348 research outputs found
Internet-based early intervention for depression and anxiety
Riper, M.M. [Promotor]Cuijpers, W.J.M.J. [Promotor]Straten, A. van [Copromotor
Reconstructing the Local Twist of Coronal Magnetic Fields and the Three-Dimensional Shape of the Field Lines from Coronal Loops in EUV and X-Ray Images
Non-linear force-free fields are the most general case of force-free fields,
but the hardest to model as well. There are numerous methods of computing such
fields by extrapolating vector magnetograms from the photosphere, but very few
attempts have so far made quantitative use of coronal morphology. We present a
method to make such quantitative use of X-Ray and EUV images of coronal loops.
Each individual loop is fit to a field line of a linear force-free field,
allowing the estimation of the field line's twist, three-dimensional geometry
and the field strength along it.
We assess the validity of such a reconstruction since the actual corona is
probably not a linear force-free field and that the superposition of linear
force-free fields is generally not itself a force-free field. To do so, we
perform a series of tests on non-linear force-free fields, described in Low &
Lou (1990). For model loops we project field lines onto the photosphere. We
compare several results of the method with the original field, in particular
the three-dimensional loop shapes, local twist (coronal alpha), distribution of
twist in the model photosphere and strength of the magnetic field. We find
that, (i) for these trial fields, the method reconstructs twist with mean
absolute deviation of at most 15% of the range of photospheric twist, (ii) that
heights of the loops are reconstructed with mean absolute deviation of at most
5% of the range of trial heights and (iii) that the magnitude of non-potential
contribution to photospheric field is reconstructed with mean absolute
deviation of at most 10% of the maximal value.Comment: submitted to Ap
Flux emergence and coronal eruption
Our aim is to study the photospheric flux distribution of a twisted flux tube
that emerges from the solar interior. We also report on the eruption of a new
flux rope when the emerging tube rises into a pre-existing magnetic field in
the corona. To study the evolution, we use 3D numerical simulations by solving
the time-dependent and resistive MHD equations. We qualitatively compare our
numerical results with MDI magnetograms of emerging flux at the solar surface.
We find that the photospheric magnetic flux distribution consists of two
regions of opposite polarities and elongated magnetic tails on the two sides of
the polarity inversion line (PIL), depending on the azimuthal nature of the
emerging field lines and the initial field strength of the rising tube. Their
shape is progressively deformed due to plasma motions towards the PIL. Our
results are in qualitative agreement with observational studies of magnetic
flux emergence in active regions (ARs). Moreover, if the initial twist of the
emerging tube is small, the photospheric magnetic field develops an undulating
shape and does not possess tails. In all cases, we find that a new flux rope is
formed above the original axis of the emerging tube that may erupt into the
corona, depending on the strength of the ambient field.Comment: 5 pages, 3 figures, accepted for publication in A&
Excitation of Oscillations in the Magnetic Network on the Sun
We examine the excitation of oscillations in the magnetic network of the Sun
through the footpoint motion of photospheric magnetic flux tubes located in
intergranular lanes. The motion is derived from a time series of
high-resolution G band and continuum filtergrams using an object-tracking
technique. We model the response of the flux tube to the footpoint motion in
terms of the Klein-Gordon equation, which is solved analytically as an initial
value problem for transverse (kink) waves. We compute the wave energy flux in
upward propagating transverse waves. In general we find that the injection of
energy into the chromosphere occurs in short-duration pulses, which would lead
to a time variability in chromospheric emission that is incompatible with
observations. Therefore, we consider the effects of turbulent convective flows
on flux tubes in intergranular lanes. The turbulent flows are simulated by
adding high-frequency motions (periods 5-50 s) with an amplitude of 1 km
s^{-1}. The latter are simulated by adding random velocity fluctuations to the
observationally determined velocities. In this case we find that the energy
flux is much less intermittent and can in principle carry adequate energy for
chromospheric heating.Comment: 11 pages, 5 figures, figure 1 is in color, all files gzippe
Vitamin K status and physical decline in older adults—The Longitudinal Aging Study Amsterdam
