Magnetic loop emergence within a granule

¹ IGAM/Kanzelhöhe Observatory, Institute of Physics, Universität Graz, Universitätsplatz 5, 8010 Graz, Austria
² Astronomical Institute, Slovak Academy of Sciences, 05960 Tatranská Lomnica, Slovakia e-mail: gomory@astro.sk
³ Instituto de Astrofísica de Canarias (IAC), via Láctea, 38205 La Laguna, Spain
⁴ Astrophysikalisches Institut Potsdam, An der Sternwarte 16, 14482 Potsdam, Germany
⁵ Kiepenheuer-Institut für Sonnenphysik, Schöneckstr. 6, 79104 Freiburg, Germany

Received: 1 July 2009
Accepted: 29 August 2009

Abstract

Aims. We investigate the temporal evolution of magnetic flux emerging within a granule in the quiet-Sun internetwork at disk center.

Methods. We combined IR spectropolarimetry of high angular resolution performed in two Fe i lines at 1565 nm with speckle-reconstructed G-band imaging. We determined the magnetic field parameters by a LTE inversion of the full Stokes vector using the SIR code, and followed their evolution in time. To interpret the observations, we created a geometrical model of a rising loop in 3D. The relevant parameters of the loop were matched to the observations where possible. We then synthesized spectra from the 3D model for a comparison to the observations.

Results. We found signatures of magnetic flux emergence within a growing granule. In the early phases, a horizontal magnetic field with a distinct linear polarization signal dominated the emerging flux. Later on, two patches of opposite circular polarization signal appeared symmetrically on either side of the linear polarization patch, indicating a small loop-like structure. The mean magnetic flux density of this loop was roughly 450 G, with a total magnetic flux of around 3 10¹⁷ Mx. During the ~12 min episode of loop occurrence, the spatial extent of the loop increased from about 1 to 2 arcsec. The middle part of the appearing feature was blueshifted during its occurrence, supporting the scenario of an emerging loop. There is also clear evidence for the interaction of one loop footpoint with a preexisting magnetic structure of opposite polarity. The temporal evolution of the observed spectra is reproduced to first order by the spectra derived from the geometrical model. During the phase of clearest visibility of the loop in the observations, the observed and synthetic spectra match quantitatively.

Conclusions. The observed event can be explained as a case of flux emergence in the shape of a small-scale loop. The fast disappearance of the loop at the end could possibly be due to magnetic reconnection.