Formation of Penumbra in a Sample of Active Regions Observed by the SDO Satellite

Recently, high-resolution observations improved our understanding of the penumbra formation process around sunspots. In particular, two aspects have been carefully investigated: whether the settlement of the penumbra can occur between the main opposite magnetic polarities where new magnetic flux is still emerging, and the establishment of the Evershed flow. In this paper, we present the analysis of twelve active regions (ARs) where both the penumbra formation and the onset of the Evershed flow were observed. We used data acquired by the Helioseismic and Magnetic Imager (HMI) instrument on board the Solar Dynamic Observatory (SDO) satellite analyzing continuum images, magnetograms, and Dopplergrams of the selected ARs. The results obtained in our sample provided the following information about the stable settlement of the penumbra: eight spots formed the first stable penumbral sector in the region between the two opposite polarities, and nine spots formed on the opposite side. Moreover, eleven sunpots showed an inverse Evershed flow (i.e., a plasma motion directed toward the protospot border) before the penumbra formation, which changes within 1-6 hr into the classical Evershed flow as soon as the penumbra forms. Comparing our results with recent observations, we are able to discriminate between the different ways of penumbra formation. Moreover, we suggest that the change from inverse Evershed flow, visible before the penumbra appears, into the classical Evershed flow may be a signature of the formation of penumbral filaments.