Observations of Flux Rope Formation in the Outer Corona

Abstract

In previous studies employing the Large Angle and Spectrometric Coronagraph (LASCO), we identified a class of white-light ejections that separate into incoming and outgoing components at distances of ~3-5 R_solar from Sun center. These events, of which up to several per month are observed during high solar activity, are generally preceded by a gradual outward expansion of faint loops over a period of a day or more. The expansion terminates when the streamer material, in the form of an elongated stalk or a sheetlike structure, suddenly tears apart. The collapsing material is sometimes recognizable as a collection of loops, while the ejected component is usually poorly resolved. Here we describe a streamer detachment observed on 2005 December 11, in which the outgoing component can be clearly identified as a cylindrical flux rope with its ends anchored in the Sun. Based on simple three-dimensional white-light reconstructions, we conclude that in/out pairs in general represent the pinching off of streamer loop arcades to form flux ropes, as seen from different viewing angles.