PPPL to Host Plasma Astrophysics Conference January 18-21
January, 2010

Accretion occurs in a binary star system when one star is paired with a sufficiently compact star such as a white dwarf, a neutron star, or a black hole (see image: Space Telescope Science Institute). Accretion disks also form during star and planet formation or at center of many active galaxies. How accretion — or the matter collection process — takes place is one of the puzzles scientists will discuss during the Workshop on Opportunities in Plasma Astrophysics at PPPL later this month.

by Tony Rothman

If a total solar eclipse is the most breathtaking spectacle that nature accords humans, then not far behind are the aurora borealis and, to the astronomers able to observe them, the eruption of a solar flare. No one who has witnessed a display of the northern lights can ever forget it; no scientist who lived through the great geomagnetic storm of 1859 — when the aurora were visible in the Caribbean, when one could read by auroral light at 1 A.M., when telegraphs worldwide failed and others operated without batteries — could doubt any longer that the aurora and magnetism were inextricably linked. In our own day, no astronomer doubts that northern lights find their origins in violent events on the sun's surface, where during the eruption of a flare its magnetic field lines cross, reconnect and release the energy of up to a billion megaton bombs. And to a modern plasma physicist, this astronomical scenario is distressingly familiar — it is the same process that causes potentially dangerous disruptions in tokamaks, the mainline approach to fusion energy.

"Plasma physics governs much of the behavior of the visible universe at all scales, from tokamaks to extra-galactic jets that are ten billion times larger than the solar system," emphasizes Stewart Prager, Director of the U.S. Department of Energy's Princeton Plasma Physics Laboratory (PPPL). It is precisely this close relationship between tokamak physics and astrophysical phenomena that provides the motivation for the Workshop on Opportunities in Plasma Astrophysics (WOPA) http://www.pppl.gov/conferences/2010/WOPA/, to be held January 18-21 at PPPL. The workshop, Prager explains, will bring together experimentalists, astronomers and computational scientists to identify the major puzzles at this intersection of laboratory physics and space science and to map out new strategies for better understanding the plasma universe.

One of those major puzzles is magnetic reconnection. PPPL's Hantao Ji, a conference organizer, has spent over a decade investigating this poorly understood phenomenon along with his colleagues Masaaki Yamada, Russell Kulsrud and other PPPL crewmembers. On the sun or in a tokamak, during magnetic reconnection field lines cross, cancel and, as the name implies, reconnect. For years scientists have assumed that the magnetic reconnection of solar flares powers the aurora borealis: the enormous energy released ejects charged particles from the sun's surface. These particles — the solar wind — eventually strike the earth's magnetosphere, where they are trapped and accelerated by the earth's magnetic field lines, exactly as ions are trapped within a fusion device. Colliding with oxygen or nitrogen atoms in the upper atmosphere, the ions excite the atoms, and when the electrons return to their original energy levels, they emit the auroral light.

As simple as this sounds, only in 2008 did the THEMIS satellites provide convincing evidence that magnetic reconnection on the sun really was the trigger. From a theoretical viewpoint reconnection is extremely difficult to understand. According to undergraduate physics, one might well conclude that it is impossible. All physics majors learn that magnetic field lines can't cross and can't break. If they did cross, a charged particle following a particular field line wouldn't know which direction to turn when it came to an intersection with another field line, and this violates the basic principle that the equations of physics uniquely determine a particle's trajectory. Breaking a field line would mean that a loose end is hanging somewhere in space, but physics majors know that field lines always form closed loops. How can magnetic reconnection even happen?

The physics involved is considerably subtler than just described, but James Drake of the University of Maryland, who will chair the WOPA session on magnetic reconnection, says, "There is still no agreed-upon mechanism that explains the breaking of field lines in astrophysical situations. Furthermore, reconnection of solar field lines takes place orders of magnitude faster than current models would suggest, and no one is certain why."

Drake, a member of the theory and modeling team for NASA's Magnetospheric Multiscale Mission (MMS), which consists of four satellites designed to fly directly through the reconnection zone of the earth's magnetosphere, says that obtaining data to explain these theoretical gaps is one of the main goals of the mission. The MMS was approved for implementation in June 2009 and launch is scheduled for 2014.

