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Mid-altitude, mid-latitude mountains are complex environments owing to their Pleistocene glacial heritage, the importance of geomorphic processes on the steep slopes, and the climatic conditions that are often close to periglacial. These factors, along with the fragmentation of the alpine habitats, enhance the topographic and floristic diversity of these environments in northeastern North America. Through case studies, this synthesis underlines the interactions between the geosphere (glacial, paraglacial, and periglacial processes), the atmosphere (climatic fluctuations), and the biosphere (vegetation establishment and evolution to the present day) that explain the low elevation of the northeastern North American alpine environment and that testify to its complexity. Vegetation established earlier in the southern ranges, following the same general trend as the Laurentian Ice Sheet recession. However, local factors such as ice retreat, response to global-scale climate changes, and paraglacial processes acted in synergy to increase the resilience and to influence the occurrence of alpine landscapes. The establishment of the latter environment can therefore be considered to be azonal. Finally, our findings highlight the lack of a conceptual framework, systemic studies, and multi-proxy reconstructions of alpine environments located at the limit of bioclimatic zones controlled by the equilibrium between biostatic and rhexistatic regimes. Les montagnes d'altitude et de latitude moyennes constituent des environnements complexes en raison de leur heritage glaciaire Pleistocene, de l'importance des processus geomorphologiques sur les pentes abruptes et des conditions climatiques qui s'approchent souvent de conditions periglaciaires. Jumeles a une fragmentation des habitats alpins, ces facteurs rehaussent la diversite topographique et floristique de ces environnements dans le nord-est de l'Amerique du Nord. Par des etudes de cas, la presente synthese souligne les interactions entre la geosphere (processus glaciaires, paraglaciaires et periglaciaires), l'atmosphere (fluctuations climatiques) et la biosphere (etablissement de la vegetation et son evolution jusqu'a maintenant) qui expliquent l'elevation peu elevee de l'environnement alpin dans le nord-est de l'Amerique du Nord et qui temoignent de sa complexite. La vegetation s'est etablie plus tot dans les etendues au sud, suivant la meme tendance generale que le retrait de l'Inlandsis laurentidien. Toutefois, des facteurs locaux, tels que le retrait glaciaire, la reaction aux changements climatiques a l'echelle mondiale et les processus paraglaciaires ont agi en synergie pour avoir une influence sur la presence des divers paysages alpins et pour augmenter leur resilience. L'etablissement de ce dernier environnement peut donc etre considere comme etant azonal. Finalement, nos resultats mettent en evidence le manque de cadre conceptuel, d'etudes systemiques et de reconstructions multi-proxy des environnements alpins situes a la limite des zones bioclimatiques controlees par l'equilibre entre les regimes biostatiques et rhexistatiques. [Traduit par la Redaction], Introduction The diverse and combined influences of many orogenies--some in the recent past on a geological timescale--with the many subsequent erosion agents, which depend on climatic variations at local and [...]

The authors investigate a hybrid constant switching frequency current controller that uses both the turn-on and the turn-off current mode and the sliding mode control techniques. It is suitable for one-, two-, or four-quadrant operations and can be used with most topologies of dc--dc converters. It presents some advantages over the conventional peak current mode control. Indeed, it allows using low compensating slope without compromising the stability and it can work in the full range of the duty cycle values. Two methods are presented to study the characteristics of the proposed current controller. First, a high frequency small-signal average model is developed which takes into account the high switching frequency. It allows getting some basics design concerns such as overflow, time response and frequency behavior of the hybrid current controller. Second, a mathematical model derived from chaos theory is developed. It allows investigating the nature of the cycle described by the state trajectory, and determining the operating switching frequency of the current controller. Besides, it will be used to study its robustness properties according to loads parameters variations. Experimental results validate the theoretical approach and prove the effectiveness of this hybrid fixed frequency current controller. Index Terms--Bifurcation's diagram, Floquet multipliers, Jacobian's matrix, peak current mode control, Poincare's surface, sliding mode control, small-signal average model.

In this paper, the modeling and study of a new hybrid current controller is presented. It ensures high dynamic response with a fixed-frequency operation mode, a zero static error, and high robustness properties in regard to system parameters variations. To model the proposed nonlinear current controller, different tools are developed. In a first step, a high-frequency average model is proposed. It allows studying the average dynamic properties (bandwidth, time response, and overflow). To investigate the behavior of the current ripple due to the switching effect, a second model, based on the construction of a 3-D bifurcation diagram and the definition of a form function, is established. This model allows studying the nature of the cycle described by the state trajectory and proving that the system operates with a fixed switching frequency. Design rules of the control parameters of this controller are explained and its robustness properties are tested by numerical simulations and validated by experimental tests. Index Terms--Average model, bifurcation diagram, controller parameters, fixed frequency, form function, modulated hysteresis controller, robustness.