Global Dynamics of Subsurface Solar Active Regions

We present three-dimensional numerical simulations of a magnetic loop evolving in either a convectively stable or unstable rotating shell. The magnetic loop is introduced in the shell in such a way that it is buoyant only in a certain portion in longitude, thus creating an \Omega-loop. Due to the action of magnetic buoyancy, the loop rises and develops asymmetries between its leading and following legs, creating emerging bipolar regions whose characteristics are similar to the ones of observed spots at the solar surface. In particular, we self-consistently reproduce the creation of tongues around the spot polarities, which can be strongly affected by convection. We moreover emphasize the presence of ring-shaped magnetic structures around our simulated emerging regions, which we call "magnetic necklace" and which were seen in a number of observations without being reported as of today. We show that those necklaces are markers of vorticity generation at the periphery and below the rising magnetic loop. We also find that the asymmetry between the two legs of the loop is crucially dependent on the initial magnetic field strength. The tilt angle of the emerging regions is also studied in the stable and unstable cases and seems to be affected both by the convective motions and the presence of a differential rotation in the convective cases.Comment: 23 pages (ApJ 2-column format), 19 figures, accepted for publication in Ap

VALORISATION OF BIOMASS-DERIVED MOLECULES BY NOBLE METAL CATALYSTS

Valorisation of different biomass derived molecules was successfully approached and studied in this PhD project. The focus of the thesis was addressed to the catalysts preparation, passing through an accurate catalytic designed, to be then tested in academic and industrially appealing reactions. This approach led to the synthesis of different but equally interesting catalytic systems for the valorisation of substrates derived from the first and second generation of biomass feedstock. An extended study, at first, was conducted on the oxidation of glycerol (1st generation of biomass related), both in alkaline (needed for gold monometallic systems) and free pH (high industrial relevance) conditions. The target reaction was approached starting from the simplest Au/C catalytic systems, to finally move to more complicated and innovative materials: bimetallic once. Initially, the Au on carbon Vulcan (with the highest graphitisation degree) SOL derived catalysts showed a remarkable initial activity (IA= 1091 h-1) in comparison with the other carbonaceous supports (Norit and X40S) and the SMAD derived catalysts. This result pointed out the importance of the protecting agent (a polymer that surrounded the nanoparticles and is solely present for the SOL synthetic route) beside the importance of the support\u2019s features. Similarly, electronic effects ascribed to the interaction with the support of the nanoparticles (i.e. the strong metal support interaction (SMSI) thermally induced on Au4Ag1/TiO2) showed to be the ruling factor to determine the oxidation state of the metals. This latter, subsequentially, influence the catalytic activity: an enhanced initial catalytic activity was detected for the Au4Ag1/TiO2 catalyst (IA= 1616 h-1), in comparison with the Au4Ag1/Al2O3 (IA= 963 h-1). The SMSI have influenced also the stability of the system, avoiding the enlargement of the nanoparticles during the thermal treatments. On the other hand, the SMSI induced the presence of Ag+ species onto the bimetallic nanoparticles titania supported, leading to a quite rapid deactivation of the catalytic system. The thermal treatments pointed out also the importance of the protecting agent (polyvinyl alcohol, PVA): on one side when it is present confers resistance to the system towards the nanoparticles aggregation, on the other when it is removed from the nanoparticles\u2019 surface (by the same thermal treatment), the catalyst acquired an enhanced initial activity. AuPt/TiO2 catalytic systems were subsequentially exploited both in alkaline and free pH conditions. The gold content positively influenced the activity of the catalytic systems in both the conditions. In particular Au9Pt1/TiO2 was the most active catalyst in the alkaline condition (IA= 7389 h 121), and Au6Pt4/TiO2 showed the highest initial activity (IA= 301 h-1) in free pH condition. For all the bimetallic system mentioned and exploited in the valorisation of glycerol, furthermore, a synergistic effect was detected. The importance of gold as modifier to confer resistance to the catalytic system by stabilizing the oxidation state of the second metal was also