THE ORIGIN OF INTRANETWORK FIELDS - A SMALL-SCALE SOLAR DYNAMO

The intranetwork magnetic fields observed on the solar surface consist of flux tubes that are thin enough for their motion to be fully determined by the drag forces exerted on them by turbulent motions. The equations governing such a passive transport of the mean magnetic flux density [B] and of the unsigned flux density [Absolute value of B] in the convective zone of the Sun (assuming a one-dimensional geometry) are derived and discussed: turbulent diffusion and turbulent pumping are found to be the main transport effects. As the timescale of the transpon is much shorter than the solar cycle, the flux density at any instant is given by an equilibrium solution of the transport equations. These solutions are computed and presented. The main conclusions are the following. 1. If no source terms are included in its transport equation, the mean flux density increases by 4 orders of magnitude from the surface to the bottom of the convective zone, showing that turbulent pumping is one of the main mechanisms confining the global dynamo to the bottom of the convective zone. 2. The observed emergence rate of magnetic flux in active regions is not sufficient to sustain the observed mean magnetic fields which must therefore be sustained and continuously refreshed on a timescale of approximately 20 days by the emergence of statistically aligned smaller bipolar flux concentrations (intranetwork fields or small ephemeral regions) from below. 3. The observed unsigned flux density of intranetwork fields is only consistent with the model if a small-scale dynamo mechanism is operating in the convective zone continuously producing unsigned flux; no other likely sources of flux of the correct order of magnitude are known. 4. Modelling the source term corresponding to the small-scale dynamo on the basis of numerical simulation results, the hidden magnetic flux density is predicted to lie between about 2.5 and 7.5 mT. This prediction may be tested by observations in the not too distant future.