Sustained magneto-shear instabilities in the solar tachocline

We present nonlinear three-dimensional simulations of the stably-stratified portion of the solar tachocline in which the rotational shear is maintained by mechanical forcing. When a broad toroidal field profile is specified as an initial condition, a clam-shell instability ensues which is similar to the freely-evolving cases studied previously. After the initial nonlinear saturation, the residual mean fields are apparently too weak to sustain the instability indefinitely. However, when a mean poloidal field is imposed in addition to the rotational shear, a statistically-steady state is achieved in which the clam-shell instability is operating continually. This state is characterized by a quasi-periodic exchange of energy between the mean toroidal field and the instability mode with a longitudinal wavenumber m=1. This quasi-periodic behavior has a timescale of several years and may have implications for tachocline dynamics and field emergence patterns throughout the solar activity cycle.Comment: 5 pages, 3 figures (eps format). Fig. 3 also in jpg format. Submitted to Astrophysical Journal Letter

Solar Dynamics, Rotation, Convection and Overshoot

We discuss recent observational, theoretical and modeling progress made in understanding the Sun's internal dynamics, including its rotation, meridional flow, convection and overshoot. Over the past few decades, substantial theoretical and observational effort has gone into appreciating these aspects of solar dynamics. A review of these observations, related helioseismic methodology and inference and computational results in relation to these problems is undertaken here.Comment: 31 pages, 10 figures, Space Science Review

Three-Dimensional Simulations of Solar and Stellar Dynamos: The Influence of a Tachocline

We review recent advances in modeling global-scale convection and dynamo processes with the Anelastic Spherical Harmonic (ASH) code. In particular, we have recently achieved the first global-scale solar convection simulations that exhibit turbulent pumping of magnetic flux into a simulated tachocline and the subsequent organization and amplification of toroidal field structures by rotational shear. The presence of a tachocline not only promotes the generation of mean toroidal flux, but it also enhances and stabilizes the mean poloidal field throughout the convection zone, promoting dipolar structure with less frequent polarity reversals. The magnetic field generated by a convective dynamo with a tachocline and overshoot region is also more helical overall, with a sign reversal in the northern and southern hemispheres. Toroidal tachocline fields exhibit little indication of magnetic buoyancy instabilities but may be undergoing magneto-shear instabilities.Comment: 14 pages, 5 color figures, to appear in Proc. GONG 2008/SOHO XXI Meeting on Solar-Stellar Dynamos as Revealed by Helio and Asteroseismology, held August 15-18, 2008, Boulder, CO, Astronomical Soc. Pac. Conf. Series, volume TB

Characterizing kinetics near a first-order catalytic-poisoning transition

We conside the kinetics near a first-order poisoning transition for simple surface reaction models which exlude nonreactive desorption. Elucidation of the development and growth of the reactive steady state from a near-poisoned state is achieved via an ‘‘oxidation epidemic’’ analysis of the evolution of an empy patch embedded in the poisoned phase. Associated critical exponents vary strongly with reaction rate, and reflect slow kinetics. The increase in epidemic survival probability with initial patch size is smooth, and described by additional exponents, rather than indicating a critical size for survival

New insights about meridional circulation dynamics from 3D MHD global simulations of solar convection and dynamo action

The solar meridional circulation is a "slow", large scale flow that transports magnetic field and plasma throughout the convection zone in the (r, theta) plane and plays a crucial role in controlling the magnetic cycle solutions presented by flux transport dynamo models. Observations indicate that this flow speed varies in anti-phase with the solar cycle at the solar surface. A possible explanation for the source of this variation is based on the fact that inflows into active regions alter the global surface pattern of the meridional circulation. In this work we examine the meridional circulation profile that emerges from a 3D global simulation of the solar convection zone, and its associated dynamics. We find that at the bottom of the convection zone, in the region where the toroidal magnetic field accumulates, the meridional circulation is highly modulated through the action of a magnetic torques and thus provides evidence for a new mechanism to explain the observed cyclic variations

Dynamo Action in the Solar Convection Zone and Tachocline: Pumping and Organization of Toroidal Fields

We present the first results from three-dimensional spherical shell simulations of magnetic dynamo action realized by turbulent convection penetrating downward into a tachocline of rotational shear. This permits us to assess several dynamical elements believed to be crucial to the operation of the solar global dynamo, variously involving differential rotation resulting from convection, magnetic pumping, and amplification of fields by stretching within the tachocline. The simulations reveal that strong axisymmetric toroidal magnetic fields (about 3000 G in strength) are realized within the lower stable layer, unlike in the convection zone where fluctuating fields are predominant. The toroidal fields in the stable layer possess a striking persistent antisymmetric parity, with fields in the northern hemisphere largely of opposite polarity to those in the southern hemisphere. The associated mean poloidal magnetic fields there have a clear dipolar geometry, but we have not yet observed any distinctive reversals or latitudinal propagation. The presence of these deep magnetic fields appears to stabilize the sense of mean fields produced by vigorous dynamo action in the bulk of the convection zone.Comment: 4 pages, 3 color figures (compressed), in press at ApJ

Meridional circulation dynamics in a cyclic convective dynamo

Surface observations indicate that the speed of the solar meridional circulation in the photosphere varies in anti-phase with the solar cycle. The current explanation for the source of this variation is that inflows into active regions alter the global surface pattern of the meridional circulation. When these localized inflows are integrated over a full hemisphere, they contribute to slowing down the axisymmetric poleward horizontal component. The behavior of this large-scale flow deep inside the convection zone remains largely unknown. Present helioseismic techniques are not sensitive enough to capture the dynamics of this weak large-scale flow. Moreover, the large time of integration needed to map the meridional circulation inside the convection zone, also masks some of the possible dynamics on shorter timescales. In this work we examine the dynamics of the meridional circulation that emerges from a 3D MHD global simulation of the solar convection zone. Our aim is to assess and quantify the behavior of meridional circulation deep inside the convection zone where the cyclic large-scale magnetic field can reach considerable strength. Our analyses indicate that the meridional circulation morphology and amplitude are both highly influenced by the magnetic field via the impact of magnetic torques on the global angular momentum distribution. A dynamic feature induced by these magnetic torques is the development of a prominent upward flow at mid-latitudes in the lower convection zone that occurs near the equatorward edge of the toroidal bands and that peaks during cycle maximum. Globally, the dynamo-generated large-scale magnetic field drives variations in the meridional flow, in stark contrast to the conventional kinematic flux transport view of the magnetic field being advected passively by the flow.Centra-ISTGRPS-UdeMNatural Sciences and Engineering Research Council of CanadaNational Science FoundationUniversity of the Algarveinfo:eu-repo/semantics/publishedVersio