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Paper: Formation of the Penumbra and Start of the Evershed Flow
Volume: 526, Solar Polarization Workshop 8
Page: 243
Authors: Murabito, M.; Romano, P.; Guglielmino, S. L.; Zuccarello, F.; Solanki, S. K.
Abstract: We analyze high-resolution observations of Active Region NOAA 11490, acquired on 2012 May 28 and 29. Spectropolarimetric measurements of the photospheric lines of Fe I at 617.3 nm and 630.25 nm were taken with the Interferometric BIdimensional Spectrometer (IBIS), mounted on NSO/DST, during about 30 minutes for each day. To study the evolution of continuum intensity, LOS velocity, inclination and strength of the magnetic field during the entire time interval, we also used data taken by SDO/HMI. We used the SIR code to invert the Stokes profiles observed with IBIS, using different initial models to take into account the physical conditions of the plasma in the region of umbra, penumbra, and quiet Sun. From the analysis of the SIR results, we found that, before the formation of the penumbra, the annular zone is characterized by downflows in the inner part. Furthermore, we observed that the onset of the classical Evershed flow occurs on a short time scale, 1–3 hours, while the penumbra is forming. In order to investigate the conditions that lead to the establishment of the classical Evershed flow, we analyzed the evolution of the continuum intensity, LOS velocity, inclination and strength of the magnetic field in a segment in the north-western part of the leading spot. In about 1 hour, we noted a clear evolution from redshift to blueshift in the penumbral filaments along the selected segment. We propose a scenario in which the penumbra is formed by magnetic flux dragged down from the canopy surrounding the initial pore: the Evershed flow starts when the sinking magnetic field dips below the solar surface and magnetoconvection sets in.
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