Plasma Astrophysics Group - Research Interests

1. Transport, Acceleration and Radiation of High Energy Particles

One of the main research interests of the Plasma Astrophysics Group is the problem of transport, acceleration and radiation of high energy particles (electrons and ions) in Astrophysics. Our goal is to estimate the energy distribution function of energetic particles as well as the corresponding radiation emitted in a variety of astrophysical systems such as the Sun, the extragalactic radio sources (jets and hot spots), the AGN's and the Supernovae.
So far work has been done on solar flares and extragalactic radio sources, using coherent (E-fields and waves) and stochastic acceleration mechanisms (Fermi acceleration and multiple shock acceleration). Numerical methods (particle simulation codes) and analytical approach (solutions of the Fokker - Planck equation) gave results for the particle distribution in remarkable agreement with the observations. Recently we have started the investigation of acceleration and transport of the energetic particles in strong turbulent flows, which are very common in Astrophysics.

2. Solar and Stellar Flares

We study the spatiotemporal evolution of solar and stellar active regions and investigate the statistical properties of solar and stellar flares.We model the evolution of an active region using an idea developed initially for complex dynamically systems, namely the concept of Self- Organised Criticality (SOC). The Self-Organisation of certain types of dynamical systems, results in the formation of fractal structures and the appearance of Self-Similarity is well known. Only recently it was argued that certain types of spatially extended dynamical systems can naturally evolve into a state in which the system is marginally stable against a disturbance. In the SOC state, a single perturbation can ignite cascades of events of all sizes with a scale- invariant behaviour, thus providing a connection between nonlinear dynamics and the emergence of Self-Similarity. Self-similar behaviour leads to the emergence of power laws in the events' size frequency distribution. Well-defined power laws are obtained by the observed peak-luminosity frequency distribution of flares. Using a "Three-Dimensional Sandpile Model" embedded in a cellular automaton, we have reproduced the observed frequency distribution of solar hard X-ray bursts. Furthermore, we have developed a model, which seems to reproduce better small events and obeys to a frequency distribution with a much steeper power cut-off. We stress that these weak events correspond to the presently unobserved nanoflares, which, due to the steep power rollover, may account for coronal heating in typical main-sequence (Sun-like) stars.

3. Dynamics of Complex Systems

We demonstrate that adding extended instability criteria to 3D critical-slope sand-pile models has the effect of considerably enhancing the self-organised critical state displayed by these models. We present two extended models, an isotropic one which shows much shorter and much more frequent energy bursts. We illuminate the multifractal nature of the self-organised critical state in both extended models by finding a nontrivial spectrum of generalised correlation dimensions from the time series and by showing that the power-law regions of the distribution functions for different lattice sizes show a scaling behaviour which is ruled by a generalised multifractal scaling transformation. Recently we have investigate the effect of a non-constant (power-law) loading mechanism on SOC.
We also emphasise that a competition process as well as a feedback mechanism are essential features of, and possibly necessary conditions for self-organised criticality. Our conclusions are based on a simple one-dimensional feedback-controlled competition model as well as on existing sandpile cellular automata. We argue that self-organised criticality can be divided into two main classes: in the one class there is a perfect symmetry between the two competing tendencies, and the distribution functions associated with both tendencies show power-law behaviour. In the other case, there is an asymmetry between the competing tendencies and the power laws in the distribution functions associated with one tendency are replaced by exponential ones, while the power-law behaviour in the distribution functions associated with the other tendency remains. A too strong feedback destroys the self-organised critical state: all power laws are replaced by exponential ones.

4. Competition Models for the Evolution of Active Regions

The evolution of active regions is an inherently complex phenomenon. Magnetic fields generated in the base of convection zone follow a chaotic evolution before reaching the surface. Turbulent convection zone splits the magnetic field in fibers (which are the magnetohydrodynamic equivalent for the fluid eddies). The statistical description of the evolution of fibers and the rate of emergence on the solar surface is rather difficult, unless computer simulation is introduced. We use a competition probabilistic model for the formation of magnetic patterns in the stellar surface. We construct a one and two dimensional grid and load initially a small percentage of grid points with magnetic field (we call them A(active)) whereas the rest remain not active (N, unmagnetised). We follow the evolution of magnetised cells in many timesteps. In each timestep a) every A can turn its neighbouring N?s into A?s with a given probability and b) every A can be turned into N with another probability which depends on the numbers of N?s in its nearest neighbourhood. Studying this process numerically in one and two dimensions we found power laws for the number of clusters of active (magnetised) cells versus their size and we calculated the fractal dimension of the patterns formed. Our results are compared with solar observations.