Flux emergence in a magnetized convection zone

We study the influence of a dynamo magnetic field on the buoyant rise and emergence of twisted magnetic flux-ropes, and their influence on the global external magnetic field. We ran 3D MHD numerical simulations using the ASH code and analysed the dynamical evolution of such buoyant flux-ropes from the bottom of the convection zone until the post-emergence phases. The global nature of this model represents very crudely and inaccurately the local dynamics of the buoyant rise, but allows to study the influence of global effects such as self-consistently generated differential rotation, meridional circulation and Coriolis forces. Although motivated by the solar context, this model cannot be thought of as a realistic model of the rise of magnetic structures and their emergence in the Sun where the local dynamics are completely different. The properties of initial phases of the buoyant rise in good agreement with previous studies. However, the effects of the interaction of the background dynamo field become increase as the flux-ropes evolve. During the buoyant rise across the CZ, the flux-rope's magnetic field strength and scales as $rho^a$, with $a\lesssim 1$. An increase of velocity, density and current precedes flux emergence at all longitudes. The geometry, latitude and relative orientation of the flux-ropes with respect to the background magnetic field influences the rise speeds, zonal flow amplitudes (which develop within the flux-ropes) and the corresponding surface signatures. This influences the morphology, duration and amplitude of the associated surface shearing and Poynting flux. The emerged flux influences the system's global polarity, leading in some cases to a polarity reversal while inhibiting background dynamo from doing so in some others. The emerged magnetic flux is slowly advected poleward, while being diffused and assimilated by the background dynamo field.Comment: Accepted for publication in Ap

The H.E.S.S. multi-messenger program

Based on fundamental particle physics processes like the production and subsequent decay of pions in interactions of high-energy particles, close connections exist between the acceleration sites of high-energy cosmic rays and the emission of high-energy gamma rays and high-energy neutrinos. In most cases these connections provide both spatial and temporal correlations of the different emitted particles. The combination of the complementary information provided by these messengers allows to lift ambiguities in the interpretation of the data and enables novel and highly sensitive analyses. In this contribution the H.E.S.S. multi-messenger program is introduced and described. The current core of this newly installed program is the combination of high-energy neutrinos and high-energy gamma rays. The search for gamma-ray emission following gravitational wave triggers is also discussed. Furthermore, the existing program for following triggers in the electromagnetic regime was extended by the search for gamma-ray emission from Fast Radio Bursts (FRBs). An overview over current and planned analyses is given and recent results are presented.Comment: In Proceedings of the 34th International Cosmic Ray Conference (ICRC2015), The Hague, The Netherland

Comparison between S. T. radar and in situ balloon measurements

A campaign for simultaneous in situ and remote observation of both troposphere and stratosphere took place near Aire-sur-l'Adour (in southeastern France) on May 4, 1984. The aim of this campaign was a better understanding of the physics of radar echoes. The backscattered signal obtained with a stratosphere-troposphere radar both at the vertical and 15 deg. off vertical is compared with the velocity and temperature measurements made in the same region (about 10 km north of the radar site) by balloon-borne ionic anenometers and temperature sensors. In situ measurements clearly indicate that the temperature fluctuations are not always consistent with the standard turbulent theory. Nevertheless, the assumptions generally made (isotropy and turbulent field in k) and the classical formulation so derived for radar reflectivity are able to reproduce the shape of the radar return power profiles in oblique directions. Another significant result is the confirmation of the role played by the atmospheric stratification in the vertical echo power. It is important to develop these simultaneous in situ and remote experiments for a better description of the dynamical and thermal structure of the atmosphere and for a better understanding of the mechanisms governing clear-air radar reflectivity

Soft X-ray emission in kink-unstable coronal loops

Solar flares are associated with intense soft X-ray emission generated by the hot flaring plasma. Kink unstable twisted flux-ropes provide a source of magnetic energy which can be released impulsively and account for the flare plasma heating. We compute the temporal evolution of the thermal X-ray emission in kink-unstable coronal loops using MHD simulations and discuss the results of with respect to solar flare observations. The model consists of a highly twisted loop embedded in a region of uniform and untwisted coronal magnetic field. We let the kink instability develop, compute the evolution of the plasma properties in the loop (density, temperature) without accounting for mass exchange with the chromosphere. We then deduce the X-ray emission properties of the plasma during the whole flaring episode. During the initial phase of the instability plasma heating is mostly adiabatic. Ohmic diffusion takes over as the instability saturates, leading to strong and impulsive heating (> 20 MK), to a quick enhancement of X-ray emission and to the hardening of the thermal X-ray spectrum. The temperature distribution of the plasma becomes broad, with the emission measure depending strongly on temperature. Significant emission measures arise for plasma at temperatures T > 9 MK. The magnetic flux-rope then relaxes progressively towards a lower energy state as it reconnects with the background flux. The loop plasma suffers smaller sporadic heating events but cools down conductively. The total thermal X-ray emission slowly fades away during this phase, and the high temperature component of emission measure distribution converges to the power-law distribution $EM\propto T^{-4.2}$. The amount of twist deduced directly from the X-ray emission patterns is considerably lower than the maximum magnetic twist in the simulated flux-ropes.Comment: submitted to A&