established. Subsequentially, completely different designed and synthesised catalysts were prepared for the valorisation of substrates related with the 2nd generation of biomass. Bare carbon nanofibers (CNFs) and functionalised CNFs (CNFs-P, CNFs-O and CNFs-N), for instance, were employed as supports for Ru nanoparticles (introduced by incipient wet impregnation). All the catalysts prepared showed activity in the valorisation of cellulose derived molecules. In particular, it was observed how N-containing functionalisation of the support, promoted by a strong interaction with the Ru nanoparticles, led to the highest catalytic activity among the set of catalysts tested for the levulinic acid (LA) hydrogenation (88 % of conversion after 3 h) with a full selectivity to y-valerolactone (GVL). On the other side, exploring the 5-hydroxymethylfurfural (HMF) valorisation, Ru/CNF-N and Ru/CNF-P showed a lower activity but also a change in selectivity. In fact, these latter two catalysts enhanced the formation of ethers due to the reaction between 2,5-dihydroxymethylfuran and/or methylfurfuryl alcohol with the solvent (2-butanol). Similar support effects were also observed in the furfural hydrogenation over platinum nanoparticles (introduced by solvated metal atoms deposition, SMAD) supported on niobia and tailor-made modified niobia. Niobia was hydrothermally synthetized pure and doped with other two different metals (W and Ti, both 10 at.%) to tune the acidity of the system. In particular, we were able to enhance to 0.191 mmolPy/gCAT (W-Nb2O5) and decrees to 0.014 mmolPy/gCAT (Ti-Nb2O5) the acidity of the pure Niobia (0.078 mmolPy/gCAT). Platinum nanoparticles, showing a narrow particle size distribution (1.1-1.2 nm) for all the supports, have allowed a proper study of the acidity effect. The acidity, indeed, showed to be the ruling factor: the most acidic material showed the highest activity coupled with a selectivity addressed to the furan ether products (acid catalysed reaction\u2019s step) at the expenses of furfuryl alcohol (highest selectivity of FA showed for the lowest acid catalyst). Unfortunately, the condition and the type of acidity (Lewis acidity) obtained were not sufficient to observe a high fraction of diols (target product, less than 10 % in selectivity), produced from the ring-opening of the substrate. Lastly, in the benzyl alcohol oxidation (model compound for the lignin) it was highlighted how gold-based materials characterised by comparable nanoparticles dimension (Au-Pd, Au-Pt, Au-Ru and Au-Cu, all supported on carbon) could change the catalytic behaviour and the bimetallic structure just by varying the second metal. For AuPd/C and AuPt/C, for example, alloyed structures were observed. On the other hand, for the case of Ru as second metal, a core-shell structure was found. When Cu was employed, bimetallic nanoparticles with Au:Cu molar ratio lower than the nominal one were detected suggesting the presence of segregated gold nanoparticles. All the catalysts were active and highly selective towards the desired and industrial appealing product (benzaldehyde, selectivity 65 99 %). Only in the case of AuPd/C and AuCu/C, however, a synergistic effect was observed. In particular, the AuPd/C bimetallic sample showed the highest activity (fully conversion of the substrate after 5 min). For the interesting Au-Cu system (the only catalysts that contain a not noble metal), furthermore, the role of the Cu was clarified and the composition effect was studied. The metals were deposited on a carbonaceous support by SMAD technique in order to avoid a protecting agent influence. More in details, it was speculated how Cu, promptly oxidised at CuO (if exposed to air), is responsible of the O2 activation, while the reaction took part at the Au-CuO interface. This reactivity is guided by a specific structure of the bimetallics particles finely characterized: Aucore-CuOshell structure. This last evidence highlighted once more the importance of having a good knowledge and control on the catalyst synthetic routes. Furthermore, synergistic effect was observed for all the active AuCu/C bimetallic systems, even when the amount of gold was very low (Au13Cu1/C, IA= 329 h-1). The highest initial activity, however, was reached with Au4Cu1/C catalysts (IA= 399 h-1). All the active AuCu bimetallic catalysts showed a high selectivity towards the desired product: benzaldehyde ( 65 95%). Good stability against deactivation was also observed. For the Cu-rich sample (Au1Cu17/C) case, distinguished by the negligible activity, it was assumed how the external copper oxide shells, by entirely covering the gold atoms, have repressed any catalytic activities

