630 research outputs found
Magnetohydrodynamics dynamical relaxation of coronal magnetic fields. I. Parallel untwisted magnetic fields in 2D
Context. For the last thirty years, most of the studies on the relaxation of
stressed magnetic fields in the solar environment have onlyconsidered the
Lorentz force, neglecting plasma contributions, and therefore, limiting every
equilibrium to that of a force-free field. Aims. Here we begin a study of the
non-resistive evolution of finite beta plasmas and their relaxation to
magnetohydrostatic states, where magnetic forces are balanced by
plasma-pressure gradients, by using a simple 2D scenario involving a
hydromagnetic disturbance to a uniform magnetic field. The final equilibrium
state is predicted as a function of the initial disturbances, with aims to
demonstrate what happens to the plasma during the relaxation process and to see
what effects it has on the final equilibrium state. Methods. A set of numerical
experiments are run using a full MHD code, with the relaxation driven by
magnetoacoustic waves damped by viscous effects. The numerical results are
compared with analytical calculations made within the linear regime, in which
the whole process must remain adiabatic. Particular attention is paid to the
thermodynamic behaviour of the plasma during the relaxation. Results. The
analytical predictions for the final non force-free equilibrium depend only on
the initial perturbations and the total pressure of the system. It is found
that these predictions hold surprisingly well even for amplitudes of the
perturbation far outside the linear regime. Conclusions. Including the effects
of a finite plasma beta in relaxation experiments leads to significant
differences from the force-free case
Transverse oscillations in solar coronal loops induced by propagating Alfvenic pulses
The propagation and the evolution of Alfvenic pulses in the solar coronal
arcades is investigated by means of MHD numerical simulations. Significant
transverse oscillations in coronal loops, triggered by nearby flare events, are
often measured in EUV lines and are generally interpreted as standing kink
modes. However, the damping times of these oscillations are typically very
short (from one to a few periods) and the physical mechanism responsible for
the decay is still a matter of debate. Moreover, the majority of the observed
cases actually appears to be better modeled by propagating, rather than
standing, modes. Here we perform 2.5-D compressible MHD simulations of
impulsively generated Alfven waves propagating in a potential magnetic arcade
(assumed as a simplified 2-D loop model), taking into account the
stratification of the solar atmosphere with height from the photosphere to the
corona. The results show a strong spreading of the initially localized pulses
along the loop, due to the variations in the Alfven velocity with height, and
correspondingly an efficient damping of the amplitude of the oscillations. We
believe that simple explanations based on the effects of wave propagation in
highly inhomogeneous media may apply to the majority of the reported cases, and
that variations of the background density and Alfven speed along the loop
should be considered as key ingredients in future models.Comment: Accepted for publication in A&A on 26 October 2004; 10 pages, 8
figure
Towards a better estimation of prevalence of female genital mutilation in the European Union : a situation analysis
Background:Female genital mutilation (FGM) is a harmful cultural practice that is predominantly documented inAfrica, but also occurs in other parts of the world. Due to migration, women who have undergone FGM can also befound in the European Union (EU). Due to a lack of systematic representative surveys on the topic in EU, theprevalence of FGM and the number of women and children subjected to the practice remains unknown. However,information on the magnitude of the problem in the EU is necessary for policy makers to design and trackpreventive measures and to determine resource allocation.Methods:Between March 2015 and May 2015, we performed a situation analysis consisting of a critical interpretivesynthesis and SWOT-analysis of available at the time peer reviewed and grey literature document on nationalprevalence studies on FGM in the EU. Studies estimating the prevalence of FGM and the number of girls and womensubjected to the practice in the EU were mapped to analyse their methodologies and identify their Strengths,Weakness, Opportunities and Threats (SWOT). Distinction was made between direct and indirect estimation methods.Results:Thirteen publications matched the prioritized inclusion criteria. The situation analysis showed that both directand indirect methodologies were used to estimate FGM prevalence and the number of girls and women subjected toFGM in the EU. The SWOT-analysis indicated that due to the large variations in the targeted population and the availablesecondary information in EU Member States, one single estimation method is not applicable in all Member States.Conclusions:We suggest a twofold method for estimating the number of girls and women who have undergoneFGMinthe EU. For countries with a low expected prevalence of women who have undergoneFGM, the indirect method will providea good enough estimation of the FGM prevalence. The extrapolation-of-FGM-countries-prevalence-data-method, based on thedocumented FGM prevalence numbers in DHS and MICS surveys, can be used for indirect estimations of girls and womensubjected to FGM in theEU. For countries with a high expected prevalence of FGM in the EU Member State, we recommendto combine both a direct estimation method (e.g. in the form ofa survey conducted in the target population) and an indirectestimation method and to use a sample design as developed bythe FGM-PREV project. The choice for a direct or indirectmethod will ultimately depend on available financial means and the purpose for the estimation
