Space Climate 7 Meeting Abstract

Melinda Nagy (ELTE)

The solar dipole moment at the time of cycle minimum is known to be a good predictor of the amplitude of the subsequent cycle maximum. This dipole moment, in turn, is built up from the contributions of individual bipolar magnetic regions (BMRs), tilted relative to the East-West direction, emerging on the solar surface during the previous cycle, which offers a way towards an early prediction of the subsequent cycle. The initial contributions of these BMRs are, however, modified by surface flux transport (SFT) processes by the end of the cycle. The relation between the initial and final contribution of a BMR to the solar dipole moment is derived in numerical 1D SFT models. Comparison with 2D models confirms that the parameter dependences derived in the 1D model are correct and fairly accurate. These dependences indicate that the final/initial contribution ratio only depends on 3 numerical parameters, without regard to model details such as meridional flow profile. These parameters correspond to the equatorial divergence D of the meridional flow, the magnetic diffusivity eta and the decay time scale tau applied. An analytic approximation of the final/initial dipole moment ratio is also derived. As found in some earlier studies for special cases, the dependence on BMR emergence latitude is approximately Gaussian; the parameters of this Gaussian are now derived as functions of D, eta and tau. In cases where the values of D, eta and tau are sufficiently well specified (e.g. in theoretical models, or potentially after applying appropriate observational constraints for the real Sun) these results allow to completely bypass the use of an SFT model, so that predictions can be made with a simple algebraic process, adding up the computed final dipole contributions of emerging BMRs. We further pose the question whether a significant deviation of the solar dipole moment from the value expected in case if BMRs showed no stochastic scatter around Spoerer's law and Joy's law can be predicted without individually accounting for a very large number of BMRs. Analyzing the effect of BMRs in the 2x2D dynamo model of Lemerle et al. (2017) we find that considering the individual properties of just about a score of BMRs in a cycle (those with the highest calculated impact on the deviation of the dipole moment from its expected value) is sufficient for a reasonably robust forecast of the amplitude of the final dipole moment.

Mode of presentation: oral (Need to be confirmed by the SOC)