The formation of sunspot penumbra

Abstract

Aims. We study the magnetic flux emergence and formation of a sunspot penumbra in the active region NOAA 11024. Methods. We simultaneously observed the Stokes parameters of the photospheric iron lines at 1089.6 nm with the TIP and 617.3 nm with the GFPI spectropolarimeters along with broad-band images using G-band and Ca ii K filters at the German VTT. The photospheric magnetic field vector was reconstructed from an inversion of the measured Stokes profiles. Using the AZAM code, we converted the inclination from line-of-sight (LOS) to the local reference frame (LRF). Results. Individual filaments are resolved in maps of magnetic parameters. The formation of the penumbra is intimately related to the inclined magnetic field. No penumbra forms in areas with strong magnetic field strength and small inclination. corresponds to the merging of one SFP per minute. We also find that, during the formation of the spot penumbra, a) the maximum magnetic field strength of the umbra does not change; b) the magnetic neutral line keeps the same position relative to the umbra; c) the new flux arrives on the emergence side of the spot while the penumbra forms on the opposite side; d) the average LRF inclination of the light bridges decreases from 50• to 37• ; and e) as the penumbra develops, the mean magnetic field strength at the spot border decreases from 1.0 to 0.8 kG.Conclusions. The SFPs associated with elongated granules are the building blocks of structure formation in active regions. During the sunspot formation, their contribution is comparable to the coalescence of pores. Besides a set of critical parameters for the magnetic field, a quiet environment in the surroundings is important for penumbral formation. As remnants of trapped granulation between merging pores, the light bridges are found to play a crucial role in the formation process. They seem to channel the magnetic flux through the spot during its formation. Light bridges are also the locations where the first penumbral filaments form.

Evolution of Active Regions

“…The penumbra forms in a bursty manner, with segments forming first on the outside of the developing sunspot, that is, away from the site of the magnetic flux emergence (Bray and Loughhead, 1964;Bumba, 1965b;Schlichenmaier et al, 2010). These first penumbral segments can be co-spatial with granular regions that become 'trapped' between emerging and merging magnetic fragments (Yang et al, 2003;Rezaei et al, 2012). The accumulation of magnetic flux as the emergence proceeds is clearly an important factor, with some work suggesting that there may be a critical flux value, Φ crit ≤ 5 × 10 20 Mx, which must be reached before the penumbra starts to form (Zwaan, 1987).…”

“…The accumulation of magnetic flux as the emergence proceeds is clearly an important factor, with some work suggesting that there may be a critical flux value, Φ crit ≤ 5 × 10 20 Mx, which must be reached before the penumbra starts to form (Zwaan, 1987). The magnetic field in the penumbra is highly inclined (≥ 60°, Rezaei et al, 2012) which suggests that as the magnetic flux in the sunspot increases due to flux emergence, so too does the inclination of the magnetic field to the vertical on the periphery of the sunspot.…”

Shear and vortex motions in a forming sunspot

“…Schlichenmaier et al (2010a,b) and Rezaei et al (2012) described the observational details and reported on the evolution of the AR, on the formation of the spot and on its magnetic properties.…”

“…The FPI observations were reconstructed with the Göttingen speckle code (de Boer 1996;Janssen 2003;Bello González et al 2007). The Stokes measurements were demodulated to yield the profiles (I λ , Q λ , U λ , V λ ) over the field of view (FOV) as described in Rezaei et al (2012).…”

Multiwavelength spectropolarimetric observations of an Ellerman bomb

“…We only used one node for all parameters except for the temperature (e.g., Beck 2008;Rezaei et al 2012). The magnetic field vector, the line-of-sight (LOS) velocity and temperature stratifications were retrieved from the inversion.…”