On the formation of a stable penumbra in a region of flux emergence in the Sun

TL;DR

This study investigates how a stable penumbra forms around a pore in a solar active region, revealing that it begins in a flux emergence area with specific magnetic and chromospheric features, using high-resolution solar observations.

Contribution

It demonstrates that penumbra formation initiates in flux emergence regions facing opposite polarity, differing from previous findings, with detailed magnetic and chromospheric analysis.

Findings

Penumbral formation starts in flux emergence regions with opposite polarity.

Magnetic flux density >900 G correlates with penumbra development.

Area of umbra increases during penumbra formation.

Abstract

We studied the formation of the first penumbral sector around a pore in the following polarity of the Active Region (AR) NOAA 11490. We used a high spatial, spectral, and temporal resolution data set acquired by the Interferometric BIdimensional Spectrometer operating at the NSO/Dunn Solar Telescope as well as data taken by the Helioseismic and Magnetic Imager onboard the Solar Dynamics Observatory satellite. On the side towards the leading polarity, elongated granules in the photosphere and an arch filament system (AFS) in the chromosphere are present, while the magnetic field shows a sea-serpent configuration, indicating a region of magnetic flux emergence. We found that the formation of a stable penumbra in the following polarity of the AR begins in the area facing the opposite polarity located below the AFS in the flux emergence region, differently from what found by Schlichenmaier and colleaguestbf. Moreover, during the formation of the first penumbral sector, the area characterized by magnetic flux density larger than 900 G and the area of the umbra increase.