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We want to study the connections between the magnetic field generated inside the Sun and the solar wind impacting Earth, especially the influence of north–south asymmetry on the magnetic and velocity fields. We study a solar-like 11-year cycle in a quasi-static way: an asymmetric dynamo field is generated through a 2.5-dimensional (2.5-D) flux-transport model with the Babcock–Leighton mechanism, and then is used as bottom boundary condition for compressible 2.5-D simulations of the solar wind. We recover solar values for the mass loss rate, the spin-down time scale and the Alfvén radius, and are able to reproduce the observed delay in latitudinal variations of the wind and the general wind structure observed for the Sun. We show that the phase lag between the energy of the dipole component and the total surface magnetic energy has a strong influence on the amplitude of the variations of global quantities. We show in particular that the magnetic torque variations can be linked to topological variations during a magnetic cycle, while variations in the mass loss rate appear to be driven by variations of the magnetic energy.
Disc galaxies forming in a LambdaCDM cosmology often experience violent mergers. The fact that disc galaxies are ubiquitous suggests that quiescent histories are not necessary. Modern cosmological simulations can now obtain realistic populations of disc galaxies, but it is still unclear how discs manage to survive massive mergers. Here we use a suite of hydrodynamical cosmological simulations to elucidate the fate of discs encountering massive mergers. We follow the changes in the post-merger disc-to-total ratios (D/T) of simulated galaxies and examine the relations between their present-day morphology, assembly history and gas fractions. We find that approximately half of present-day disc galaxies underwent at least one merger with a satellite more massive the host's stellar component and a third had mergers with satellites three times as massive. These mergers lead to a sharp, but often temporary, decrease in the D/T of the hosts, implying that discs are usually disrupted but then quickly re-grow. To do so, high cold gas fractions are required post-merger, as well as a relatively quiescent recent history (over a few Gyrs before z = 0). Our results show that discs can form via diverse merger pathways and that quiescent histories are not the dominant mode of disc formation.
The largest solar flares, of class X and above, are often associated with strong energetic particle acceleration. Based on the self-similar distribution of solar flares, self-organized criticality models such as sandpiles can be used to successfully reproduce their statistics. However, predicting strong (and rare) solar flares turns out to be a significant challenge. We build here on an original idea based on the combination of minimalistic flare models (sandpiles) and modern data assimilation techniques (4DVar) to predict large solar flares. We discuss how to represent a sandpile model over a reduced set of eigenfunctions to improve the efficiency of the data assimilation technique. This improvement is model-independent and continues to pave the way towards efficient near real-time solutions for predicting solar flares.
The origin of magnetic cycles in the Sun and other cool stars is one of the great theoretical challenge in stellar astrophysics that still resists our understanding. Ab-initio numerical simulations are today required to explore the extreme turbulent regime in which stars operate and sustain their large-scale, cyclic magnetic field. We report in this work on recent progresses made with high performance numerical simulations of global turbulent convective envelopes. We rapidly review previous prominent results from numerical simulations, and present for the first time a series of turbulent, global simulations producing regular magnetic cycles whose period varies systematically with the convective envelope parameters (rotation rate, convective luminosity). We find that the fundamentally non-linear character of the dynamo simulated in this work leads the magnetic cycle period to be inversely proportional to the Rossby number. These results promote an original interpretation of stellar magnetic cycles, and could help reconcile the cyclic behaviour of the Sun and other solar-type stars.
The VIMOS VLT Deep Survey (VVDS) is underway to study the evolution of galaxies, large scale structures and AGNs, from the measurement of more than 100 000 spectra of faint objects. We present here the results from the first epoch observations of more than 20000 spectra. The main challenge of the program, the redshift measurements, is described, in particular entering the “redshift desert” in the range 1.5 < z < 3 for which only very weak features are detected in the observed wavelength range. The redshift distribution of a magnitude limited sample brighter than IAB = 24 is presented for the first time, showing a peak at a low redshift z ∼ 0.7, and a tail extending all the way above z = 4. The evolution of the luminosity function out to z = 1.5 is presented, with the LF of blue star forming galaxies carrying most of the evolution, with L* changing by more than two magnitudes for this sub-sample.
14C measurements were made on present-day plant material with short integration times (tree leaves and sprouts) in the Eifel area, western Germany, where ancient volcanism produces gaseous emanations of considerable yield. Plants growing near sources emanating 14C-free CO2 show a significant depletion in the period of their growth. The same effect is found in the 14C content of recent samples from the Thera (Santorini) Archipelago/Greece. This mixing of “dead” CO2 may lead to pseudo ages in archaeologic or geologic samples of up to 1600 years in samples from the vicinity of CO2 emanating sources.
