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The writhe of helical structures in the solar corona  

Tibor Toeroek   Submitted: 2010-04-23 06:11

Helicity is a fundamental property of magnetic fields, conserved in ideal MHD. In flux rope topology, it consists of twist and writhe helicity. Despite the common occurrence of helical structures in the solar atmosphere, little is known about how their shape relates to the writhe, which fraction of helicity is contained in writhe, and how much helicity is exchanged between twist and writhe when they erupt. Here we perform a quantitative investigation of these questions relevant for coronal flux ropes. The decomposition of the writhe of a curve into local and nonlocal components greatly facilitates its computation. We use it to study the relation between writhe and projected S shape of helical curves and to measure writhe and twist in numerical simulations of flux rope instabilities. The results are discussed with regard to filament eruptions and coronal mass ejections (CMEs). We conclude that the writhe is useful in interpreting S shaped coronal structures and in constraining models of eruptions.

Authors: Tibor Toeroek, Mitchell A. Berger, Bernhard Kliem
Projects: None

Publication Status: Accepted by Astronomy and Astrophysics
Last Modified: 2010-04-23 08:59
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Fan-spine topology formation through two-step reconnection driven by twisted flux emergence  

Tibor Toeroek   Submitted: 2009-09-14 08:55

We address the formation of 3D nullpoint topologies in the solar corona by combining Hinode/XRT observations of a small dynamic limb event, which occurred beside a non-erupting prominence cavity, with a 3D zero-beta MHD simulation. To this end, we model the boundary-driven kinematic emergence of a compact, intense, and uniformly twisted flux tube into a potential field arcade that overlies a weakly twisted coronal flux rope. The expansion of the emerging flux in the corona gives rise to the formation of a nullpoint at the interface of the emerging and the pre-existing fields. We unveil a two-step reconnection process at the nullpoint that eventually yields the formation of a broad 3D fan-spine configuration above the emerging bipole. The first reconnection involves emerging fields and a set of large-scale arcade field lines. It results in the launch of a torsional MHD wave that propagates along the arcades, and in the formation of a sheared loop system on one side of the emerging flux. The second reconnection occurs between these newly formed loops and remote arcade fields, and yields the formation of a second loop system on the opposite side of the emerging flux. The two loop systems collectively display an anenome pattern that is located below the fan surface. The flux that surrounds the inner spine field line of the nullpoint retains a fraction of the emerged twist, while the remaining twist is evacuated along the reconnected arcades. The nature and timing of the features which occur in the simulation do qualititatively reproduce those observed by XRT in the particular event studied in this paper. Moreover, the two-step reconnection process suggests a new consistent and generic model for the formation of anemone regions in the solar corona.

Authors: T. Toeroek, G. Aulanier, B. Schmieder, K.K. Reeves, L. Golub
Projects: Hinode/XRT

Publication Status: ApJ (in press)
Last Modified: 2009-09-14 09:33
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Numerical simulations of fast and slow coronal mass ejections  

Tibor Toeroek   Submitted: 2007-05-16 05:33

Solar coronal mass ejections (CMEs) show a large variety in their kinematic properties. CMEs originating in active regions and accompanied by strong flares are usually faster and accelerated more impulsively than CMEs associated with filament eruptions outside active regions and weak flares. It has been proposed more than two decades ago that there are two separate types of CMEs, fast (impulsive) CMEs and slow (gradual) CMEs. However, this concept may not be valid, since the large data sets acquired in recent years do not show two distinct peaks in the CME velocity distribution and reveal that both fast and slow CMEs can be accompanied by both weak and strong flares. We present numerical simulations which confirm our earlier analytical result that a flux-rope CME model permits describing fast and slow CMEs in a unified manner. We consider a force-free coronal magnetic flux rope embedded in the potential field of model bipolar and quadrupolar active regions. The eruption is driven by the torus instability which occurs if the field overlying the flux rope decreases sufficiently rapidly with height. The acceleration profile depends on the steepness of this field decrease, corresponding to fast CMEs for rapid decrease, as is typical of active regions, and to slow CMEs for gentle decrease, as is typical of the quiet Sun. Complex (quadrupolar) active regions lead to the fastest CMEs.

Authors: Toeroek T., Kliem B.
Projects: None

Publication Status: Astronomische Nachrichten 328, No.8 (in press)
Last Modified: 2007-05-16 10:04
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Confined and ejective eruptions of kink-unstable flux ropes  

Tibor Toeroek   Submitted: 2005-07-13 09:20

The ideal helical kink instability of a force-free coronal magnetic flux rope, anchored in the photosphere, is studied as a model for solar eruptions. Using the flux rope model of Titov and Démoulin (1999) as the initial condition in MHD simulations, both the development of helical shape and the rise profile of a confined (or failed) filament eruption (on 2002 May 27) are reproduced in excellent agreement with the observations. By modifying the model such that the magnetic field decreases more rapidly with height above the photosphere, a full (or ejective) eruption of the flux rope is obtained in excellent agreement with the developing helical shape and the exponential-to-linear rise profile of a fast coronal mass ejection (CME) (on 2001 May 15). This confirms that the helical kink instability of a twisted magnetic flux rope can be the mechanism of the initiation and the initial driver of solar eruptions. The agreement of the simulations with properties that are characteristic of many eruptions suggests that they are often triggered by the kink instability. The decrease of the overlying field with height is a main factor in deciding whether the instability leads to a confined event or to a CME.

Authors: Tibor Toeroek and Bernhard Kliem
Projects: None

Publication Status: ApJL (submitted)
Last Modified: 2005-07-13 09:20
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Abstracts by Author
The writhe of helical structures in the solar corona
Fan-spine topology formation through two-step reconnection driven by twisted flux emergence
Numerical simulations of fast and slow coronal mass ejections
Confined and ejective eruptions of kink-unstable flux ropes

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