Exploring the PcycP_{cyc} vs ProtP_{rot} relation with flux transport dynamo models of solar-like stars

Aims: To understand stellar magnetism and to test the validity of the Babcock-Leighton flux transport mean field dynamo models with stellar activity observations Methods: 2-D mean field dynamo models at various rotation rates are computed with the STELEM code to study the sensitivity of the activity cycle period and butterfly diagram to parameter changes and are compared to observational data. The novelty is that these 2-D mean field dynamo models incorporate scaling laws deduced from 3-D hydrodynamical simulations for the influence of rotation rate on the amplitude and profile of the meridional circulation. These models make also use of observational scaling laws for the variation of differential rotation with rotation rate. Results: We find that Babcock-Leighton flux transport dynamo models are able to reproduce the change in topology of the magnetic field (i.e. toward being more toroidal with increasing rotation rate) but seem to have difficulty reproducing the cycle period vs activity period correlation observed in solar-like stars if a monolithic single cell meridional flow is assumed. It may however be possible to recover the $P_{cyc}$ vs $P_{rot}$ relation with more complex meridional flows, if the profile changes in a particular assumed manner with rotation rate. Conclusions: The Babcock-Leighton flux transport dynamo model based on single cell meridional circulation does not reproduce the $P_{cyc}$ vs $P_{rot}$ relation unless the amplitude of the meridional circulation is assumed to increase with rotation rate which seems to be in contradiction with recent results obtained with 3-D global simulations.Comment: 12 pages, 8 figures, accepted for publication by A&A 1: AIM, CEA/DSM-CNRS-Univ. Paris 7, IRFU/SAp, France, 2: D.A.M.T.P., Centre for Mathematical Sciences, Univ. of Cambridge, UK, 3: JILA and Department of Astrophysical and Planetary Sciences, Univ. of Colorado, US

Polar cap magnetic field reversals during solar grand minima: could pores play a role?

We study the magnetic flux carried by pores located outside active regions with sunspots and investigate their possible contribution to the reversal of the global magnetic field of the Sun. We find that they contain a total flux of comparable amplitude to the total magnetic flux contained in polar caps. The pores located at distances of 40--100~Mm from the closest active region have systematically the correct sign to contribute to the polar cap reversal. These pores can predominantly be found in bipolar magnetic regions. We propose that during grand minima of solar activity, such a systematic polarity trend, akin to a weak magnetic (Babcock-Leighton-like) source term could still be operating but was missed by the contemporary observers due to the limited resolving power of their telescopes.Comment: 11 pages, 9 figures, accepted for publication in Astronomy&Astrophysic

Le morcellement informel du foncier en Algérie

Etude originaleInternational audienceThe agricultural reform adopted in Algeria in 1987 dictated the division of the Socialist Agricultural Domains and the creation of Collective and Individual Farms (EAC and EAI), the State keeping land ownership. In spite of the rules imposed by this reform, the obligation of the collective mode of production for the EAC and the banning of tenant farming, the informal division of the farms and farming by tenants have spread very quickly. The qualitative investigation carried out on a sample of 48 farmers in some EAC in the irrigated perimeter of Western Mitidja showed that the failure of the collective production form can be explained by the difficulties encountered by the beneficiaries because of the sudden change from the socialist system to a form of autonomous collective management for which preparation would have been required. Thereafter, the evolution towards an informal division of farms and the development of farming by tenants, both being illegal, resulted in behavioural diversity among the beneficiaries (ten types of behaviours).La réforme agricole adoptée en Algérie en 1987 a décidé le partage des domaines agricoles socialistes (DAS) et la création des exploitations agricoles collectives (EAC) et exploitations agricoles individuelles (EAI), en laissant la propriété de la terre à l'État. En dépit des règles imposées par cette réforme - obligation du mode de production collectif aux EAC et interdiction du mode de faire valoir indirect - le morcellement informel des exploitations et le mode de faire-valoir indirect se sont généralisés très rapidement. L'enquête qualitative réalisée auprès d'un échantillon de 48 agriculteurs dans des EAC du périmètre irrigué de la Mitidja Ouest a montré que l'échec de la forme de production collective s'explique par les difficultés rencontrées par les attributaires à cause du passage brutal du système socialiste à une forme de gestion collective autonome qui aurait nécessité une préparation. Par la suite, l'évolution vers un morcellement informel des exploitations et le développement du mode de faire-valoir indirect, tous deux interdits par la loi, se sont traduits par une diversité comportementale des attributaires (dix types de comportements)