The solar convection zone exhibits a differential rotation with radius and latitude that poses major theoretical challenges. Helioseismology has revealed that a smoothly varying pattern of decreasing angular velocity Ω with latitude long evident at the surface largely prints through much of the convection zone, encountering a region of strong shear called the tachocline at its base, below which the radiative interior is nearly in uniform solid body rotation. Helioseismic observations with MDI on SOHO and with GONG have also led to the detection of significant variations in Ω with 1.3 yr period in the vicinity of the tachocline. There is another shearing layer just below the solar surface, and that region exhibits propagating bands of zonal flow. Such rich dynamical behavior requires theoretical explanations, some of which are beginning to emerge from detailed 3-D simulations of turbulent convection in rotating spherical shells. We discuss some of the properties exhibited by such numerical models. Although these simulations are highly simplified representations of much of the complex physics occurring within the convection zone, they are providing a very promising path for understanding the solar differential rotation and its temporal variations.
Mass losses originating from supraglacial ice cliffs at the lower tongues of debris-covered glaciers are a potentially large component of the mass balance, but have rarely been quantified. In this study, we develop a method to estimate ice cliff volume losses based on high-resolution topographic data derived from terrestrial and aerial photogrammetry. We apply our method to six cliffs monitored in May and October 2013 and 2014 using four different topographic datasets collected over the debris-covered Lirung Glacier of the Nepalese Himalayas. During the monsoon, the cliff mean backwasting rate was relatively consistent in 2013 (3.8 ± 0.3 cm w.e. d−1) and more heterogeneous among cliffs in 2014 (3.1 ± 0.7 cm w.e. d−1), and the geometric variations between cliffs are larger. Their mean backwasting rate is significantly lower in winter (October 2013–May 2014), at 1.0 ± 0.3 cm w.e. d−1. These results are consistent with estimates of cliff ablation from an energy-balance model developed in a previous study. The ice cliffs lose mass at rates six times higher than estimates of glacier-wide melt under debris, which seems to confirm that ice cliffs provide a large contribution to total glacier melt.
The possibility that magnetic torques may participate in close-in planet migration has recently been postulated. We develop three dimensional global models of magnetic star-planet interaction under the ideal magnetohydrodynamic (MHD) approximation to explore the impact of magnetic topology on the development of magnetic torques. We conduct twin numerical experiments in which only the magnetic topology of the interaction is altered. We find that magnetic torques can vary by roughly an order of magnitude when varying the magnetic topology from an aligned case to an anti-aligned case. Provided that the stellar magnetic field is strong enough, we find that magnetic migration time scales can be as fast as ~100 Myr. Hence, our model supports the idea that magnetic torques may participate in planet migration for some close-in star-planet systems.
Magnetic interactions between a close-in planet and its host star have been postulated to be a source of enhanced chromospheric emissions. We develop three dimensional global models of star-planet systems under the ideal magnetohydrodynamic (MHD) approximation to explore the impact of magnetic topology on the energy fluxes induced by the magnetic interaction. We conduct twin numerical experiments in which only the magnetic topology of the interaction is altered. We find that the Poynting flux varies by more than an order of magnitude when varying the magnetic topology from an aligned case to an anti-aligned case. This provides a simple and robust physical explanation for on/off enhanced chromospheric emissions induced by a close-in planet on time-scales of the order of days to years.
In this chapter we briefly summarize how angular momentum is being transported and exchanged between convective and radiative zones in stars. We discuss what physical processes influence the internal rotation history of stars on short to long (secular) time scales.
The astrophysical context
Stars are rotating magnetic bodies with complex internal and external dynamics. Observations using helioseismology (e.g., García et al., 2007), asteroseismology (e.g., Deheuvels et al., 2014), and spectropolarimetry (e.g., Donati and Land street, 2009) techniques put more and more constraints on this intricate dynamics. To get a complete and coherent picture of dynamical processes in stars and of the associated transport of angular momentum that goes beyond the “standard” modeling of stellar structure and evolution (Maeder, 2009) one needs to develop new models by introducing an improved physical description of these time-dependent processes. However, to simulate such processes in a star in full detail requires treating spatial and temporal scales spanning about 10 orders of magnitude. This is clearly not yet feasible, even with the most powerful computers available today. Therefore, one can choose to describe what occurs on a dynamical time scale (such as a convective turnover time or stellar magnetic cycles) or on the long-term evolution where the typical characteristic time scale is the dominant nuclear reactions. The same applies for spatial scales. One has to choose which relevant scale one needs to model in order to accurately describe the spatial dependence of the physical processes (convection motions, MHD instabilities, transport and mixing processes, surface dynamics).This is the reason why it is nowadays necessary to use and couple 1D, 2D, and 3D models to get a global picture of macroscopic MHD transport processes in stars over short to secular time scales.
In this chapter, we report on the state of the art of the modeling of the transport of angular momentum in stars both in convection and in radiation zones and we present our main contributions to this field of research.
Here we report the first membrane-less biofuel cell made by using three-dimensional carbonaceous foam electrodes. We first developed a new synthetic pathway to produce a new carbonaceous foam electrode material with increased porosity both in the meso and macroporous scale. We proved that by increasing the porosity of our three-dimensional foams we could increase the current density of our modified electrodes. Then, by choosing the right combination of enzyme and mediator, and the right loading of active components, we achieved unprecedentedly high current densities for an anodic system. Finally, we combined the improved cathode and anode to build a new membrane-less hybrid enzymatic biofuel cell consisting of a mediated anode and a mediator-less cathode.
