A study of the causal relationship between the emergence of a twisted magnetic flux rope and a small Hα two-ribbon flare

¹ Kwasan and Hida Observatories, Kyoto University, Yamashinaku, Kyoto 607-8471, Japan
² Institute of Space and Astronautical Sciences, Sagamihara, Kanagawa 229-8510, Japan
³ Lockheed Martin Solar and Astrophysics Lab, O/L9-41, B/252, 3251 Hannover Street, Palo Alto, CA 94304, USA

Corresponding author: D. H. Brooks, dbrooks@kwasan.kyoto-u.ac.jp

Received: 9 December 2002
Accepted: 19 August 2003

Abstract

We present results from an analysis of a small two-ribbon flare which occurred above emerging flux in solar active region NOAA 8218 on 1998, May 13th and which was observed by the Swedish Vacuum Solar Telescope (SVST) on the island of La Palma, Spain. The relatively simple magnetic morphology and small size of the flare together with the high quality of the SVST observations allow us to examine the essential properties of flares in emerging flux regions in greater detail than before. 
In this paper we compare and contrast the flaring emerging flux region simultaneously with a non-flaring emerging flux region within the same field of view. Unusual magnetic footpoint motions are observed in the flaring region, coincident with the Hα kernels, which result in a high level of shearing of the magnetic neutral line between opposite polarities. The Hα images show dark filament structures which form an inverted S-like shape immediately prior to the flare and then separate after the energy release disrupts the magnetic field. We interpret the motions and structures as strong evidence for the emergence of a twisted magnetic flux rope which developed a sheared configuration with the overlying magnetic field. In contrast the companion region shows separating footpoints, with apparent arch-like filament connections in the Hα images, consistent with the expected appearance of emerging flux. The observations imply that the attachment of the inverted S-shaped structure may be an observational consequence of the magnetic reconnection or untwisting of the field which triggered the flare. We also find some evidence that the increase in magnetic flux is faster in the flaring region.
Finally, we propose a simple schematic model of the emergence of a twisted magnetic flux rope and attached branch which can account for the observed footpoint motions and Hα structures of the flaring region. Such a model can, in principle, induce partial magnetic reconnection in the overlying coronal field and we found some evidence of coronal loop footpoint brightenings which support our conclusions. Our high resolution study supports the results of previous authors that even a small twisted structure in an emerging flux tube can be important for flare production.