Magnetorotational instability driven dynamos at low magnetic Prandtl numbers

Numerical simulations of the magnetorotational instability (MRI) with zero initial net flux in a non-stratified isothermal cubic domain are used to demonstrate the importance of magnetic boundary conditions. In fully periodic systems the level of turbulence generated by the MRI strongly decreases as the magnetic Prandtl number (Pm), which is the ratio of kinematic viscosity and magnetic diffusion, is decreased. No MRI or dynamo action below Pm=1 is found, agreeing with earlier investigations. Using vertical field conditions, which allow magnetic helicity fluxes out of the system, the MRI is found to be excited in the range 0.1

Sensory auditory processing and intuitive sound detection : an investigation of musical experts and nonexperts

The auditory system can detect occasional changes (deviants) in acoustic regularities without the need for subjects to focus their attention on the sound material. Deviant detection is reflected in the elicitation of the mismatch negativity component (MMN) of the event-related potentials. In the studies presented in this thesis, the MMN is used to investigate the auditory abilities for detecting similarities and regularities in sound streams. To investigate the limits of these processes, professional musicians have been tested in some of the studies. The results show that auditory grouping is already more advanced in musicians than in nonmusicians and that the auditory system of musicians can, unlike that of nonmusicians, detect a numerical regularity of always four tones in a series. These results suggest that sensory auditory processing in musicians is not only a fine tuning of universal abilities, but is also qualitatively more advanced than in nonmusicians. In addition, the relationship between the auditory change-detection function and perception is examined. It is shown that, contrary to the generally accepted view, MMN elicitation does not necessarily correlate with perception. The outcome of the auditory change-detection function can be implicit and the implicit knowledge of the sound structure can, after training, be utilized for behaviorally correct intuitive sound detection. These results illustrate the automatic character of the sensory change detection function.

Passive vector turbulence

This thesis consists of three articles on passive vector fields in turbulence. The vector fields interact with a turbulent velocity field, which is described by the Kraichnan model. The effect of the Kraichnan model on the passive vectors is studied via an equation for the pair correlation function and its solutions. The first paper is concerned with the passive magnetohydrodynamic equations. Emphasis is placed on the so called "dynamo effect", which in the present context is understood as an unbounded growth of the pair correlation function. The exact analytical conditions for such growth are found in the cases of zero and infinite Prandtl numbers. The second paper contains an extensive study of a number of passive vector models. Emphasis is now on the properties of the (assumed) steady state, namely anomalous scaling, anisotropy and small and large scale behavior with different types of forcing or stirring. The third paper is in many ways a completion to the previous one in its study of the steady state existence problem. Conditions for the existence of the steady state are found in terms of the spatial roughness parameter of the turbulent velocity field.

Sustainable forest management : ecologically sound and socially accepted

This dissertation explored the ecological dimension of ecologically sustainable forest management in boreal forests, and factors of the socio-cultural dimension that affect how the concept of ecologically sustainable forest management is defined. My approach was problem-oriented and generalistic-holistic. I examined associations between the abundances of wildlife groups (grouse, large predators, small predators, ungulates) and Siberian flying squirrels, and their co-occurrence with tree structural characteristics at the regional level. The trade-offs between ecological, social and economic sustainability in forestry were explored at the regional scale. I identified a potential 'shopping basket' of regional indicators for ecologically sustainable forest management, combining the relative abundance of Siberian flying squirrels, a wildlife richness index (WRI) for grouse, diversity indices of saw-timber trees, tree age classes and the proportion of old-growth (> 120 yr) forests. I suggest that the close association between forestry activity, the proportion of young forests (< 40 yr) and a WRI for small predators can be considered as potential 'alarm bells' for regions in which the creation of trade-offs (negative relationships) between economic and ecological components of sustainable forestry is ongoing. Explorative analyses revealed negative relationships between forestry activity and a WRI of 16 game species, the WRI for grouse and tree age diversity. Socially sustainable communities compete less intensively with ecological components of forests than communities where forestry is important. Interestingly, forest ownership types (farmers, other private forest owners, the forestry industry, the State) correlated significantly with the co-occurrence of flying squirrels, grouse and diverse forest structural characteristics rather than, for instance, with the total number of protection areas, suggesting that private forest ownership can lead to increased ecological sustainability. I examined forest actors’ argumentation to identify characteristics that affect the interpretation of ecologically sustainable forest management. Four argumentation frame types were constructed: information, work, experience and own position based. These differed in terms of their emphasis on external experts or own experiences. The closer ecologically sustainable forest management is to the forest actor’s daily life, the more profiled policy tools (counselling, learning through experiences) are needed to guide management behaviour to become more ecologically sound. I illustrated that forest actors interpret, use and understand information through meaningful framing. I analysed the extent to which ecological research information has been perceived in the Forestry Development Centre TAPIO’s recommendations and revised PEFC Finland criteria. We noticed that the political value for decaying wood was much lower in PEFC Finland critera (4 m3) than could be expected as a socially acceptable level (9 m3) or ecologically sound (10-20 m3). I consider it important for scientists to join political discourses and become involved in policy making concerning sustainable forest management to learn to present their results in a way that is reasonable from the user’s perspective.

