The magnetic structure of surges in small-scale emerging flux regions^⋆

¹ School of Mathematics and Statistics, University of Glasgow, Glasgow, G12 8QW, UK
e-mail: david.mactaggart@glasgow.ac.uk
² White Space Research, Abertay University, Dundee, DD1 1HG, UK
³ INAF–Osservatorio Astrofisico di Catania, via S. Sofia 78, 95123 Catania, Italia
⁴ Dipartimento di Fisica e Astronomia – Sezione Astrofisica, Università di Catania, via S. Sofia 78, 95123 Catania, Italia
⁵ School of Mathematics and Statistics, University of St Andrews, St Andrews, Fife, KY16 9SS, UK

Received: 21 July 2014
Accepted: 30 January 2015

Abstract

Aims. We investigate the relationship between surges and magnetic reconnection during the emergence of small-scale active regions. In particular, to examine how the large-scale geometry of the magnetic field, shaped by different phases of reconnection, guides the flowing of surges.

Methods. We present three flux emergence models. The first model, and the simplest, consists of a region emerging into a horizontal ambient field that is initially parallel to the top of the emerging region. The second model is the same as the first but with an extra smaller emerging region which perturbs the main region. This is added to create a more complex magnetic topology and to test how this complicates the development of surges compared to the first model. The last model has a non-uniform ambient magnetic field to model the effects of emergence near a sunspot field and impose asymmetry on the system through the ambient magnetic field. At each stage, we trace the magnetic topology to identify the locations of reconnection. This allows for field lines to be plotted from different topological regions, highlighting how their geometry affects the development of surges.

Results. In the first model, we identify distinct phases of reconnection. Each phase is associated with a particular geometry for the magnetic field and this determines the paths of the surges. The second model follows a similar pattern to the first but with a more complex magnetic topology and extra eruptions. The third model highlights how an asymmetric ambient field can result in preferred locations for reconnection, subsequently guiding the direction of surges.

Conclusions. Each of the identified phases highlights the close connection between magnetic field geometry, reconnection and the flow of surges. These phases can now be detected observationally and may prove to be key signatures in determining whether or not an emerging region will produce a large-scale (CME-type) eruption.