From chromosphere to corona through ground-based to space observatories

The solar photosphere-chromosphere, as observed from ground-based observatories, is a very thick atmosphere where phenomena related to magnetic fields, differential rotation, waves, take place. The transition region between the thick atmosphere and the extended thin corona can be observed in the ultraviolet from space observatories such as SOHO and TRACE. In this paper we present some results which show the structuring of the transition region as a result from phenomena having their source in the lower atmosphere and we point out some connections with the corona. An overview of the main characteristics of the transition region obtained from space observations evidences the progress in our understanding of the onset of the solar wind and allows to put constraints on the coronal heating

Policy Gradients for CVaR-Constrained MDPs

We study a risk-constrained version of the stochastic shortest path (SSP) problem, where the risk measure considered is Conditional Value-at-Risk (CVaR). We propose two algorithms that obtain a locally risk-optimal policy by employing four tools: stochastic approximation, mini batches, policy gradients and importance sampling. Both the algorithms incorporate a CVaR estimation procedure, along the lines of Bardou et al. [2009], which in turn is based on Rockafellar-Uryasev's representation for CVaR and utilize the likelihood ratio principle for estimating the gradient of the sum of one cost function (objective of the SSP) and the gradient of the CVaR of the sum of another cost function (in the constraint of SSP). The algorithms differ in the manner in which they approximate the CVaR estimates/necessary gradients - the first algorithm uses stochastic approximation, while the second employ mini-batches in the spirit of Monte Carlo methods. We establish asymptotic convergence of both the algorithms. Further, since estimating CVaR is related to rare-event simulation, we incorporate an importance sampling based variance reduction scheme into our proposed algorithms

Working memory, strategy execution, and strategy selection in mental arithmetic

A total of 72 participants estimated products of complex multiplications of two-digit operands (e.g., 63 x 78), using two strategies that differed in complexity. The simple strategy involved rounding both operands down to the closest decades (e.g., 60 _ 70), whereas the complex strategy required rounding both operands up to the closest decades (e.g., 70 _ 80). Participants accomplished this estimation task in two conditions: a no-load condition and a working-memory load condition in which executive components of working memory were taxed. The choice/no-choice method was used to obtain unbiased strategy execution and strategy selection data. Results showed that loading working memory resources led participants to poorer strategy execution. Additionally, participants selected the simple strategy more often under working-memory load. We discuss the implications of the results to further our understanding of variations in strategy selection and execution, as well as our understanding of the impact of working-memory load on arithmetic performance and other cognitive domains

Generalizing a definition of Lusternik and Schnirelmann to model categories

AbstractLusternik-Schnirelmann category is an important numerical homotopy invariant, defined originally for topological spaces but employed since in other categories in which there is a good notion of homotopy. In the interest of unification, Doeraene defined a J-category to be a category satisfying a certain set of axioms which ensure that an LS-type invariant can be reasonably defined. He provided two equivalent definitions of LS-category for J-categories and showed that these generalized definitions agree with previous definitions in specific categories. Extending his results, we provide a third equivalent definition of the LS-category of an object in a J-category, analogous to the original topological definition of Lusternik and Schnirelmann

Normal stress differences in non-Brownian fiber suspensions

International audienceIn this paper, we present an experimental study of the normal stress differences that arise in non-Brownian rigid fiber suspensions subject to a shear flow. While early measurements of the normal stress in fiber suspensions in Newtonian fluids measured only N 1 − N 2 , the recent work of Snook et al. J. Fluid Mech. 758 486 (2014) and the present paper provide the first measurements of N 1 and N 2 separately. Snook et al perform such measurements with a gap that is very wide compared with the fiber length, whereas the present paper explores the effects of confinement when the gap is 4-10 times the fiber length. The first and the second normal stress differences are measured using a single experiment which consists of determining the radial profile of the second normal stress, along the velocity gradient direction, Σ 22 , in a torsional flow between two parallel discs. Suspensions are made of monodisperse fibers immersed in a neutrally buoyant Newtonian fluid. Two fiber lengths and three aspect ratios a r = L/d, and a wide range of concentrations have been tested. N 1 is found to be positive while N 2 is negative and the magnitude of both normal stress differences increases when nL 2 d increases, n being the number fraction of fibers. The magnitude of N 2 is found to be much smaller than N 1 only for high aspect ratios and low fiber concentrations

The Ly-alpha profile and center-to-limb variation of the quiet Sun

We study the emission of the hydrogen Lyman-a line in the quiet Sun, its center-to-limb variation (CLV), and its radiance distribution. We also compare quasi-simultaneous Ly-a and Ly-b line profiles. We used the high spectral and spatial resolution of the SUMER spectrometer and completed raster scans at various locations along the disk. For the first time, we used a method to reduce the incoming photon flux to a 20%-level by partly closing the aperture door. We also performed a quasi-simultaneous observation of both Ly-a and Ly-b at Sun center in sit-and-stare mode. We infer the flow characteristic in the Ly-a map from variations in the calibrated 1206 Si III line centroids. We present the average profile of Ly-a, its radiance distribution, its CLV behaviour, and the signature of flows on the line profiles. Little CLV and no limb brightening are observed in the profiles of the Ly-a line. In contrast to all other lines of the Lyman series, which have a stronger red-horn, Ly-a has a dominating blue-horn asymmetry. There appears to be a brightness-to-asymmetry relationship. A similar and even clearer trend is observed in the downflow-to-asymmetry relationship. This important result is consistent with predictions from models that include flows. However, the absence of a clear CLV in the profiles may be more indicative of an isotropic field than a radial flow. It appears that the ubiquitous hydrogen behaves similar to a filter that dampens all signatures of the line formation by processes in the chromosphere and transition region.Comment: 4 pages, 4 figure

Improving the detection of low concentration metabolites in magnetic resonance spectroscopy by digital filtering

In vivo detection and quantitation of metabolites is often limited by their low concentration. As far as magnetic resonance spectroscopy (MRS) is concerned, detection and quantitation can be significantly improved by reduction of the observed spectral width (SW). The reduction is limited to the spreading of resonances in the bandwidth unless high performance digital filters are used. Indeed, these filters avoid the folding of unwanted resonances such as water peak into the main frequency spectrum and therefore allow reduction of the spectral width to its optimal value. These filters are now available on most MRS systems but their use is not common even if, as we show in the particular case of proton MRS, a significant increase in signal-to-noise ratio (two-fold factor for SW reduction from 5000 Hz to 1351 Hz) can be achieved. This signal-to-noise improvement allows better quantitation accuracy

Oscillations in the expression of a self-repressed gene induced by a slow transcriptional dynamics

We revisit the dynamics of a gene repressed by its own protein in the case where the transcription rate does not adapt instantaneously to protein concentration but is a dynamical variable. We derive analytical criteria for the appearance of sustained oscillations and find that they require degradation mechanisms much less nonlinear than for infinitely fast regulation. Deterministic predictions are also compared with stochastic simulations of this minimal genetic oscillator

Oscillations in the expression of a self-repressed gene induced by a slow transcriptional dynamics

We revisit the dynamics of a gene repressed by its own protein in the case where the transcription rate does not adapt instantaneously to protein concentration but is a dynamical variable. We derive analytical criteria for the appearance of sustained oscillations and find that they require degradation mechanisms much less nonlinear than for infinitely fast regulation. Deterministic predictions are also compared with stochastic simulations of this minimal genetic oscillator