Local numerical modelling of magnetoconvection and turbulence - implications for mean-field theories

During the last decades mean-field models, in which large-scale magnetic fields and differential rotation arise due to the interaction of rotation and small-scale turbulence, have been enormously successful in reproducing many of the observed features of the Sun. In the meantime, new observational techniques, most prominently helioseismology, have yielded invaluable information about the interior of the Sun. This new information, however, imposes strict conditions on mean-field models. Moreover, most of the present mean-field models depend on knowledge of the small-scale turbulent effects that give rise to the large-scale phenomena. In many mean-field models these effects are prescribed in ad hoc fashion due to the lack of this knowledge. With large enough computers it would be possible to solve the MHD equations numerically under stellar conditions. However, the problem is too large by several orders of magnitude for the present day and any foreseeable computers. In our view, a combination of mean-field modelling and local 3D calculations is a more fruitful approach. The large-scale structures are well described by global mean-field models, provided that the small-scale turbulent effects are adequately parameterized. The latter can be achieved by performing local calculations which allow a much higher spatial resolution than what can be achieved in direct global calculations. In the present dissertation three aspects of mean-field theories and models of stars are studied. Firstly, the basic assumptions of different mean-field theories are tested with calculations of isotropic turbulence and hydrodynamic, as well as magnetohydrodynamic, convection. Secondly, even if the mean-field theory is unable to give the required transport coefficients from first principles, it is in some cases possible to compute these coefficients from 3D numerical models in a parameter range that can be considered to describe the main physical effects in an adequately realistic manner. In the present study, the Reynolds stresses and turbulent heat transport, responsible for the generation of differential rotation, were determined along the mixing length relations describing convection in stellar structure models. Furthermore, the alpha-effect and magnetic pumping due to turbulent convection in the rapid rotation regime were studied. The third area of the present study is to apply the local results in mean-field models, which task we start to undertake by applying the results concerning the alpha-effect and turbulent pumping in mean-field models describing the solar dynamo.

On global solar dynamo simulations

Global dynamo simulations solving the equations of magnetohydrodynamics (MHD) have been a tool of astrophysicists who try to understand the magnetism of the Sun for several decades now. During recent years many fundamental issues in dynamo theory have been studied in detail by means of local numerical simulations that simplify the problem and allow the study of physical effects in isolation. Global simulations, however, continue to suffer from the age-old problem of too low spatial resolution, leading to much lower Reynolds numbers and scale separation than in the Sun. Reproducing the internal rotation of the Sun, which plays a crucual role in the dynamo process, has also turned out to be a very difficult problem. In the present paper the current status of global dynamo simulations of the Sun is reviewed. Emphasis is put on efforts to understand how the large-scale magnetic fields, i.e. whose length scale is greater than the scale of turbulence, are generated in the Sun. Some lessons from mean-field theory and local simulations are reviewed and their possible implications to the global models are discussed. Possible remedies to some of the current issues of the solar simulations are put forward.