The reversal of the Sun's magnetic field in cycle 24

TL;DR

This paper analyzes the stochastic nature of solar polar magnetic field reversals during cycle 24, highlighting irregularities and factors that complicate modeling and prediction of solar magnetic activity.

Contribution

It demonstrates that the polar field reversals are influenced by random surface magnetic flux emergence and activity asymmetries, challenging classical deterministic models.

Findings

Polar field reversals exhibit stochastic elements.

Large active regions and coronal holes influence magnetic evolution.

Surface magnetic activity during cycle 24 is highly irregular.

Abstract

Analysis of synoptic data from the Vector Stokes Magnetograph (VSM) of the Synoptic Optical Long-term Investigations of the Sun (SOLIS) and the NASA/NSO Spectromagnetograph (SPM) at the NSO/Kitt Peak Vacuum Telescope facility shows that the reversals of solar polar magnetic fields exhibit elements of a stochastic process, which may include the development of specific patterns of emerging magnetic flux, and the asymmetry in activity between northern and southern hemispheres. The presence of such irregularities makes the modeling and prediction of polar field reversals extremely hard if possible. In a classical model of solar activity cycle, the unipolar magnetic regions (UMRs) of predominantly following polarity fields are transported polewards due to meridional flows and diffusion. The UMRs gradually cancel out the polar magnetic field of the previous cycle, and re-build the polar field of opposite polarity setting the stage for the next cycle. We show, however, that this deterministic picture can be easily altered by the developing of a strong center of activity, or by the emergence of an extremely large active region, or by a "strategically placed" coronal hole. We demonstrate that the activity occurring during the current cycle 24 may be the result of this randomness in the evolution of the solar surface magnetic field.