Flux emergence within mature solar active regions

Abstract

Aims. Recent insterest in flux emergence within mature active regions has led to several observational studies. Our aim is to model such a scenario and investigate the evolution of the system. Methods. We solve the 3D MHD equations numerically with a Lagrangian-remap scheme. The mature active region is modelled, in the initial condition, with a potential field. The smaller emerging region is a twisted flux tube and is placed between the two polarities of the mature region. The polarities of the new flux are aligned the same way as those of the mature region. The new flux emerges closer to the main negative polarity than the main positive polarity. To investigate the effects of reconnection, the distribution of the parallel electric field is calculated throughout the simulation. The topology of the magnetic field is then studied in regions of interest indicated by the electric field distribution. Results. The expansion of the new negative polarity is restricted due to the curvature of the overlying field and also because it is of the same sign. Reconnection is found to be strongest at low heights (below the corona) and along the outer side of the new positive polarity and its magnetic tongue. The effect of reconnection, in combination with the pressure between the two flux systems, is to resist the expansion of the new flux. The system then relaxes. Large-scale eruptions, such as CMEs, are not expected from the setup considered. At the new negative polarity, the high magnetic pressure can generate strong parallel electric fields which may lead to localized reconnection. The results of the model are in qualitative agreement with observations.

Evidence of Twisted Flux-Tube Emergence in Active Regions

“…They were originally reported by López Fuentes et al (2000) and observed later in other works (see, e.g., Li et al, 2007;Chandra et al, 2009;Luoni et al, 2011). Their presence in emerging flux-ropes has been found by numerical simulations (Archontis and Hood, 2010;MacTaggart, 2011;Jouve, Brun, and Aulanier, 2013). Luoni et al (2011) compared the helicity-sign inferred from the orientation of magnetic tongues, observed in line-of-sight magnetograms, with other known helicity sign proxies for a sample of 40 ARs.…”

Twisted versus braided magnetic flux ropes in coronal geometry

“…One area of significant interest is the emergence of both twisted and untwisted flux ropes through the photosphere into the lower corona, and how they affect the existing field in this region (MacTaggart & Hood 2009a,b;Fang et al 2010;Moreno-Insertis & Galsgaard 2013;MacTaggart 2011). One might ask whether a braided/tangled flux tube would be able to emerge from the convection zone?…”

“…This is based on the numerical experiments of Wilmot-Smith et al (2009, 2011, where a family of braided magnetic fields were created using series of n opposing pairs of rotations through an angle π rad at staggered distances along the tube's length. In this case we use 4 pairs of opposing twist so this would be a B4 field (in the notation of Wilmot-Smith et al (2009) this would be an E4 field, but we prefer "B" to indicate "braided").…”

Flux Emergence (Theory)

“…Many of them use boundary conditions to insert the emerging flux in the domain (Chen and Shibata, 2000;Dubey et al, 2006;Zuccarello et al, 2008Zuccarello et al, , 2009Kusano et al, 2012), whereas others solve the buoyant rise of the emerging flux from below the photosphere self-consistently (MacTaggart and Hood, 2009a;MacTaggart, 2011;Leake et al, 2010). …”