Detection of ultra-weak magnetic fields in Am stars: beta UMa and theta Leo

An extremely weak circularly polarized signature was recently discovered in spectral lines of the chemically peculiar Am star Sirius A. A weak surface magnetic field was proposed to account for the observed polarized signal, but the shape of the phase-averaged signature, dominated by a prominent positive lobe, is not expected in the standard theory of the Zeeman effect. We aim at verifying the presence of weak circularly polarized signatures in two other bright Am stars, beta UMa and theta Leo, and investigating the physical origin of Sirius-like polarized signals further. We present here a set of deep spectropolarimetric observations of beta UMa and theta Leo, observed with the NARVAL spectropolarimeter. We analyzed all spectra with the Least Squares Deconvolution multiline procedure. To improve the signal-to-noise ratio and detect extremely weak signatures in Stokes V profiles, we co-added all available spectra of each star (around 150 observations each time). Finally, we ran several tests to evaluate whether the detected signatures are consistent with the behavior expected from the Zeeman effect. The line profiles of the two stars display circularly polarized signatures similar in shape and amplitude to the observations previously gathered for Sirius A. Our series of tests brings further evidence of a magnetic origin of the recorded signal. These new detections suggest that very weak magnetic fields may well be present in the photospheres of a significant fraction of intermediate-mass stars. The strongly asymmetric Zeeman signatures measured so far in Am stars (featuring a dominant single-sign lobe) are not expected in the standard theory of the Zeeman effect and may be linked to sharp vertical gradients in photospheric velocities and magnetic field strengths

Les agricultures périurbaines méditerranéennes à l'épreuve de la multifonctionnalité : comment fournir aux villes une nourriture et des paysages de qualité ?

International audienceL'agriculture périurbaine occupe une place centrale pour l'approvisionnement des villes mais aussi comme élément de l'aménagement urbain et de paysage pour les citadins. Mais cette agriculture est mise en péril, au nord et au sud de la Méditerranée, par l'accélération de l'urbanisation et l'étalement spatial des villes. Des travaux de recherche réalisés dans une dizaine de pays méditerranéens montrent la capacité d'adaptation de l'agriculture périurbaine qui développe des formes très différenciées mais soulignent aussi que les externalités négatives de ce processus sont nombreuses et variées. Pour répondre aux exigences sociales contemporaines, l'agriculture est mise au défi de développer toutes ses fonctions, non seulement la production pour l'alimentation, mais aussi l'entretien des paysages et la protection de l'environnement. Sous condition de multifonctionnalité, l'agriculture pourrait s'intégrer à des projets de territoire et contribuer au renforcement du lien entre villes et campagnes, notamment par la valorisation des productions traditionnelles locales

Gold-Silver Catalysts: Ruling Factors for Establishing Synergism

DPU and SOL immobilisation have been used to prepare 1 %AuAg/TiO2 with internal ratio 1 : 1 and 4 : 1 which have been studied as fresh, calcined in air at 300 \ub0C and reduced at 550 \ub0C in H2. TEM-EDS, XPS, UV-Vis and CO-DRIFT allowed to characterize the samples in terms of particle size, particle composition, exposure and oxidation state of metals. Correlating these characteristics to the catalytic behaviour we concluded that only Au-rich catalysts show synergistic effect, silver in bimetallic systems appears more resistant to oxidation than in monometallic one, thermal treatment enhances the SMSI thus producing (regardless to the post-treatment) almost the same amount of Au\u3b4+ and also Ag\u3b4+. Catalysts prepared by DPU (calcined in air or reduced in H2) are more active than SOL (fresh or calcined) probably due to the higher presence of gold at the surface