Computer model for nuclear fusion explains solar prominences

15 September 2011
At the surface of the sun, prominences are constantly developing. These arches of magnetised plasma continuously float through the rarefied solar atmosphere due to magnetic forces. A prominence is much larger than the earth and up to several hundred times denser and colder than its surroundings. Plasma physicist Dr. Jan-Willem Blokland from the FOM Institute Rijnhuizen and Prof. Rony Keppens from the University of Leuven now present the first model to describe the moving solar plasma in detail. The two physicists have published two articles about their results in the renowned journal Astronomy & Astrophysics.

Prominence turning into a coronal mass ejection, observed by NASA's Solar Dynamics Observatory

The plasma that our sun and other stars are made up of is material of such a high temperature that its atoms break up into charged particles. Plasma flows like a gas and it also responds strongly to electrical and magnetic fields. The plasma’s own movement also generates such fields. It is almost impossible to compute all of these interactions in a computer model. Therefore researchers often describe plasma with simplified mathematical models in which, for example, variations in the pressure or in the surrounding magnetic field are not included. Blokland and Keppens have managed to realistically model all of these phenomena with the linked computer models FINESSE and PHOENIX.

Cut away diagram of a prominence (plasma arch on the sun's surface). In grey: two condensates, regions with higher pressure than their surroundings. The blue cross-section indicates the pressure in the entire prominence. The model shows that the magnetic lines (red) in a prominence rotate in a helix around the areas with higher pressure.
credit: A&A / J-W Blokland

Seismology – vibrating plasma

A good example of the complex solar plasma are the prominences that float above the surface of the sun. These are enormous arches of extremely hot plasma that are captured in the sun’s magnetic fields. According to theoretical physicist Jan-Willem Blokland, plasma arches are made up of vibrating tubes of charged particles with hot and cold inner shells. "An awful lot happens in these prominences," explains Blokland. "Just as in a guitar string, vibrations occur throughout such a prominence. Everything responds to everything else: wave movements of the plasma, but also those of the electromagnetic fields. The result is a whole series of modes, excited states of the arched plasma."

In the Journal Astronomy & Astrophysics, Blokland and Keppens provide calculations for how the complex interactions in a prominence can be described. With these calculations they can predict how such a tense plasma arch can vibrate and move: it is like having the first catalogue of the various vibrations that are generated by an earthquake on earth. Using such prominence seismology the two researchers think that they will be able to gain a better understanding of the behaviour of our star: "Once you know how vibrations and movements occur in such a tensioned string of plasma then you can deduct far more from satellite observations about what exactly is happening on the sun.”

Cross section of a prominence – the contours indicate the varying pressure in the prominence. The two structures ('condensates') inside the prominence have a higher pressure and lower temperature than the surrounding plasma.
credit: A&A / J-W Blokland
 

Computer model from nuclear fusion

The FINESSE / PHOENIX calculation model that Blokland and Keppens use to describe prominences originates from the world of nuclear fusion. Fusion researchers are trying to simulate the energy source of the sun as a new form of clean, safe energy. In a tokamak reactor they heat up their fuel to a plasma at hundreds of millions of degrees Celsius. Instabilities and turbulences spontaneously arise in that hot plasma. These influence how easily heat dissipates to the edge of the plasma. Being able to control these instabilities is vital for ensuring that the fusion reactor works efficiently. Therefore fusion experts have developed detailed mathematical models to describe the movement in the plasma down to the finest details. Those mathematical models have also proven suitable for understanding the plasma of the sun.

"Fortunately, the natural laws of plasma are scale-invariant: with the same set of equations you can describe both the enormous masses of plasma on the sun and the few cubic metres of plasma in a fusion reactor. Using the same equations you can also model a prominence on the sun. And the converse is also true: by testing our theory on the behaviour of the sun, we might be able to learn more about the plasma in fusion reactors."

Read more

Toward detailed prominence seismology
Astronomy & Astrophysics, Volume 532, August 2011

I. Computing accurate 2.5D magnetohydrodynamic equilibria
    DOI     10.1051/0004-6361/201117013
    Web    http://tinyurl.com/3gy98x6

II. Charting the continuous magnetohydrodynamic spectrum
    DOI    10.1051/0004-6361/201117014
    Web    http://tinyurl.com/3g452cy