicative random walks c goes to 0, do not commute:
for any finite while (which is the common range
in empirical systems) for any positive . The time evolution of exhibits intermittent fluctuations parametrized by a (truncated)
L\'evy-stable distribution with the same index . This
non-trivial relation between the distribution of the 's at a given time
and the temporal fluctuations of their average is examined and its relevance to
empirical systems is discussed.Comment: 7 pages, 4 figure
Geriatric Screening, Triage Urgency, and 30-Day Mortality in Older Emergency Department Patients
BACKGROUND: Urgency triage in the emergency department (ED) is important for early identification of potentially lethal conditions and extensive resource utilization. However, in older patients, urgency triage systems could be improved by taking geriatric vulnerability into account. We investigated the association of geriatric vulnerability screening in addition to triage urgency levels with 30-day mortality in older ED patients. DESIGN: Secondary analysis of the observational multicenter Acutely Presenting Older Patient (APOP) study. SETTING: EDs within four Dutch hospitals. PARTICIPANTS: Consecutive patients, aged 70 years or older, who were prospectively included. MEASUREMENTS: Patients were triaged using the Manchester Triage System (MTS). In addition, the APOP screener was used as a geriatric screening tool. The primary outcome was 30-day mortality. Comparison was made between mortality within the geriatric high- and low-risk screened patients in every urgency triage category. We calculated the difference in explained variance of mortality by adding the geriatric screener (APOP) to triage urgency (MTS) by calculating Nagelkerke R2. RESULTS: We included 2,608 patients with a median age of 79 (interquartile range = 74-84) years, of whom 521 (20.0%) patients were categorized as high risk accor
Predicting mortality in acutely hospitalized older patients: a retrospective cohort study
Acutely hospitalized older patients have an increased risk of mortality, but at the moment of presentation this risk is difficult to assess. Early identification of patients at high risk might increase the awareness of the physician, and enable tailored decision-making. Existing screening instruments mainly use either geriatric factors or severity of disease for prognostication. Predictive performance of these instruments is moderate, which hampers successive interventions. We conducted a retrospective cohort study among all patients aged 70 years and over who were acutely hospitalized in the Acute Medical Unit of the Leiden University Medical Center, the Netherlands in 2012. We developed a prediction model for 90-day mortality that combines vital signs and laboratory test results reflecting severity of disease with geriatric factors, represented by comorbidities and number of medications. Among 517 patients, 94 patients (18.2 %) died within 90 days after admission. Six predictors of mortality were included in a model for mortality: oxygen saturation, Charlson comorbidity index, thrombocytes, urea, C-reactive protein and non-fasting glucose. The prediction model performs satisfactorily with an 0.738 (0.667–0.798). Using this model, 53 % of the patients in the highest risk decile (N = 51) were deceased within 90 days. In conclusion, we are able to predict 90-day mortality in acutely hospitalized older patients using a model with directly available clinical data describing disease severity and geriatric factors. After further validation, such a model might be used in clinical decision making in older patients
The Potts Fully Frustrated model: Thermodynamics, percolation and dynamics in 2 dimensions
We consider a Potts model diluted by fully frustrated Ising spins. The model
corresponds to a fully frustrated Potts model with variables having an integer
absolute value and a sign. This model presents precursor phenomena of a glass
transition in the high-temperature region. We show that the onset of these
phenomena can be related to a thermodynamic transition. Furthermore this
transition can be mapped onto a percolation transition. We numerically study
the phase diagram in 2 dimensions (2D) for this model with frustration and {\em
without} disorder and we compare it to the phase diagram of the model with
frustration {\em and} disorder and of the ferromagnetic model.
Introducing a parameter that connects the three models, we generalize the exact
expression of the ferromagnetic Potts transition temperature in 2D to the other
cases. Finally, we estimate the dynamic critical exponents related to the Potts
order parameter and to the energy.Comment: 10 pages, 10 figures, new result
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