Observation of bipolar moving magnetic features streaming out from a naked spot

Abstract

Context. Mechanisms responsible for active-region formation, evolution, and decay have been investigated by many authors and several common features have been identified. In particular, a key element in the dispersal of the magnetic field seems to be the presence of magnetic elements, called moving magnetic features (MMFs). Aims. We analyze the short-lived sunspot group NOAA 10977, which appeared on the solar disk between 2 and 8 December 2007, to study the details of its emergence and decay phases. Methods. We performed a multi wavelength analysis of the region using images at visible (G band and Hα) and near-IR (Ca ii) wavelengths acquired by both the IBIS instrument and SOT/HINODE, EUV images (17.1 nm) acquired by TRACE, and MDI and SOT magnetograms. Results. The observed region exhibits some peculiarities. During the emergence phase the formation of the f-pore was initially observed, while the p-polarity later formed a naked spot, i.e., a sunspot without a penumbra. We measured a moat flow around this spot, and observed some MMFs streaming out from it during the decay phase. The characteristics of these MMFs allowed us to classify them as type I (U-shaped) MMFs. They were also cospatial with sites of increased brightness both in the photosphere and the chromosphere. Conclusions. The presence of bipolar MMFs in a naked spot indicates that current interpretation of bipolar MMFs, as extensions of the penumbral filaments beyond the sunspot outer boundaries, should be revised, to take into account this observational evidence. We believe that our results provide new insights into improving models of sunspot evolution.