Flux-tube geometry and solar wind speed during an activity cycle

The solar wind speed at 1 AU shows variations in latitude and in time which reflect the evolution of the global background magnetic field during the activity cycle. It is commonly accepted that the terminal wind speed in a magnetic flux-tube is anti-correlated with its expansion ratio, which motivated the definition of widely-used semi-empirical scaling laws relating one to the other. In practice, such scaling laws require ad-hoc corrections. A predictive law based solely on physical principles is still missing. We test whether the flux-tube expansion is the controlling factor of the wind speed at all phases of the cycle and at all latitudes using a very large sample of wind-carrying open magnetic flux-tubes. We furthermore search for additional physical parameters based on the geometry of the coronal magnetic field which have an influence on the terminal wind flow speed. We use MHD simulations of the corona and wind coupled to a dynamo model to provide a large statistical ensemble of open flux-tubes which we analyse conjointly in order to identify relations of dependence between the wind speed and geometrical parameters of the flux-tubes which are valid globally (for all latitudes and moments of the cycle). Our study confirms that the terminal speed of the solar wind depends very strongly on the geometry of the open magnetic flux-tubes through which it flows. The total flux-tube expansion is more clearly anti-correlated with the wind speed for fast rather than for slow wind flows, and effectively controls the locations of these flows during solar minima. Overall, the actual asymptotic wind speeds attained are also strongly dependent on field-line inclination and magnetic field amplitude at the foot-points. We suggest ways of including these parameters on future predictive scaling-laws for the solar wind speed.Comment: Accepted for publicaton on Astronomy & Astrophysic

A quantitative assessment of the ecological value of sycamore maple habitats in the French Alps

La naturalité est un critère important pour l'évaluation de mesures conservatoires des écosystèmes. Au niveau local, une telle évaluation doit être basée sur des indicateurs objectifs et quantifiables sur le terrain. Dans cette étude, nous avons utilisé une méthode multicritères basée sur la différence entre Valeur Naturelle (NV) et Valeur Conservatoire (CV) pour quantifier la valeur écologique des érablaies de versant à érable sycomore (Acer pseudoplatanus L.) par comparaison avec les peuplements mixtes de hêtraie-sapinière-pessière avoisinants. En effet, les naturalistes ainsi que l'Union Européenne considèrent que les érablaies de versant ont une valeur de conservation et de naturalité élevée. Nos résultats montrent que les valeurs naturelle et de conservation sont significativement plus élevées pour l'érablaie que pour la forêt mixte avoisinante et que cette évaluation ne dépend pas de facteurs abiotiques tels que l'altitude ou l'exposition. En fait, la naturalité de structure et de composition des érablaies de versant sont plus fortes que celles des forêts mixtes et permettent de différencier les deux habitats en termes de valeur écologique. Les gestionnaires peuvent facilement utiliser cette méthode pour évaluer la valeur écologique de petits habitats en zone de montagne, ce qui permet d'établir des orientations sylvicoles pour une gestion conservatoire et proche de la nature. / Naturalness is an important criterion in nature conservation assessment. At the stand-level, such assessment must be based on objective and quantifiable indicators measurable in the field. In this study, we used a multi-criterion method based on the difference between a Natural Value(NV) and a Conservation Value (CV) to quantify the ecological value of sycamore maple patches compared to the surrounding mixed forests. Indeed, sycamore habitats are considered of high natural and conservation value both by naturalists and by European institutions. Our results showed that the natural and conservation values were significantly higher for the sycamore forests than for the surrounding mixed forests and that this assessment did not depend on abiotic factors such as elevation or aspect. Actually, naturalness of structure and composition in the sycamore habitats was higher than for mixed forests and allowed us to differentiate between the two habitats. Managers could easily use this method in order to assess the ecological value of small habitats in mountainous regions and to provide guidelines for close-to-nature and conservation-related silviculture.FORET DE MONTAGNE;ECOLOGIE FORESTIERE;EVALUATION;PROTECTION DE LA NATURE;ACER PSEUDOPLATANUS;METHODOLOGIE;HABITAT;VALEUR ECOLOGIQUE;NATURALITE;ALPES FRANCAISES;CHARTREUSE MASSIF;ACER PSEUDOPLATANUS;CHARTREUSE;INDICATORS;NATURALNESS;ECOLOGICAL VALUE

A Bayesian approach to evaluate confidence intervals in counting experiments with background

In this paper we propose a procedure to evaluate Bayesian confidence intervals in counting experiments where both signal and background fluctuations are described by the Poisson statistics. The results obtained when the method is applied to the calculation of upper limits will also be illustrated.Comment: 19 pages, 6 figure

Study of the heating effect contribution to the nonlinear dielectric response of a supercooled liquid

We present a detailed study of the heating effects in dielectric measurements carried out on a liquid. Such effects come from the dissipation of the electric power in the liquid and give a contribution to the nonlinear third harmonics susceptibility chi_3 which depends on the frequency and temperature. This study is used to evaluate a possible `spurious' contribution to the recently measured nonlinear susceptibility of an archetypical glassforming liquid (Glycerol). Those measurements have been shown to give a direct evaluation of the number of dynamically correlated molecules temperature dependence close to the glass transition temperature T_g~190K (Crauste-Thibierge et al., Phys. Rev. Lett 104,165703(2010)). We show that the heating contribution is totally negligible (i) below 204K at any frequency; (ii) for any temperature at the frequency where the third harmonics response chi_3 is maximum. Besides, this heating contribution does not scale as a function of f/f_{\alpha}, with f_{\alpha}(T) the relaxation frequency of the liquid. In the high frequency range, when f/f_{\alpha} >= 1, we find that the heating contribution is damped because the dipoles cannot follow instantaneously the temperature modulation due to the heating phenomenon. An estimate of the magnitude of this damping is given.Comment: 25 pages, 10 figures, Accepted for publication in Journal of Chemical Physic