Bipolar magnetic spots from dynamos in stratified spherical shell turbulence
by
MsSarah Jabbari(PhD student)
→
Europe/Stockholm
122:026
122:026
Description
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.