Understanding the turbulence of gases in planet-forming protoplanetary disks

Apr 02, 2013 by Scott Gibson
Diagram of a protoplanetary disk around a young star. Angular momentum is transported outward and mass is transported inward and onto the star via disk turbulence. This turbulence is studied using local simulations of a small patch of disk (the 3D box on the lower right; this shows the gas density in the turbulent state). The presence of an external magnetic field similar to the one shown acts to enhance this turbulence in the outer disk regions.

(Phys.org) —Many newly formed stars are surrounded by what are called protoplanetary disks, swirling masses of warm dust and gas that can constitute the core of a developing solar system. Proof of the existence of such disks didn't come until 1994, when the Hubble telescope examined young stars in the Orion Nebula.

Protoplanetary disks may potentially become such as planets and asteroids. But just how they make that transformation will remain a mystery to science until researchers can get a grasp on the disordered movement, or , that characterizes the constituent gases of the disks. Turbulence is what some people regard as "the last great problem."

"By understanding the nature of the gases, we can learn something about how small particles interact with each other, coagulate to become larger particles and then ultimately form planets," says Jake Simon of the University of Colorado, principal investigator of a research project currently taking on two primary challenges in the quest to understand protoplanetary disk turbulence.

Details of the research are contained in an article titled, "Turbulence in the Outer Regions of Protoplanetary Disks. I. Weak Accretion with No Vertical ," in Astrophysical Journal 764 (2013) 66–82.

Correct Models, Accurate Algorithms

The first challenge is developing correct models for the simulations. They have not yet been completely worked out and are subject to considerable uncertainty, Simon explains.

"Our oftentimes have to assume a particular model for the structure of the disk—that is, how density and with distance away from the star," he says. "We must also make assumptions as to the structure and strength of the magnetic field that is present, and for the ionization structure of the disk, which would entail, for example, finding where in the disk the temperature might be hot enough, or some source of radiation powerful enough, to knock off electrons from molecules and atoms, thus creating ions (positively charged)."

This video is not supported by your browser at this time.
Video showing the turbulence in both the gas density and the magnetic field in a protoplanetary disk patch.

The ionization structure is particularly important, Simon explains, because turbulence will be more vigorous where the disk gas is ionized and seeking the balance of electrons (negatively charged).

The second challenge the team is addressing has to do with a technical issue related to the simulations.

"In a particular region in these disks, the electrons are tied to magnetic fields, while the ions are not. This leads to something called the Hall effect and currently, our numerical algorithms cannot accurately capture the nature of this effect," he says.

Discovered by American physicist Edwin Hall in 1879, the Hall effect refers to a voltage-difference that occurs across an electrical conductor. The voltage difference is crossways to an electrical current in the conductor and a magnetic field that is perpendicular to the current.


While Simon and his research team are striving to solve the problems that impede progress, they are also experiencing success. One of the accomplishments has involved gaining an understanding of what is known as ambipolar diffusion, in which both the electrons and the positively charged ions in protoplanetary disks are dragged along by a magnetic field. Ambipolar diffusion is important at low gas densities, which occur at large distances from the star, Simon explains.

This diagram depicts the electric current, or current density (the current per unit volume), that results from all the magnetic turbulence that is induced by the magnetorotational instability. The particular simulation is not subject to ambipolar diffusion but instead represents the ideal limit where no diffusion effects are important.

"If the ions and electrons don't collide with the neutrals frequently enough, ambipolar diffusion acts to damp out the turbulence," he says. "The degree to which this happens has been explored with our high-resolution numerical simulations that we have run on the Kraken supercomputer. We believe we now have a much better understanding of how disks behave in their outer regions, far from the central star."

Simon and his team have used more than 4 million service units (compute hours) on Kraken so far, including an average of approximately 585 cores per run and a single-run high of 18,432 cores. The National Science Foundation's Extreme Science and Engineering Discovery Environment (known as XSEDE) has provided the compute time allocation for the project on Kraken, one of the most powerful supercomputers in academia. Kraken is housed at Oak Ridge National Laboratory and managed by the University of Tennessee's National Institute for Computational Sciences.

