The naked emergence of solar active regions observed with SDO/HMI

TL;DR

This study uses SDO/HMI observations to analyze the emergence of solar active regions, revealing the dynamics of magnetic fields, plasma flows, and the formation of moving dipolar features during early development.

Contribution

It provides detailed observational insights into the magnetic and plasma dynamics during the initial emergence of active regions on the Sun.

Findings

Horizontal fields are dragged to the surface by upflows and linked to brighter elongated granulation.

Main magnetic footpoints are characterized by vertical fields and downflows.

Moving dipolar features (MDFs) are common and linked to magnetic reconnection processes.

Abstract

We take advantage of the HMI/SDO instrument to study the naked emergence of active regions from the first imprints of the magnetic field on the solar surface. To this end, we followed the first 24 hours in the life of two rather isolated ARs that appeared on the surface when they were about to cross the central meridian. We analyze the correlations between Doppler velocities and the orientation of the vector magnetic field finding, consistently, that the horizontal fields connecting the main polarities are dragged to the surface by relatively-strong upflows and are associated to elongated granulation that is, on average, brighter than its surroundings. The main magnetic footpoints, on the other hand, are dominated by vertical fields and downflowing plasma. The appearance of moving dipolar features, MDFs, (of opposite polarity to that of the AR) in between the main footpoints, is a rather common occurrence once the AR reaches a certain size. The buoyancy of the fields is insufficient to lift up the magnetic arcade as a whole. Instead, weighted by the plasma that it carries, the field is pinned down to the photosphere at several places in between the main footpoints, giving life to the MDFs and enabling channels of downflowing plasma. MDF poles tend to drift towards each other, merge and disappear. This is likely to be the signature of a reconnection process in the dipped field lines, which relieves some of the weight allowing the magnetic arcade to finally rise beyond the detection layer of the HMI spectral line.