Nordita Astrophysics seminars

Bipolar magnetic spots from dynamos in stratified spherical shell turbulence

by Ms Sarah Jabbari (PhD student)

122:026 ()


Recent work by Mitra et al. (2014) has shown that in strongly stratified forced turbulence with helicity and corresponding large-scale dynamo action in the lower part, a magnetic field occurs in the upper parts in the form of sharply bounded bipolar magnetic spots. Here we extend this model to spherical wedge geometry covering the northern hemisphere up to 75◦ latitude and an azimuthal extent of 180◦. The kinetic helicity is maximum at low latitudes, which is therefore also where the magnetic field is strongest. For moderately strong stratification, several bipolar spots form that fill eventually the full longitudinal extent. At early times, the polarity of spots reflects the orientation of the underlying azimuthal field, as expected from Ω-shaped flux loops. At late times their tilt changes such that there is a radial field of opposite orientation at different latitudes separated by about 10◦. Our model demonstrates for the first time the spontaneous formation of spots that are much larger than the pressure scale height. Their surface-filling tendency is argued to be reminiscent of highly active stars.