Optimization of surface flux transport models for the solar polar magnetic field

Context. The choice of free parameters in surface flux transport (SFT) models describing the evolution of the large-scale poloidal magnetic field of the Sun is critical for the correct reproduction of the polar magnetic flux built up during a solar cycle, which is known to be a good predictor of the amplitude of the upcoming cycle. Aims. For an informed choice of parameters it is important to understand the effects of and interplay among the various parameters and to optimize the models for the polar magnetic field. Methods. Here we present the results of a large-scale systematic study of the parameter space in an SFT model where the source term representing the net effect of tilted flux emergence was chosen to represent a typical, average solar cycle as described by observations. Results. Comparing the results with observational constraints on the spatiotemporal variation of the polar magnetic field, as seen in magnetograms for the last four solar cycles, we mark allowed and excluded regions in the 3D parameter space defined by the flow amplitude u0, the magnetic diffusivity η and the decay time scale τ, for three different assumed meridional flow profiles. Conclusions. Without a significant decay term in the SFT equation (i.e., for τ > 10 yr) the global dipole moment reverses too late in the cycle for all flow profiles and parameters, providing independent supporting evidence for the need of a decay term, even in the case of identical cycles. An allowed domain is found to exist for τ values in the 5–10 yr range for all flow profiles considered. Generally higher values of η (500–800 km2 s−1) are preferred though some solutions with lower η are still allowed.

Are there local dynamo in solar polar region?

Polar magnetic field, as a component produced by the global dynamo, is thought to be the remant of toroidal magnetic field transported poleward from Sun’s active belt. With the improvement of instruments, more and more observations are challenging the viewpoint. Recently, we identify the bipolar magnetic emergences (BMEs) in the polar region, and find that the distribution of the magnetic axes for these BMEs shows random state, which does not follow the Joy’s law of active region. The result implies the possible existence of local dynamo in the solar polar region.

The Sun’s polar magnetic field: datasets, proxies and theoretical issues

The polar magnetic field of the Sun is a manifestation of certain aspects of the dynamo process and is a good precursor for predicting a sunspot cycle before its onset. Although actual synoptic measurements of this field exist only from the mid-1970s, it has now been possible to determine its evolution from the beginning of the twentieth century with the help of various proxies. The recently developed 3D kinematic dynamo model can study the build-up of the Sun’s polar magnetic field more realistically than the earlier surface flux transport model.

Reversal of Sun’s polar magnetic field in solar cycle

The connection of solar activity expressed by international sunspot (Wolf) numbers in the northern and southern hemispheres of the Sun in the current 24th cycle with the time of polar magnetic field reversal in the corresponding hemisphere is investigated. It was obtained that: – The change of the sign of the polar magnetic field at the southern pole occurs almost a year later than in the north. – The polar magnetic field reversals do not coincide with the maximum activity in each of the hemispheres. In the northern hemisphere, the activity maximum was observed almost one and a half years earlier than the first polar field reversal and two and a half years earlier than the third or final one. In the southern hemisphere, the activity maximum was observed almost a year earlier from the change of the field sign at the pole. – The maximum of the 24th cycle almost coincides with the time of the change of the sign of the magnetic field at the northern pole. – In each of the hemispheres, the change in the sign of a magnetic field in the polar zone above 55 degrees occurred almost two years earlier than the final polar field reversal. The second and third changes of the sign of the total field in the polar zone above 55 degrees occurred shortly after the corresponding polar field reversal. – In the northern hemisphere, the polar field reversals occur at the time of maximum values of the inclination of the heliospheric current sheet, and in the south – almost two years after the maximum inclination of the HCS. – Three-fold polar field reversal at the northern pole occurs at small values of polar magnetic field measured at the Wilcox Solar Observatory while single reversal at the southern pole occurs at sufficiently high value of the corresponding measured polar field.