Emergence of Embedded Magnetic Flux and Its Connections to the Solar Dynamo

Abstract

A strong (~ few × 105 G) and slightly twisted magnetic field (of flux 1023 Mx) can be stored in a dynamo flux tube within the overshoot region provided that the tube has a slightly lower temperature than the background gas, which has a stratification profile γ(d ln ρ/d ln P) - 1 ≥ B2/8πP, where P, ρ, B and γ are the gas pressure, density, magnetic field strength, and adiabatic index. Pumped by the intense vortical downdraft, the outer layer of dynamo flux tube is proposed to possess opposite magnetic helicity from that in the tube interior, as a direct result of the conservation of magnetic helicity. When the field-line twist in the outer layer exceeds some threshold, we show that the helicity-reversal layer can be subject to MHD instabilities that trigger partial flux eruption. In the nonlinear stage, flux ropes of 1022 Mx with magnetic helicity of a definite sign are expected to be ejected into the convection zone, leaving magnetic helicity of the opposite sign to accumulate in the main flux tube. Such a helicity removal mechanism breaks the conservation of magnetic helicity within the dynamo flux tube and can circumvent the recently discovered difficulty (the so-called α quench) for the generation of large-scale fields. We argue that the dynamo α is likely to resume a value of the same order as that of the kinematic turbulent dynamo. We further suggest that the helical flux expulsion is an indispensable component for the solar dynamo to operate properly. A natural prediction of this scenario is that strong sunspots in the same hemisphere should possess magnetic helicity of the same sign, regardless of the solar cycles. Specifically, the helical field lines in strong sunspots should obey the left-hand rule in the northern hemisphere and the right-hand rule in the southern hemisphere.