Photospheric Flow Field Related to the Evolution of the Sun's Polar Magnetic Patches Observed by Hinode SOT

TL;DR

This study uses high-resolution Hinode observations to analyze how photospheric plasma motions influence the formation, evolution, and decay of polar magnetic patches, revealing converging flows as a key factor.

Contribution

It provides new evidence linking converging supergranulation flows to the clustering and formation of polar magnetic patches, enhancing understanding of polar magnetic field evolution.

Findings

Polar magnetic patches are surrounded by strong converging flows.

Converging flows are more evident in deeper photospheric Doppler profiles.

Magnetic patches decay through fragmentation and cancellation processes.

Abstract

We investigated the role of photospheric plasma motions in the formation and evolution of polar magnetic patches using time-sequence observations with high spatial resolution. The observations were obtained with the spectropolarimeter on board the Hinode satellite. From the statistical analysis using 75 magnetic patches, we found that they are surrounded by strong converging, supergranulation associated flows during their apparent life time and that the converging flow around the patch boundary is better observed in the Doppler velocity profile in the deeper photosphere. Based on our analysis we suggest that the like-polarity magnetic fragments in the polar region are advected and clustered by photospheric converging flows thereby resulting in the formation of polar magnetic patches. Our observations show that, in addition to direct cancellation magnetic patches decay by fragmentation followed by unipolar disappearance or unipolar disappearance without fragmentation. It is possible that the magnetic patches of existing polarity fragment or diffuse away into smaller elements and eventually cancel out with opposite polarity fragments that reach the polar region around solar cycle maximum. This could be one of the possible mechanisms by which the existing polarity decay during the reversal of the polar magnetic field.