arXiv subjects

Eurico Covas

Publications and source records attributed to Eurico Covas.

26 records · Page 2Linked to original sources

arXiv1999

Intermittency as a possible underlying mechanism for solar and stellar variability

We briefly discuss the status of the intermittency hypothesis, according to which the grand minima type variability in solar-type stars may be understood in terms of dynamical intermittency. We review concrete examples which establish this hypothesis in the mean-field setting. We discuss some difficulties and open problems regarding the establishment of this hypothesis in more realistic settings as well as its operationally decidability.

astro-ph↗

arXiv1998

In-out intermittency in PDE and ODE models of axisymmetric mean-field dynamos

Employing some recent results in dynamics of systems with invariant subspaces we find evidence in both truncated and full axisymmetric mean-field dynamo models of a recently discovered type of intermittency, referred to as in-out intermittency. This is a generalised form of on-off intermittency that can occur in systems that are not skew products. As far as we are aware this is the first time detailed evidence has been produced for the occurrence of a particular form of intermittency for such deterministic PDE models and their truncations. The specific signatures of this form of intermittency make it possible in principle to look for such behaviour in solar and stellar observations. Also in view of its generality, this type of intermittency is likely to occur in other physical models with invariant subspaces.

chao-dyn↗

arXiv1998

Transverse instability for non-normal parameters

We consider the behaviour of attractors near invariant subspaces on varying a parameter that does not preserve the dynamics in the invariant subspace but is otherwise generic, in a smooth dynamical system. We refer to such a parameter as ``non-normal''. If there is chaos in the invariant subspace that is not structurally stable, this has the effect of ``blurring out'' blowout bifurcations over a range of parameter values that we show can have positive measure in parameter space. Associated with such blowout bifurcations are bifurcations to attractors displaying a new type of intermittency that is phenomenologically similar to on-off intermittency, but where the intersection of the attractor by the invariant subspace is larger than a minimal attractor. The presence of distinct repelling and attracting invariant sets leads us to refer to this as ``in-out'' intermittency. Such behaviour cannot appear in systems where the transverse dynamics is a skew product over the system on the invariant subspace. We characterise in-out intermittency in terms of its structure in phase space and in terms of invariants of the dynamics obtained from a Markov model of the attractor. This model predicts a scaling of the length of laminar phases that is similar to that for on-off intermittency but which has some differences.

chao-dyn↗

arXiv1997

Axisymmetric mean field dynamos with dynamic and algebraic $α$--quenchings

We study axisymmetric mean field spherical and spherical shell dynamo models, with both dynamic and algebraic $α$--quenchings. Our results show that there are qualitative as well as quantitative differences and similarities between these models. Regarding similarities, both groups of models exhibit symmetric, antisymmetric and mixed modes of behaviour. As regards differences, the important feature in the full sphere models is the occurrence of chaotic behaviour in the algebraic $α$--quenching models. For the spherical shell models with dynamic $α$ the main features include the possibility of multi-attractor regimes with final state sensitivity with respect to small changes in the magnitude of $α$ and the initial parity. We find the effect of introducing a dynamic $α$ is likely to be complicated and depend on the region of the parameter space considered, rather than a uniform change towards simplicity or complexity.

astro-ph↗

arXiv1997

Non-normal parameter blowout bifurcation: an example in a truncated mean field dynamo model

We examine global dynamics and bifurcations occurring in a truncated model of a stellar mean field dynamo. This model has symmetry-forced invariant subspaces for the dynamics and we find examples of transient type I intermittency and blowout bifurcations to transient on-off intermittency, involving laminar phases in the invariant submanifold. In particular, our model provides examples of blowout bifurcations that occur on varying a non-normal parameter; that is, the parameter varies the dynamics within the invariant subspace at the same time as the dynamics normal to it. As a consequence of this we find that the Lyapunov exponents do not vary smoothly and the blowout bifurcation occurs over a range of parameter values rather than a point in the parameter space.

chao-dyn↗

arXiv1997

Crisis-induced intermittency in truncated mean field dynamos

We investigate the detailed dynamics of a truncated $αω$ dynamo model with a dynamic $α$ effect. We find the presence of multiple attractors, including two chaotic attractors with a fractal basin boundary which merge to form a single attractor as the control parameter is increased. By considering phase portraits and the scaling of averaged times of transitions between the two attractors, we demonstrate that this merging is accompanied by a crisis-induced intermittency. We also find a range of parameter values over which the system has a fractal parameter dependence for fixed initial conditions. This is the first time this type of intermittency has been observed in a dynamo model and it could be of potential importance in accounting for some forms of intermittency in the solar and stellar output.

astro-ph↗

arXiv1997

Dynamos with different formulations of a dynamic alpha-effect

We investigate the behaviour of $αΩ$ dynamos with a dynamic $α$, whose evolution is governed by the imbalance between a driving and a damping term. We focus on truncated versions of such dynamo models which are often studied in connection with solar and stellar variability. Given the approximate nature of such models, it is important to study how robust they are with respect to reasonable changes in the formulation of the driving and damping terms. For each case, we also study the effects of changes of the dynamo number and its sign, the truncation order and initial conditions. Our results show that changes in the formulation of the driving term have important consequences for the dynamical behaviour of such systems, with the detailed nature of these effects depending crucially on the form of the driving term assumed, the value and the sign of the dynamo number and the initial conditions. On the other hand, the change in the damping term considered here seems to produce little qualitative effect.

astro-ph↗

arXiv1997

Robustness of truncated alpha-Omega dynamos with a dynamic alpha

In a recent work (Covas et al. 1997), the behaviour and the robustness of truncated $αΩ$ dynamos with a dynamic $α$ were studied with respect to a number of changes in the driving term of the dynamic $α$ equation, which was considered previously by Schmalz and Stix (1991) to be of the form $\sim A_ϕB_ϕ$. Here we review and extend our previous work and consider the effect of adding a quadratic quenching term of the form $α|B|^2$. We find that, as before, such a change can have significant effects on the dynamics of the related truncated systems. We also find intervals of (negative) dynamo numbers, in the system considered by Schmalz and Stix (1991), for which there is sensitivity with respect to small changes in the dynamo number and the initial conditions, similar to what was found in our previous work. This latter behaviour may be of importance in producing the intermittent type of behaviour observed in the sun.

astro-ph↗