Linking dissipation-induced instabilities with nonmodal growth: the case of helical magnetorotational instability

The helical magnetorotational instability is known to work for resistive rotational flows with comparably steep negative or extremely steep positive shear. The corresponding lower and upper Liu limits of the shear are continuously connected when some axial electrical current is allowed to flow through the rotating fluid. Using a local approximation we demonstrate that the magnetohydrodynamic behavior of this dissipation-induced instability is intimately connected with the nonmodal growth and the pseudospectrum of the underlying purely hydrodynamic problem.Comment: 5 pages, 4 figure

La fiction comme réplique immédiate à l'Histoire. L'oeuvre d'Oscar-Paul Gilbert entre 1940 et 1944

Les fictions qui ont paru durant les années de l’Occupation nazie, avaient, sans aucun doute, un impact très important sur le public. Nous nous intéresserons, dans ce contexte, aux textes de fictions d’Oscar-Paul Gilbert (1898-1972) qui n’est pas seulement un écrivain, mais un journaliste. Dans cette perspective, le roman est quelque part agencé à la manière du reportage où l’auteur joue sur ces deux techniques : celle du romancier et celle du journaliste. Nous interrogerons essentiellement deux romans du cycle Bauduin – "Carpant" (1941) et "Madeleine Bauduin" (1943), avec quelques parallèles avec le texte non-fictionnel "La Légion des vivants" (1941). Les trois œuvres sont composées dans un contexte historique et politique identique et retracent parallèlement l’immédiat de l’événement

Vortices in self-gravitating disks

Vortices are believed to greatly help the formation of km sized planetesimals by collecting dust particles in their centers. However, vortex dynamics is commonly studied in non-self-gravitating disks. The main goal here is to examine the effects of disk self-gravity on the vortex dynamics via numerical simulations. In the self-gravitating case, when quasi-steady gravitoturbulent state is reached, vortices appear as transient structures undergoing recurring phases of formation, growth to sizes comparable to a local Jeans scale, and eventual shearing and destruction due to gravitational instability. Each phase lasts over 2-3 orbital periods. Vortices and density waves appear to be coupled implying that, in general, one should consider both vortex and density wave modes for a proper understanding of self-gravitating disk dynamics. Our results imply that given such an irregular and rapidly changing, transient character of vortex evolution in self-gravitating disks it may be difficult for such vortices to effectively trap dust particles in their centers that is a necessary process towards planet formation.Comment: to appear in the proceedings of Cool Stars, Stellar Systems and The Sun, 15th Cambridge Workshop, St. Andrews, Scotland, July 21-25, 200

Linear stability analysis of magnetized relativistic jets: the nonrotating case

We perform a linear analysis of the stability of a magnetized relativistic non-rotating cylindrical flow in the aproximation of zero thermal pressure, considering only the m = 1 mode. We find that there are two modes of instability: Kelvin-Helmholtz and current driven. The Kelvin-Helmholtz mode is found at low magnetizations and its growth rate depends very weakly on the pitch parameter. The current driven modes are found at high magnetizations and the value of the growth rate and the wavenumber of the maximum increase as we decrease the pitch parameter. In the relativistic regime the current driven mode is splitted in two branches, the branch at high wavenumbers is characterized by the eigenfunction concentrated in the jet core, the branch at low wavenumbers is instead characterized by the eigenfunction that extends outside the jet velocity shear region.Comment: 22 pages, 13 figures, MNRAS in pres

Linear stability analysis of magnetized relativistic rotating jets

We carry out a linear stability analysis of a magnetized relativistic rotating cylindrical jet flow using the approximation of zero thermal pressure. We identify several modes of instability in the jet: Kelvin-Helmholtz, current driven and two kinds of centrifugal-buoyancy modes -- toroidal and poloidal. The Kelvin-Helmholtz mode is found at low magnetization and its growth rate depends very weakly on the pitch parameter of the background magnetic field and on rotation. The current driven mode is found at high magnetization, the values of its growth rate and the wavenumber, corresponding to the maximum growth, increase as we decrease the pitch parameter of the background magnetic field. This mode is stabilized by rotation, especially, at high magnetization. The centrifugal-buoyancy modes, arising due to rotation, tend also to be more stable when magnetization is increased. Overall, relativistic jet flows appear to be more stable with respect to their non-relativistic counterpart.Comment: 15 pages, 15 figures, accepted for pubblication in MNRA