An X-ray image of a solar coronal structure from Japan's Hinode spacecraft (left) is overlaid with an illustration of the magnetic field lines and the reconnection region (dotted box) that produces it. Computer simulations of electron populations in reconnection regions (right) have been key in deciphering more about how this event unfolds. (Left image is courtesy of the Japanese Space Agency, right image is courtesy of James Drake, University of Maryland, and Michael Shay, University of Delaware.)

Understanding magnetic reconnection is of more than academic interest for the fusion community. As the plasma physicist watching the solar flare knows, reconnection causes "sawtooth" disruptions in tokamaks, in which electrons, heated to a critical temperature, suddenly crash to a lower temperature; plasma confinement is lost, causing potential damage to the vessel wall. Preventing large-scale disruptions will be crucial for ITER, the international fusion experiment being built in France, and in the operation of any commercial fusion plant.

To unravel the basic physics of reconnection, PPPL maintains the small Magnetic Reconnection Experiment (MRX) http://mrx.pppl.gov/, which allows researchers like Yamada and Ji to create reconnection within a plasma. Using the MRX the Princeton team investigates all the outstanding issues in the field: the basic reconnection mechanism, the speed of reconnection and its relationship to solar physics.

Reconnection is hardly the only topic that will be discussed at the upcoming workshop. Another area of great interest is magnetorotational instability. Today, astrophysicists believe that the enormous energy output of quasars (which can exceed an entire galaxy's) is due to matter accreting onto a black hole. As the matter falls in, it forms a disk around the hole, and the disk's properties are crucial in determining how this occurs and how much energy ultimately escapes.

Modeling the accretion disks, though, has proven to be a decades-long endeavor. Naively speaking, no matter whatsoever should fall into a black hole, for the same reason that the planets do not fall into the sun: each planet has an angular momentum, which as any physics major knows, must remain constant; therefore planets maintain their orbits. For matter to accrete into a black hole, it must lose angular momentum. To accomplish this, theorists add friction, or viscosity, to the system, which dissipates angular momentum. A few years ago, however, a team of researchers on PPPL's Magnetorotational Instability Experiment (MRI) http://mri.pppl.gov/, including Ji, Michael Burnin (now at California State University-San Marcos), Ethan Schartman (now at Nova Photonics) and Princeton University's Jeremy Goodman, showed that adding viscosity was itself insufficient to explain the behavior of accretion disks.

What is required is apparently magnetorotational instability (hence MRI). First discovered in 1959 by Evgeny Velikov (and by Chandrasekhar in 1960), who was investigating laboratory plasmas, magnetorotational instability is related to the famous satellite paradox: Friction in the atmosphere causes satellites to lose energy, and so they fall to a lower orbit. But according to Keplerian physics, a satellite on a lower orbit must have a higher velocity. Paradoxically, friction causes satellites to speed up, not slow down. If one imagines two satellites at slightly different orbits connected by a spring, the lower satellite wants to move faster, but the stretched spring slows it down, like friction. The satellite loses energy and falls to an even lower orbit. But the spring also tugs on the slower, upper satellite, tending to speed it up. So this one gains energy and moves to a higher orbit. Thus the separation between the two satellites grows with time.

In 1991 Steven Balbus and John Hawley proposed that such an instability occurs in any astrophysical accretion disk, where the magnetic field acts like the spring, and researchers now generally invoke magnetorotational instability to explain the behavior of quasars.

Whether it is shock waves, turbulence, plasma jets or magnetic reconnection, virtually any phenomenon that takes place in a laboratory fusion experiment takes place somewhere in outer space. All these topics and more will receive attention at the upcoming workshop. "The purpose for bringing together scientists from various disciplines is to heighten interest in jointly solving astrophysical puzzles through plasma physics. This conference will help us identify opportunities for working together and will be a step toward expanding the field of plasma astrophysics," Ji says.

It might be much to expect that an astrophysicist's investigation of plasmas in a black hole accretion disk will make fusion practical, but as Prager adds, "Let's not forget that the entire fusion program sprung from looking toward the stars."