Computational Astrophysics > Research

Research

The Solar Corona
The most strange thing about the Solar Corona is its high temperature of more than a million degree Kelvin. This is several hundred times the temperature of the underlying layer; the optical surface layer of the Sun. Astrophysicists at the NBI has been involved in the research of the Corona of the Sun since the beginning of the 1990's, and different mechanism to release free magnetic energy from stressed magnetic fields have been investigated using 3D Magneto-Hydro-Dynamical numerical simulations.

Particle Acceleration and Shocks
To our present knowledge Gamma Ray Bursts (GRB) are the most energetic events in the universe. GRBs are thought to be violent explosions of massive stars, which inevitably will be followed by ultra relativistic expanding ejecta. Using a relativistic particle-in-cell (PIC) code, we investigate the micro-physics that governs collisionless shocks. We find that magnetic field generation and particle acceleration may well be generic and unavoidable in collisionless relativistic shocks.

Stellar Dynamos and Magnetism
Magnetic fields are found throughout the Universe; in planets, stars and galaxies. The magnetic fields are thought to be generated and maintained by dynamo mechanisms in which kinetic energy is being converted into magnetic energy. In astrophysical dynamos the non-linear interaction between churning turbulent motions and an initially weak magnetic field first amplifies and later saturates the magnetic activity. The most proximate and well observed example of an astrophysical dynamo is the solar dynamo with its sunspot cycle. Hence the Sun a stepping stone to the understanding of magnetic activity in other astrophysical contexts. We build detailed numerical models of both the Sun and other stars, attempting to understand magnetism across the H-R diagram, from cool dwarf stars and super-giants, to the Sun and rapidly rotating peculiar stars.

Stellar Convection and Oscillations
The main method of this ongoing project is to use the known detailed physics of stellar surface layers (equation of state, opacities, radiative transfer), to obtain a correct entropy jump in the modelled convection zone. The results include ab initio predictions of stellar convection zone depths that agrees to within 0.1% with helioseismic measurements in the solar case. Furthermore, the 3-D average structure is different from (any) 1-D model. The models resolve a major part of a solar oscillation frequency discrepancy.

Magnetic Flux Tubes
Buoyant magnetic flux tubes are an essential part of the framework of the current theories of dynamo action in both the Sun and solar-like stars: it is believed that when formed near the bottom of the convection zone, by a combination of rotation and turbulent convection, toroidal flux tubes buoyantly ascend, in the form of tubular Omega-shaped loops. Rising under the influence of buoyancy and rotational forces, they finally emerge after a few months as slightly asymmetric and tilted bipolar magnetic regions at the surface. Many models of buoyant magnetic flux tubes are based on the thin flux tube approximation that treats the tubes as strings, much thinner than e.g. the local pressure scale height, moving subjected the Coriolis and drag forces. We study the ascent and emergence of buoyant flux tubes under solar-like conditions.

TWiki work area
As a convenient tool, which we use in collaborations with external scientiest, we have a work area, which uses the TWiki mechanism for content and access management.