Tuesday, December 13, 2016

Coronal Mass Ejections Successfully Reproduced in Laboratory

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Image of CME successfully produced in laboratory experiment

For the first time ever solar researchers have successfully replicated an artificial coronal mass ejection in the laboratory as part of a plasma experiment. The researchers, Ha and Bellan, used a plasma gun in concert with an artificial strapping field to create their own flux ropes in their lab and from these CME-eruptions inside a vacuum chamber (see image).   Read more details of their work here, in the abstract to the Ph.D. thesis of Bao N. Quoc Ha:

http://thesis.library.caltech.edu/9061/


The strapping field itself  (that field which secures or holds back any emerging flux ropes) had to be carefully computed so that it decayed as a precise function of height. The reason is that the pair needed to generate the most likely instability - called the "torus instability". This is believed to occur in the Sun's upper atmosphere - namely the corona- when the growth of a flux rope is held back by a magnetic field structure that subsequently decays with increasing altitude.  With ultimate dissipation of the overlying field the eruption of the flux rope and CME then becomes imminent.  One such theoretical model proposed by DeMoulin and Titov exhibits a geometry such as depicted below:
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Fig. 1

The plane can be visualized as the Sun's photosphere and the entire "ring" a solar loop structure with the shaded part extending into the solar atmosphere (e.g. corona) and the lower part submerged. In many respects it is reflective of the solar loop structure shown below:
















Fig. 2

The loop circuit analog shown is for a line-tied arch(loop). By "line tied" I mean that the loop feet are firmly anchored in the photosphere at the level where the plasma beta approaches or exceeds 1. This can engender a "dynamo effect" whereby convective motions spawn electric currents which in turn create powerful magnetic fields in the loop of the solar loop what we call the "force free" region is depicted in cross section with the loop bearing magnetically-confined x-ray plasma.  In the model geometry, above (Fig. 1), the force free coronal magnetic field above the photosphere is constructed using 3 sources: a circular flux rope with thin total current, I,  a pair of charges q and (-q), the latter the image charge of the first below the photosphere, and a line current Io below the surface.  

One model type for a loosened strapping field giving rise to a disrupted prominence which might have a CME associated with it is shown below, after Tsuneta:

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Visible is a pre-existing flux rope (now in 2D) which loses stability and causes a plasma eruption. The CME is then "blown off" via the rising plasmoid. In like manner, the two researchers recreated the conditions in the lab, demonstrating a transition from slow rise to rapid acceleration.  Assume then the detailed toroidal structure is initially in equilibrium, what happens to trigger the torus instability? Well, according to one variation proposed by Yuhong Fan, the configuration becomes unstable to an expansion  D R, when the toroidal radius R attains a critical dimension relative to the separation of the charges +q and -q.

Specifically, one will be looking at the decline of the potential field   B q  with R and when it becomes critically deep.   This is evaluated using a decay index with torus instability occurring when the condition holds:

n = - d ln B q   / d ln R > 1.5

In respect of their CME -generating experiment, Ha and Bellan would likely have taken these considerations into account to a suitable scale to create their strapping magnetic field that decays with increasing altitude (e.g. z).  It is important to note that the experimentally generated CMEs followed the pattern of slow rise giving way to rapid acceleration.(In the case of the mini-CME  the distance between the plasma footpoints, e.g. +y to -y from the toroid geometry, was 8.0 cm.

These results as they stand inspire confidence that similar - and perhaps larger scale experiments in the future - could improve predictions of potentially destructive CMEs.

An encouraging aspect of the current work is an earlier finding that, indeed, the torus instability can be a trigger mechanism for CMEs. See, for example, the paper:

https://arxiv.org/pdf/1401.5936v1.pdf


Again, further demonstrations in the laboratory setting, particularly for the confined plasma under progressively different loop conditions, would go a long way toward supporting these models.

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