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)

A synchronized two-dimensional α−Ω\alpha-\Omega model of the solar dynamo

We consider a conventional $\alpha-\Omega$-dynamo model with meridional circulation that exhibits typical features of the solar dynamo, including a Hale cycle period of around 20 years and a reasonable shape of the butterfly diagram. With regard to recent ideas of a tidal synchronization of the solar cycle, we complement this model by an additional time-periodic $\alpha$-term that is localized in the tachocline region. It is shown that amplitudes of some dm/s are sufficient for this $\alpha$-term to become capable of entraining the underlying dynamo. We argue that such amplitudes of $\alpha$ may indeed be realistic, since velocities in the range of m/s are reachable, e.g., for tidally excited magneto-Rossby waves.Comment: 14 pages, 6 figure

Buoyancy-induced time delays in Babcock-Leighton flux-transport dynamo models

The Sun is a magnetic star whose cyclic activity is thought to be linked to internal dynamo mechanisms. A combination of numerical modelling with various levels of complexity is an efficient and accurate tool to investigate such intricate dynamical processes. We investigate the role of the magnetic buoyancy process in 2D Babcock-Leighton dynamo models, by modelling more accurately the surface source term for poloidal field. Methods. To do so, we reintroduce in mean-field models the results of full 3D MHD calculations of the non-linear evolution of a rising flux tube in a convective shell. More specifically, the Babcock-Leighton source term is modified to take into account the delay introduced by the rise time of the toroidal structures from the base of the convection zone to the solar surface. We find that the time delays introduced in the equations produce large temporal modulation of the cycle amplitude even when strong and thus rapidly rising flux tubes are considered. Aperiodic modulations of the solar cycle appear after a sequence of period doubling bifurcations typical of non-linear systems. The strong effects introduced even by small delays is found to be due to the dependence of the delays on the magnetic field strength at the base of the convection zone, the modulation being much less when time delays remain constant. We do not find any significant influence on the cycle period except when the delays are made artificially strong. A possible new origin of the solar cycle variability is here revealed. This modulated activity and the resulting butterfly diagram are then more compatible with observations than what the standard Babcock-Leighton model produces.Comment: 13 pages, 10 figures, accepted for publication in A&

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

Estimation of intake and digestibility of kleingrass from in situ parameters measured in sheep

The voluntary intake and digestibility of kleingrass (Panicum coloratum cv. Verde) were measured in seven periods, comprising one full year. The forage was offered twice a day ad Iibitum to .seven rams, frtted with faces collection bags; each experimental period was composed of 8 days for adaptation and 8 days for collection of data. The dry matter (DM) Intake (DMI) varied from 36.1 to 64.9 glkg MO.75, the digestible dry-matter Intake (DDMI), from 17.4 to 41.9 glkg MO.75 and the in vivo dry matter apparent digestibility (DMD), from 0.471 to 0.667. Daily samples of offered forage were taken and pooled subsamples from each of the seven periods were incubated in nylon bags in the rumen of three Hereford steers. The data were fitted to the exponential equation: p= a + b (1-e~) to estimate p (the proportionate loss of DM at time t of incubation); effective degradability (ED) and lag time (L) were also estimated. The parameters obtained in situ were related to the in vivo results by simple and multiple regression. The correlation coefficients of the rate of degradation {9 and ED with DMI, DDMI and DMD were, respectively, 0.96 and 0.97; 0.97 and 0.96; 0.86 and 0.88. By including, Q, Q and k into a multiple regression analysis, the coefficients of determination (R2) were: DMI: 0.99; DDMI: 0.99 and DMD: 0.91. Within the conditions of this study, the parameters obtained in situ were reasonable estimators of voluntary intake and digestibility.Resúmenes de Trabajos presentados en otras publicaciones (por docentes de la UNLPam.) Publicado en Animal Science, 67: 535-540, 1998