The formation of a penumbra as observed with the German VTT and SoHO/MDI

TL;DR

This study documents the formation of a penumbra in a proto-spot through multi-wavelength solar observations, revealing flux accumulation, flow changes, and magnetic field reorganization during penumbra development.

Contribution

It provides detailed observational insights into the magnetic flux emergence and penumbra formation process in active solar regions.

Findings

Magnetic flux increases during penumbra formation.

Evershed flow initiates with penumbral filaments.

Counter Evershed flows precede stable outflows.

Abstract

The generation of magnetic flux in the solar interior and its transport to the outer solar atmosphere will be in the focus of solar physics research for the next decades. One key-ingredient is the process of magnetic flux emergence into the solar photosphere, and the reorganization to form the magnetic phenomena of active regions like sunspots and pores. On July 4, 2009, we observed a region of emerging magnetic flux, in which a proto-spot without penumbra forms a penumbra within some 4.5 hours. This process is documented by multi-wavelength observations at the German VTT: (a) imaging, (b) data with high resolution and temporal cadence acquired in Fe I 617.3 nm with the 2D imaging spectropolarimter GFPI, and (c) scans with the slit based spectropolarimeter TIP in Fe I 1089.6 nm. MDI contiuum maps and magnetograms are used to follow the formation of the proto-spot, and the subsequent evolution of the entire active region. During the formation of the penumbra, the area and the magnetic flux of the spot increases. The additional magnetic flux is supplied by the adjacent region of emerging magnetic flux: As emerging bipole separate, the poles of the spot polarity migrate towards the spot, and finally merge with it. As more and more flux is accumulated, a penumbra forms. From inversions we infer maps for the magnetic field and the Doppler velocity (being constant along the line-of-sight). We calculate the magnetic flux of the forming spot and of the bipole footpoints that merge with the proto-spot. We witness the onset of the Evershed flow and the associated enhance of the field inclination as individual penumbral filaments form. Prior to the formation of individual penumbral sectors we detect the existence of 'counter' Evershed flows. These in-flows turn into the classical radial Evershed outflows as stable penumbra segments form.