"In our simulations, we do not need to simulate the entire disk, but only a small patch of it," he says. "However, this patch does have to be somewhat large and has to have a certain number of resolution points per unit length. This equates to calculations that can be performed only on the largest computers by distributing parts of the calculation across multiple CPUs. Furthermore, we had to run quite a few different simulations to explore different models and magnetic field geometries. With HPC resources like Kraken, we can run several of these high-resolution computer simulations simultaneously. This speeds up our research considerably."

Results, Implications

Simon explains that so far the most important outcome of his team's research into protoplanetary disk turbulence is the discovery that for turbulence levels to be large enough to agree with what observations suggest, a magnetic field perpendicular to the disk must be present; otherwise, the turbulence is rather weak because the ambipolar diffusion quenches the turbulence. He says the perpendicular creates more vigorous turbulence that can overpower the ambipolar diffusion in some regions.

"Another implication of our research is that the turbulent motion of the gas increases rapidly away from the mid-plane of the disk," he says. "Methods now exist that can observe how fast these turbulent motions are in real disks. By comparing these current and coming observations with our theoretical predictions, we will be able to verify our understanding of how disk turbulence works."

Explore further: Partial solar eclipse over the U.S. on Thursday, Oct. 23

More information: dx.doi.org/10.1088/0004-637X/764/1/66

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1 / 5 (6) Apr 02, 2013
This guy needs to find himself a plasma physicist. You don't call a plumber to wire your house, just as you don't call an astrophysicist if you want to now anything..., about anything!
5 / 5 (2) Apr 03, 2013
That guy IS a plasma physicist, you don't even know one when you see one. Note that video shows both the density and magnetic field while the colour coding in the still is the current density. The complexity of these structures is far beyond the toy models you see on crank EU sites.
3 / 5 (6) Apr 03, 2013
This guy needs to find himself a plasma physicist. You don't call a plumber to wire your house, just as you don't call an astrophysicist if you want to now anything..., about anything!

Not withstanding that this work was done by a plasma physicist (as Fleetfoot points out), why would ya find fault with this particular article?

Ya always decry and lament the fact that astrophysics never incorporates plasma science in the science. If only 1.0% of the plasma physicists hold to your world view then ya will only find their work presented in 1.0% of the journals.

So maybe ya need to leave off the astrophysicists and start working on the plasma physicists, they are the ones holding your EU & Plasma Cosmology back. The astrophysical community is only following their leadership (the 99% of plasma physicists) in plasma physics.
1 / 5 (4) Apr 03, 2013
How about a real plasma physicist then, one that knows how to apply the physics of plasma that actually fits the phenomenon. One that has been confused by astrophysicists assumption that this plasma behaves as "ideal gas" or ideal MHD models. Along with those electric currents, which must flow in a closed circuit, there will also be double layers, sheets, instabilities, and other variables that add to the complexity. Not the domain for "elegant" mathematical theories.

As Alfvén pointed out, time after time, the underlying assumptions of cosmologists today "are developed with the most sophisticated mathematical methods and it is only the plasma itself which does not 'understand' how beautiful the theories are and absolutely refuses to obey them."
2.6 / 5 (5) Apr 03, 2013
How about a real plasma physicist then, one that knows how to apply the physics of plasma that actually fits the phenomenon.

But see that is where ya are missing the problem. Ya are saying that the astrophysicists are not up to par with plasma physics. When your problem is the fact that 99% of the plasma physicists are leading them (astray?).

So instead of railing against the astrophysicists, maybe ya should start by getting the plasma physicists on board first. Then work on the astrophysicists who refuse to follow.

