Magnetic flux pumping and the structure of a sunspot penumbra

We propose an overall scenario for the development and maintenance of a sunspot penumbra, in which turbulent magnetic flux pumping plays a key role. Recent high‐resolution observations have revealed arched, “returning” magnetic flux tubes that emerge in the inner or middle penumbra, dive back down below the solar surface near the outer edge of the penumbra, and carry much of the Evershed flow. Some mechanism is required to keep the outer parts of the returning flux tubes submerged in spite of their magnetic buoyancy. We have proposed that the relevant mechanism is downward turbulent pumping of magnetic flux by granular convection in the moat outside the sunspot. This mechanism is demonstrated by means of an appropriate three‐dimensional numerical simulation of turbulent compressible convection in the strongly superadiabatic granulation layer. We suggest that a penumbra first forms through a convectively driven instability at the outer edge of a growing pore. The nonlinear development of this instability produces the filamentary penumbra with its interlocking‐comb magnetic field geometry. Downward flux pumping of some of the nearly horizontal magnetic flux in the dark filaments produces the returning flux tubes, with their associated Evershed flow, and also establishes the subcritical nature of the bifurcation that produces the filamentary penumbra, which explains why there are pores larger than the smallest sunspots.