Can surface flux transport account for the weak polar field in cycle 23?

TL;DR

This study investigates whether surface flux transport models can explain the weak polar magnetic fields observed during cycle 23, highlighting the impact of meridional flow and sunspot tilt angle variations.

Contribution

The paper demonstrates that adjusting meridional flow speed or sunspot tilt angles in models can reproduce the weak polar fields of cycle 23.

Findings

Increasing meridional flow by 55% reproduces observed polar fields.

Reducing sunspot tilt angles by 28% also matches observations.

Alterations in these parameters affect the timing of polar field reversals.

Abstract

To reproduce the weak magnetic field on the polar caps of the Sun observed during the declining phase of cycle 23 poses a challenge to surface flux transport models since this cycle has not been particularly weak. We use a well-calibrated model to evaluate the parameter changes required to obtain simulated polar fields and open flux that are consistent with the observations. We find that the low polar field of cycle 23 could be reproduced by an increase of the meridional flow by 55% in the last cycle. Alternatively, a decrease of the mean tilt angle of sunspot groups by 28% would also lead to a similarly low polar field, but cause a delay of the polar field reversals by 1.5 years in comparison to the observations.