Reza Rezaei, Nazaret Bello Gonzalez, Rolf Schlichenmaier
We study the formation of a sunspot penumbra in the active region NOAA11024.
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 CaIIK filters at the German VTT. The
formation of the penumbra is intimately related to the inclined magnetic field.
Within 4.5 h observing time, the magnetic flux of the penumbra increases from
9.7E+20 to 18.2E+20 Mx, while the magnetic flux of the umbra remains constant
at about 3.8E+20 Mx. Magnetic flux in the immediate surroundings is
incorporated into the spot, and new flux is supplied via small flux patches
(SFPs), which on average have a flux of 2-3E+18 Mx. The spot's flux increase
rate of 4.2E+16 Mx/s 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 deg,
and e) as the penumbra develops, the mean magnetic field strength at the spot
border decreases from 1.0 to 0.8 kG. 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. 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.
View original:
http://arxiv.org/abs/1111.3189
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