Ya can't have it both ways. The astrophysicists don't know plasma physics. But 99% of the plasma physicists are wrong. If ya get the 99% of the plasma guys straightened out, then your job of rehabilitating the astro guys will be much easier.
1 / 5 (3) Apr 03, 2013
Here is a story Q, Wal Thornhill states the reason why this theoretical plasma physicist can't seem to get a grip on this plasma...
"Plasma physics started along two parallel lines. One of them was the hundred-year-old investigation into what was called 'electric discharges in gases'. To a high degree, this approach was experimental and phenomenological, and only very slowly did it reach some degree of theoretical sophistication. Most theoretical physicists looked down on this field which was complicated and awkward. The plasma exhibited striations, double layers, and an assortment of oscillations and instabilities. The electron temperature was often found to be one or two orders of magnitude larger than the gas temperature, with the ion temperature intermediate. In short, it was a field which was not well suited for mathematically elegant theories. The other approach came from the highly developed kinetic theory of ordinary gases." (con't)
1 / 5 (3) Apr 03, 2013
"The theories were mathematically elegant and claimed to derive all of the properties of a plasma from first principles. In reality this was not true. Because of the complexity of the problem, a number of approximations were necessary which were not always appropriate. The theories had very little contact with experimental physics: all awkward and complicated phenomena observed in the laboratory were simply neglected... Theories about plasmas, at the time called ionized gases, were developed without any contact with laboratory plasma work. In spite of this -- or perhaps because of this -- belief in the theories was so strong that they were applied directly to space. One of the results was the Chapman-Ferraro theory (for a review see Akasofu and Chapman, 1972) which became accepted to such an extent that Birkeland's approach was almost completely forgotten." (con't)
1 / 5 (3) Apr 03, 2013
"For thirty or forty years, Birkland's results were often ignored in textbooks and surveys, and all attempts to revive and develop them were neglected."

"The crushing victory of the theoretical approach over the experimental approach lasted only until the theory was to make experimentally verifiable predictions. From the theory, it was concluded that in the laboratory, plasmas could easily be confined in magnetic fields and heated to such temperatures as to make thermonuclear release of energy possible. When attempts were made to construct thermonuclear reactors, a confrontation between the theories and reality was unavoidable -- the results were catastrophic. Although the theories were generally accepted, the plasma itself refused to believe them. This is not to say that Juergens' theory that the sun is an anode is valid. His observation was that the sun appears to violate the 2nd law of thermodynamics in that the heat transfer in the wrong way."
(still con't)
1 / 5 (3) Apr 03, 2013
(almost done)
" My friend Leroy, if I recall correctly, once attempted to explain this by an analogy of a man with a cigarette lighter in his extended arm. Neither suggestion is correct as the sun is not a collection of ordinary gas. It a collection of matter in the plasma form and as such the temperature of the electrons is orders of magnitude higher than the rest of the body (A normal condition for a plasma).

"The approach which Alfven suggested must ignore the elegant and simplistic ordinary gases theory as the electromagnetic forces within a plasma dominate."

You see, there is a difference between the "theoretical plasma physicist" and one who has some experimental background, for me it's a rather simple decision as to who's argument is more valid. As Alfven once said of plasma;
"In order to understand the phenomena in a certain plasma region, it is necessary to map not only the magnetic but also the electric field and the electric currents."
Clearly overlooked above.
5 / 5 (2) Apr 07, 2013
The ability of the ionization structure to turn on, and off, the magnetic-rotational instability is important. It is likely the way that the inner disk water content is constrained so terrestrials don't drown in water. And it seems to be a generic mechanism, meaning there are many Earth analogs out there:

"This is very exciting – after t~1 million years, there is a growing ice-free region right around the Earth's orbit! This resolves the discrepancy of previous models, and provides ample time for an ice-free Earth to evolve. Further work will be necessary to validate this model. If it proves consistent, then we may have reconciled planet formation theory with the water-poor Earth: Regions of low turbulence in the protoplanetary disk allow formation of water-poor terrestrial planets."

5 / 5 (1) Apr 07, 2013
As for the EU magic guy, he was especially cute not knowing plasma physicists when presented with them. Also, why complain when this is one of the few places in the universe where EM effects are important?

"Birk[e]land" [sic!]. Cute, but why confirm this is BS since we know that Thornhill is wrong (ITER will achieve closure)?

Also, currents were not "overlooked", the Hall effect and ambipolar diffusion (mostly neutral currents) were explicitly mentioned.
1 / 5 (2) Apr 08, 2013
"Birk[e]land" [sic!]. Cute, but why confirm this is BS since we know that Thornhill is wrong (ITER will achieve closure)?

Do you know who Kristian BIRKELAND is? Probably the greatest scientist Norway has produced, a man who was to be nominated for the Nobel had it not been for his untimely death. You can be sure his name is spelled BIRKELAND, as is the current that bears his name. It's clear why you don't want to try and prove the theory wrong, you haven't even passed the first branch on the knowledge tree. But hey, you're a Laplander, what should we expect?