Intensification of tilted atmospheric vortices by asymmetric diabatic heating

P\"aschke et al. (JFM, 701, 137--170 (2012)) studied the nonlinear dynamics of strongly tilted vortices subject to asymmetric diabatic heating by asymptotic methods. They found, i.a., that an azimuthal Fourier mode 1 heating pattern can intensify or attenuate such a vortex depending on the relative orientation of tilt and heating asymmetries. The theory originally addressed the gradient wind regime which, asymptotically speaking, corresponds to vortex Rossby numbers of order O(1) in the limit. Formally, this restricts the appicability of the theory to rather weak vortices in the near equatorial region. It is shown below that said theory is, in contrast, uniformly valid for vanishing Coriolis parameter and thus applicable to vortices up to hurricane strength. The paper's main contribution is a series of three-dimensional numerical simulations which fully support the analytical predictions.Comment: 22 pages, 11 figure

Torsional Oscillations in a Global Solar Dynamo

We characterize and analyze rotational torsional oscillations developing in a large-eddy magnetohydrodynamical simulation of solar convection (Ghizaru, Charbonneau, and Smolarkiewicz, Astrophys. J. Lett., 715, L133 (2010); Racine et al., Astrophys. J., 735, 46 (2011)) producing an axisymmetric large-scale magnetic field undergoing periodic polarity reversals. Motivated by the many solar-like features exhibited by these oscillations, we carry out an analysis of the large-scale zonal dynamics. We demonstrate that simulated torsional oscillations are not driven primarily by the periodically-varying large-scale magnetic torque, as one might have expected, but rather via the magnetic modulation of angular-momentum transport by the large-scale meridional flow. This result is confirmed by a straightforward energy analysis. We also detect a fairly sharp transition in rotational dynamics taking place as one moves from the base of the convecting layers to the base of the thin tachocline-like shear layer formed in the stably stratified fluid layers immediately below. We conclude by discussing the implications of our analyses with regards to the mechanism of amplitude saturation in the global dynamo operating in the simulation, and speculate on the possible precursor value of torsional oscillations for the forecast of solar cycle characteristics.Comment: 31 pages, 12 figure

The very many faces of presenilins and the γ-secretase complex

Presenilin is a central, catalytic component of the γ-secretase complex which conducts intramembrane cleavage of various protein substrates. Although identified and mainly studied through its role in the development of amyloid plaques in Alzheimer disease, γ-secretase has many other important functions. The complex seems to be evolutionary conserved throughout the Metazoa, but recent findings in plants and Dictyostelium discoideum as well as in archeons suggest that its evolution and functions might be much more diversified than previously expected. In this review, a selective survey of the multitude of functions of presenilins and the γ-secretase complex is presented. Following a brief overview of γ-secretase structure, assembly and maturation, three functional aspects are analyzed: (1) the role of γ-secretase in autophagy and phagocytosis; (2) involvement of the complex in signaling related to endocytosis; and (3) control of calcium fluxes by presenilins

On the role of tachoclines in solar and stellar dynamos

Rotational shear layers at the boundary between radiative and convective zones, tachoclines, play a key role in the process of magnetic field generation in solar-like stars. We present two sets of global simulations of rotating turbulent convection and dynamo. The first set considers a stellar convective envelope only; the second one, aiming at the formation of a tachocline, considers also the upper part of the radiative zone. Our results indicate that the resulting mean-flows and dynamo properties like the growth rate, saturation energy and mode depend on the Rossby (Ro) number. For the first set of models either oscillatory (with ~2 yr period) or steady dynamo solutions are obtained. The models in the second set naturally develop a tachocline which, in turn, leads to the generation of strong mean magnetic field. Since the field is also deposited into the stable deeper layer, its evolutionary time-scale is much longer than in the models without a tachocline. Surprisingly, the magnetic field in the upper turbulent convection zone evolves in the same time scale as the deep field. These models result in either an oscillatory dynamo with ~30 yr period or in a steady dynamo depending on Ro. In terms of the mean-field dynamo coefficients computed using FOSA, the field evolution in the oscillatory models without a tachocline seems to be consistent with dynamo waves propagating according to the Parker-Yoshimura sign rule. In the models with tachoclines the dynamics is more complex involving other transport mechanisms as well as tachocline instabilities.Comment: 42 pages, 9 figures. Accepted for publication in Ap

What sets the magnetic field strength and cycle period in solar-type stars?

Two fundamental properties of stellar magnetic fields have been determined by observations for solar-like stars with different Rossby numbers (Ro), namely, the magnetic field strength and the magnetic cycle period. The field strength exhibits two regimes: 1) for fast rotation it is independent of Ro, 2) for slow rotation it decays with Ro following a power law. For the magnetic cycle period two regimes of activity, the active and inactive branches, also have been identified. For both of them, the longer the rotation period, the longer the activity cycle. Using global dynamo simulations of solar like stars with Rossby numbers between ~0.4 and ~2, this paper explores the relevance of rotational shear layers in determining these observational properties. Our results, consistent with non-linear alpha^2-Omega dynamos, show that the total magnetic field strength is independent of the rotation period. Yet at surface levels, the origin of the magnetic field is determined by Ro. While for Ro<1 it is generated in the convection zone, for Ro>1 strong toroidal fields are generated at the tachocline and rapidly emerge towards the surface. In agreement with the observations, the magnetic cycle period increases with the rotational period. However, a bifurcation is observed for Ro~1, separating a regime where oscillatory dynamos operate mainly in the convection zone, from the regime where the tachocline has a predominant role. In the latter the cycles are believed to result from the periodic energy exchange between the dynamo and the magneto-shear instabilities developing in the tachocline and the radiative interior.Comment: 43 pages, 14 figures, accepted for publication in The Astrophysical Journa