Source of Sunspots?
Because the tachocline is where solid rotation meets differential rotation, possible source of magnetic flux tubes.
Radiative interior likely contains a "fossil" poloidal magnetic field.
As poloidal field meets differential rotation, field lines are curved into toroidal field.
Toroidal field then undergoes α-effect to produce sunspots on surface.
Due to stability of tachocline, large magnetic flux can be stored for long periods of time.