Divergent Horizontal Sub-surface Flows within Active Region 11158

TL;DR

This study investigates the complex horizontal subsurface flows within active region NOAA 11158, revealing divergent flows, their variation with magnetic polarity, and their changes during solar flares using ring-diagram analysis.

Contribution

It provides detailed analysis of the depth-dependent horizontal flow variations and their relation to magnetic structures and flare activity in a rapidly emerging active region.

Findings

Leading and trailing polarity regions move faster than mixed-polarity regions.

Horizontal flow components show opposite signs in different polarity regions, indicating divergence.

Flow velocities decrease sharply during major flares, possibly due to magnetic reorganization.

Abstract

We measure the horizontal subsurface flow in a fast emerging active region (NOAA 11158) using the ring-diagram technique and the HMI high-spatial resolution Dopplergrams. This active region had a complex magnetic structure and displayed significant changes in the morphology during its disk passage. Over the period of six days from 2011 February 11 to 16, the temporal variation in the magnitude of total velocity is found to follow the trend of magnetic field strength. We further analyze regions of individual magnetic polarity within AR 11158 and find that the horizontal velocity components in these sub-regions have significant variation with time and depth. The leading and trailing polarity regions move faster than the mixed-polarity region. Further, both zonal and meridional components have opposite signs for trailing and leading polarity regions at all depths showing divergent flows within the active region. We also find a sharp decrease in the magnitude of total horizontal velocity in deeper layer around major flares. It is suggested that the re-organization of magnetic fields during flares combined with the sunspot rotation decreases the magnitude of horizontal flows or that the flow kinetic energy has been converted into the energy released by flares. After the decline in flare activity and the sunspot rotation, the flows tend to follow the pattern of the magnetic activity.We also observe less variation in the velocity components near the surface but these tend to increase with depth, further demonstrating that the deeper layers are more affected by the topology of active regions.