KIS Picture of the Month (december 2002):

Turbulent dynamo action in fully convective stars

Red dwarf stars of type M5 or smaller are fully convective, i.e. the whole star is turbulent, while for the Sun turbulence occupies only an outer shell. This turbulent motion generates and enhances magnetic fields, which manifest themselves in the form of solar (or stellar) spots, or flares -- little bursts seen in the whole spectrum from radio to X-rays. We have simulated the evolution of magnetic fields in fully convective stars using a computer model which embeds the star into a cubic box; the picture above shows the magnetic field lines obtained in the simulation.

The following animation shows the spatial structure of temperature in several shells within the star. Blue signifies low temperature, red represents high temperature. It is obvious that the hot matter is concentrated in small spots only, which indicates thin, hot plumes in which matter rises from the core.

The next graph shows that the magnetic field grows even in the absence of rotation (orange line), albeit slower than for moderate (black line) or moderately fast rotation (blue line):

Another representation of the magnetic field shows surfaces of constant magnetic field strength. This represenation shows that the magnetic fields are mainly organized in strongly bent magnetic loops.

Model: Wolfgang Dobler, Michael Stix, Axel Brandenburg

Text: Wolfgang Dobler