The effect of activity-related meridional flow modulation on the strength of the solar polar magnetic field
We studied the effect of the perturbation of the meridional flow in the
activity belts detected by local helioseismology on the development and
strength of the surface magnetic field at the polar caps. We carried out
simulations of synthetic solar cycles with a flux transport model, which
follows the cyclic evolution of the surface field determined by flux emergence
and advective transport by near-surface flows. In each hemisphere, an
axisymmetric band of latitudinal flows converging towards the central latitude
of the activity belt was superposed onto the background poleward meridional
flow. The overall effect of the flow perturbation is to reduce the latitude
separation of the magnetic polarities of a bipolar magnetic region and thus
diminish its contribution to the polar field. As a result, the polar field
maximum reached around cycle activity minimum is weakened by the presence of
the meridional flow perturbation. For a flow perturbation consistent with
helioseismic observations, the polar field is reduced by about 18% compared to
the case without inflows. If the amplitude of the flow perturbation depends on
the cycle strength, its effect on the polar field provides a nonlinearity that
could contribute to limiting the amplitude of a Babcock-Leighton type dynamo.Comment: 16 pages, 5 figures, accepted by Ap
Nationwide Analysis of the Heat-and Cold-Related Mortality Trends in Switzerland between 1969 and 2017: The Role of Population Aging
BACKGROUND: Because older adults are particularly vulnerable to nonoptimal temperatures, it is expected that the progressive population aging will amplify the health burden attributable to heat and cold due to climate change in future decades. However, limited evidence exists on the contribution of population aging on historical temperature-mortality trends. OBJECTIVES: We aimed to a) assess trends in heat- and cold-related mortality in Switzerland between 1969 and 2017 and b) to quantify the contribution of population aging to the observed patterns. METHODS: We collected daily time series of all-cause mortality by age group ( /=80-y-old age group. Cold-related mortality rates decreased across all ages, but annual cold-related deaths still increased among the >/=80, due to the increase in the population at risk. We estimated that heat- and cold-related deaths would have been 52.7% and 44.6% lower, respectively, in the most recent decade in the absence of population aging. DISCUSSION: Our findings suggest that a substantial proportion of historical temperature-related impacts can be attributed to population aging. We found that population aging has attenuated the decrease in cold-related mortality and amplified heat-related mortality
Minimizing the stabbing number of matchings, trees, and triangulations
The (axis-parallel) stabbing number of a given set of line segments is the
maximum number of segments that can be intersected by any one (axis-parallel)
line. This paper deals with finding perfect matchings, spanning trees, or
triangulations of minimum stabbing number for a given set of points. The
complexity of these problems has been a long-standing open question; in fact,
it is one of the original 30 outstanding open problems in computational
geometry on the list by Demaine, Mitchell, and O'Rourke. The answer we provide
is negative for a number of minimum stabbing problems by showing them NP-hard
by means of a general proof technique. It implies non-trivial lower bounds on
the approximability. On the positive side we propose a cut-based integer
programming formulation for minimizing the stabbing number of matchings and
spanning trees. We obtain lower bounds (in polynomial time) from the
corresponding linear programming relaxations, and show that an optimal
fractional solution always contains an edge of at least constant weight. This
result constitutes a crucial step towards a constant-factor approximation via
an iterated rounding scheme. In computational experiments we demonstrate that
our approach allows for actually solving problems with up to several hundred
points optimally or near-optimally.Comment: 25 pages, 12 figures, Latex. To appear in "Discrete and Computational
Geometry". Previous version (extended abstract) appears in SODA 2004, pp.
430-43
A Generating Function for all Semi-Magic Squares and the Volume of the Birkhoff Polytope
We present a multivariate generating function for all n x n nonnegative
integral matrices with all row and column sums equal to a positive integer t,
the so called semi-magic squares. As a consequence we obtain formulas for all
coefficients of the Ehrhart polynomial of the polytope B_n of n x n
doubly-stochastic matrices, also known as the Birkhoff polytope. In particular
we derive formulas for the volumes of B_n and any of its faces.Comment: 24 pages, 1 figure. To appear in Journal of Algebraic Combinatoric
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