Objective: We examined the association between vitamin K status and physical functioning over 13 years in the Longitudinal Aging Study Amsterdam. Study design: Longitudinal cohort study of 633 community-dwelling adults from the Longitudinal Aging Study Amsterdam (LASA) aged 55–65 years (54% women). Main outcome measures: At baseline (2002–2003), plasma desphospho-uncarboxylated matrix Gla protein (dp-ucMGP) was measured with a sandwich ELISA as a marker of vitamin K status. The outcome measures handgrip strength, calf circumference, self-reported functional limitations and functional performance were obtained at baseline and four follow-up examinations. We used generalized estimating equations to determine the relationship between dp-ucMGP tertiles and the various outcome measurements after adjusting for potential confounders. The lowest dp-ucMGP tertile reflects a high vitamin K status and was the reference. Results: Mean dp-ucMGP was 376 ± 233 pmol/L and mean follow-up was 11.1 years. Participants showed a decline in the outcome measures over time. Compared with the lowest tertile, the highest dp-ucMGP tertile had: lower handgrip strength, 1.1 kg (95% confidence interval (−2.1, −0.1; P-trend <0.001); smaller calf circumference, −0.5 cm (−0.9 −0.1; P-trend = 0.018); and, only among women, a 0.7-point poorer functional performance score (−1.1, −0.3; P-interaction = 0.002). Dp-ucMGP was not related to self-reported functional limitations. No interaction effects between time and dp-ucMGP were observed. Conclusions: Low vitamin K status was associated with lower handgrip strength, smaller calf circumference, and, in women only, with poorer functional performance score. A low vitamin K status was however not related to the 13-year decline in these measures
Unravelling transmission trees of infectious diseases by combining genetic and epidemiological data
Knowledge on the transmission tree of an epidemic can provide valuable insights into disease dynamics. The transmission tree can be reconstructed by analysing either detailed epidemiological data (e.g. contact tracing) or, if sufficient genetic diversity accumulates over the course of the epidemic, genetic data of the pathogen. We present a likelihood-based framework to integrate these two data types, estimating probabilities of infection by taking weighted averages over the set of possible transmission trees. We test the approach by applying it to temporal, geographical and genetic data on the 241 poultry farms infected in an epidemic of avian influenza A (H7N7) in The Netherlands in 2003. We show that the combined approach estimates the transmission tree with higher correctness and resolution than analyses based on genetic or epidemiological data alone. Furthermore, the estimated tree reveals the relative infectiousness of farms of different types and sizes. (Résumé d'auteur
Geen basis voor verlaging onderste leeftijdsgrens van bevolkingsonderzoek op baarmoederhalskanker
Doel:\ud
Nagaan of de onderste leeftijdsgrens van het bevolkingsonderzoek op baarmoederhalskanker verlaagd moet worden.\ud
\ud
Opzet:\ud
Retrospectieve data-analyse.\ud
\ud
Methode:\ud
Uit de Nederlandse Kankerregistratie werden alle gevallen geselecteerd van invasieve baarmoederhalskanker, gediagnosticeerd in de periode 1989-2003. Voor de leeftijdsgroep van 25-39 jaar waren ook gegevens beschikbaar over 2004 en 2005. Gegevens over sterfte werden verkregen via het Centraal Bureau voor de Statistiek. Trends werden beschreven met behulp van de geschatte jaarlijkse procentuele verandering en ‘joinpoint’-analyse.\ud
\ud
Resultaten:\ud
Bij de 25-29-jarigen varieerde het aantal gevallen van baarmoederhalskanker van 0 tot 9 per jaar. De incidentie van baarmoederhalskanker daalde in de leeftijdsgroepen van 35-39 en 45-49 jaar (respectievelijk p < 0,001 en p = 0,012). Het aantal vrouwen dat stierf aan baarmoederhalskanker fluctueerde per jaar en daalde in de leeftijdsgroepen van 30-34 en 35-39 jaar (respectievelijk p = 0,01 en p = 0,03).\ud
\ud
Conclusie:\ud
De incidentie- en sterftecijfers van baarmoederhalskanker onder vrouwen jonger dan 30 jaar zijn heel laag en stijgen niet. Met het verlagen van de onderste leeftijdsgrens van het bevolkingsonderzoek zouden veel afwijkingen worden opgespoord die normaliter in regressie zouden gaan. Omdat de voordelen van het verlagen van de leeftijdsgrens niet opwegen tegen de nadelen in termen van overbehandeling en angst heeft deze verlaging volgens ons op dit moment geen zin
The Three-dimensional Evolution of Rising, Twisted Magnetic Flux Tubes in a Gravitationally Stratified Model Convection Zone
We present three-dimensional numerical simulations of the rise and
fragmentation of twisted, initially horizontal magnetic flux tubes which evolve
into emerging Omega-loops. The flux tubes rise buoyantly through an
adiabatically stratified plasma that represents the solar convection zone. The
MHD equations are solved in the anelastic approximation, and the results are
compared with studies of flux tube fragmentation in two dimensions. We find
that if the initial amount of field line twist is below a critical value, the
degree of fragmentation at the apex of a rising Omega-loop depends on its
three-dimensional geometry: the greater the apex curvature of a given
Omega-loop, the lesser the degree of fragmentation of the loop as it approaches
the photosphere. Thus, the amount of initial twist necessary for the loop to
retain its cohesion can be reduced substantially from the two-dimensional
limit. The simulations also suggest that as a fragmented flux tube emerges
through a relatively quiet portion of the solar disk, extended crescent-shaped
magnetic features of opposite polarity should form and steadily recede from one
another. These features eventually coalesce after the fragmented portion of the
Omega-loop emerges through the photosphere.Comment: 17 pages, 17 figures, uses AAS LaTeX macros v5.0. ApJ, in pres
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