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Recent work by Mitra et al. (2014) has shown that in strongly stratifiedforced two-layer turbulence with helicity and corresponding large-scale dynamoaction in the lower layer, a magnetic field occurs in the upper layer in theform of sharply bounded bipolar magnetic spots. Here we extend this model tospherical wedge geometry covering the northern hemisphere up to 75{\deg}latitude and an azimuthal extent of 180{\deg}. The kinetic helicity andtherefore also the large-scale magnetic field are strongest at low latitudes.For moderately strong stratification, several bipolar spots form that filleventually the full longitudinal extent. At early times, the polarity of spotsreflects the orientation of the underlying azimuthal field, as expected from{\Omega}-shaped flux loops. At late times their tilt changes such that there isa radial field of opposite orientation at different latitudes separated byabout 10{\deg}. Our model demonstrates the spontaneous formation of spots ofsizes much larger than the pressure scale height. Their tendency to producefilling factors close to unity is argued to be reminiscent of highly activestars. We confirm that strong stratification and strong scale separation areessential ingredients behind magnetic spot formation, which appears to beassociated with downflows at larger depths.

Bipolar magnetic spots from dynamos in stratified spherical shell turbulence