Nine european national metrology institutes (NMIs) are collaborating in a new project funded by the european metrology research programme (EMRP) to establish traceable dynamic measurement of the mechanical quantities force, pressure, and torque. The aim of this joint research project (JRP) is to develop appropriate calibration methods, mathematical models, and uncertainty evaluation. The duration of the project is 3 years for a global amount of €3.6 million. It began in September 2011.
The one pot-synthesis and use of monolithic biohybrid foams in a continuous flow device reported inhere presents the advantages of covalent stabilization of the enzymes, together with a low steric hindrance between proteins and substrates, an optimized mass transport due to the interconnected macroporous network and a rather simplicity in regard of the column in-situ synthetic path. Those features, concerning transesterification (biodiesel production) enzyme- based catalyzed reaction, provide high enzymatic activity addressed with bio-hybrid catalysts bearing unprecedented endurance of continuous catalysis for a two months period of time.
This work concerns the search for new electrode materials for efficient biofuel cells applications. Using a hard templating method we prepared carbonaceous electrodes modified further with Glucose Oxidase and Os polymer. The glucose electrooxidation current is 13-fold bigger on the porous electrode than on flat glassy carbon for the same enzyme loading. These electrodes are three dimensional and posses hierarchical porosity, to optimize the need for both surface area and efficient fuel delivery Although, the dependence of the catalytic current with the rotation rate suggests that the size and quantity of the macropores is not yet fully optimized, the electrode preparation protocol is simple and low cost, and can be easily adapted to tune the pore sizes. The mechanical strength and the synthetic route allow for the external shape and size of the electrodes to be designed on demand, an important feature to incorporate electrodes into devices.
Pd and Zn - tetraphenyltetrabenzoporphyrins (PdTPTBP and ZnTPTBP), hexacyanin 3 (HITCI), and substituted phthalocyanines were incorporated in solid-state matrices (xerogels) using a sol-gel process. Nonlinear reverse saturable absorption was observed with those materials when they were illuminated with nanosecond laser pulses at 532 nm, or other wavelengths in the visible spectrum (between 450 nm and 630 nm). PdTPTBP doped xerogels exhibit a nonlinear activation threshold of about 10 mJ/cm , which is much lower than the value of 80 mJ/cm2 obtained under similar conditions with classical Al phthalocyanine chloride, or HITCI molecules. Solid state “red active” reverse saturable absorbers can be obtained with substituted phthalocyanines doped xerogels. The different experimental results are discussed using classical 4- energy level diagrams.
We study how the solar magnetic field evolves from antisymmetric (dipolar) to symmetric (quadrupolar) state during the course of its 11-yr cycle. We show that based on equatorial symmetries of the induction equation, flux transport solar mean field dynamo models excite mostly the antisymmetric (dipolar) family whereas a decomposition of the solar magnetic field data reveals that both families should be excited to similar amplitude levels. We propose an alternative solar dynamo solution based on North-South asymmetry of the meridional circulation to better reconcile models and observations.
We briefly present recent progress using the ASH code to model in 3-D the solar convection, dynamo and its coupling to the deep radiative interior. We show how the presence of a self-consistent tachocline influences greatly the organization of the magnetic field and modifies the thermal structure of the convection zone leading to realistic profiles of the mean flows as deduced by helioseismology.
Phase-change materials undergo a change in bonding mechanism upon crystallization, which leads to pronounced modifications of the optical properties and is accompanied by an increase in average bond lengths as seen by extended x-ray absorption fine structure (EXAFS), neutron and x-ray diffraction. The reversible transition between a crystalline and an amorphous phase and its related property contrast are already employed in non-volatile data storage devices, such as rewritable optical discs and electronic memories. The crystalline phase of the prototypical material GeSb2Te4 is characterized by resonant bonding and pronounced disorder, which help to understand their optical and electrical properties, respectively. A change in bonding, however, should also affect the thermal properties, which will be addressed in this study. Based on EXAFS data analyses it will be shown that the thermal and static atomic displacements are larger in the meta-stable crystalline state. This indicates that the bonds become softer in the crystalline phase. At the same time, the bulk modulus increases upon crystallization. These observations are confirmed by the measured densities of phonon states (DPS), which reveal a vibrational softening of the optical modes upon crystallization. This demonstrates that the change of bonding upon crystallization in phase-change materials also has a profound impact on the lattice dynamics and the resulting thermal properties.
Using a hard exotemplate procedure, hierarchically structured carbonaceous foams have been designed, using silica monolith as inorganic template and phenolic resin as carbon precursor. The open cell carbonaceous monoliths exhibit specific surface areas from 500 to 800 m2.g-1, essentially based on microporosity and macropores ranging from 0.05 up to 50 μm. Application as electrochemical energy storage devices have been checked and discuss inhere.