Granular Scale Magnetic Flux Cancellations in the Photosphere

TL;DR

This study examines small-scale magnetic flux cancellations near sunspots, revealing the importance of subsurface motions and showing that flux cancellation events are highly time-dependent and occur at very small spatial scales.

Contribution

It provides detailed observations of granular-scale magnetic flux cancellations, highlighting the roles of subsurface flows and the variability of horizontal magnetic fields during cancellations.

Findings

Converging surface flows facilitate magnetic element approach.

Horizontal magnetic fields appear in only one of the observed events.

Flux cancellation events are highly time-dependent and occur at sub-pixel scales.

Abstract

We investigate the evolution of 5 granular-scale magnetic flux cancellations just outside the moat region of a sunspot by using accurate spectropolarimetric measurements and G-band images with the Solar Optical Telescope aboard Hinode. The opposite polarity magnetic elements approach a junction of the intergranular lanes and then they collide with each other there. The intergranular junction has strong red shifts, darker intensities than the regular intergranular lanes, and surface converging flows. This clearly confirms that the converging and downward convective motions are essential for the approaching process of the opposite-polarity magnetic elements. However, motion of the approaching magnetic elements does not always match with their surrounding surface flow patterns in our observations. This suggests that, in addition to the surface flows, subsurface downward convective motions and subsurface magnetic connectivities are important for understanding the approach and collision of the opposite polarity elements observed in the photosphere. We find that the horizontal magnetic field appears between the canceling opposite polarity elements in only one event. The horizontal fields are observed along the intergranular lanes with Doppler red shifts. This cancellation is most probably a result of the submergence (retraction) of low-lying photospheric magnetic flux. In the other 4 events, the horizontal field is not observed between the opposite polarity elements at any time when they approach and cancel each other. These approaching magnetic elements are more concentrated rather than gradually diffused, and they have nearly vertical fields even while they are in contact each other. We thus infer that the actual flux cancellation is highly time dependent events at scales less than a pixel of Hinode SOT (about 200 km) near the solar surface.