Understanding Magnetic Explosions

On December 6, 2006, Global Positioning System (GPS) devices suddenly started malfunctioning all over the Earth. The culprit: a solar flare. Solar flares can eject a billion tons of charged particles into space at a speed of 1 million km per hour, disrupting navigation and communications satellites, and sometimes even electrical grids on Earth, while producing bright auroras in the polar regions.

How so much energy can be released so quickly has perplexed scientists for decades. In 1946 Ronald Giovanelli conceived the idea of magnetic reconnection to explain solar flares. The basic idea is that the churning of ionized gas amplifies the magnetic fields in a plasma by twisting and folding them—kinetic energy being converted into magnetic energy. When the magnetic field lines touch or cross, they break, reconnect, and reverse direction (Figure 1). The process may take months or, in the case of a solar flare, as little as 30 minutes, in which case vast amounts of magnetic energy are converted back to kinetic energy with explosive force.

“Magnetic reconnection differs from a conventional explosion in that the energy is not released equally in all directions,” explained James F. Drake, Professor of Physics at the University of Maryland, whose recent research has focused on this subject. “Instead, the plasma flows in from one direction and flows out in another.

Figure 1. (Left) Glowing loops of plasma illuminate the magnetic field structure around a sunspot. The planet Earth would easily fit under one of these loops. (Right) Constantly in motion, the field lines sometimes touch or cross and reverse direction in a process called magnetic reconnection. Open field lines instead of loops show that plasma is being ejected outward as a solar flare. (Images courtesy of NASA) (Click images for larger view)

“Magnetic reconnection has broad importance for almost all areas of plasma physics, including solar flares, storms in the Earth’s magnetosphere, and disruptions in laboratory fusion experiments,” Drake added. “It’s a fascinating topic and a challenging research area.”

One of the puzzles in this sudden release of massive energy is how that much energy could have built up in the first place. If reconnection were always fast and occurred frequently, the magnetic fields would never be strong enough to reach explosive force. A long period of slow reconnections might allow the magnetic energy to accumulate. But why would reconnection happen at two different speeds?

The first mathematical model for magnetic reconnection, known as the Sweet-Parker model, was developed in the late 1950s. This model generates a slow and steady release of energy, but not the explosive events that the theory is supposed to explain. In this model the electrons and ions move together, and the heavier ions slow down the plasma flow. The more recent Hall reconnection model suggests that the movements of ions become decoupled from electrons and magnetic fields in the boundary layers where magnetic field lines reconnect. The result is much faster plasma flow. The signatures of the Hall model have been confirmed by satellite measurements in the magnetosphere and by laboratory experiments, but this model still does not explain the origin of the magnetic explosion.

In two recent papers by Paul Cassak, Drake, and Michael Shay, the two models have converged in a self-consistent model for the spontaneous onset of fast reconnection.[1] The researchers’ calculations showed that slow reconnection can continue for a long time, during which magnetic stresses continue to build up. As progressively stronger magnetic fields are drawn into the reconnection region, when the available free energy crosses a critical threshold, the system abruptly transitions to fast reconnection, manifested as a magnetic explosion.

This new model is consistent with solar flare observations. For example, extreme-ultraviolet observations of the sun’s corona have shown one instance of slow reconnection lasting for 24 hours, followed by fast reconnection lasting for 3 hours. The change was sudden, with no visible trigger mechanism, and the energy released during fast reconnection was comparable to the energy accumulated during slow reconnection.

Figure 2. Computer simulations of island formation and electron acceleration during magnetic reconnection. The electron current is shown at two time steps in (a) and (b); (c) shows the electron temperature, with intense heating caused by electron acceleration along the rims of the islands. (Click image for larger view)

Solar observations have suggested that at least 50% of the energy released during flares is in the form of energetic electrons, and energetic electrons have also been measured during disruptions in laboratory nuclear fusion experiments. The source of these energetic electrons has been a puzzle. Large numbers of these low-mass particles travel at speeds far higher than can be explained by the energy of the moving magnetic field lines that propel them. Drake and Shay, along with Michael Swisdak and Haihong Che, proposed an answer to this question in a paper published in Nature.[2]

In their simulations of magnetic reconnection, the process is more turbulent than it was once thought to be—magnetic islands form, grow, contract, and merge as the field lines converge (Figure 2). The electrons gain speed by reflecting off the ends of contracting islands, just as a ball would gain speed if it were bouncing off two walls that were moving toward one another. But as the temperature in an island goes up, back pressure slows down the shrinking, thus slowing down reconnection and converting more of the magnetic energy into electron acceleration. The repeated interactions of electrons with many islands allow them to be accelerated to high speeds.

“Ours is the first mechanism that explains why electrons gain so much energy during magnetic reconnection,” said Drake. “From a practical standpoint, these new findings can help scientists to better predict which solar storms pose the greatest threat to communications and other satellites. And they may give us a better understanding of how to control plasmas in fusion reactors.”

Drake explained that the strongest confirming evidence for the new theory was the surprising agreement between the model and data from NASA’s WIND satellite. “We were as surprised as the WIND scientists when the distribution of energetic electrons seen by their spacecraft popped right out of our model. Such a match isn’t something you see very often,” he said.

Drake computes at NERSC under the project “Turbulence, Transport and Magnetic Reconnection in High Temperature Plasma,” led by William Dorland. In addition to magnetic reconnection, this project also studies the mechanisms by which plasma particles, energy, and momentum are transported across, rather than along, magnetic field lines—the so-called “anomalous transport” problem.

[1] P. A. Cassak, M. A. Shay, and J. F. Drake, “Catastrophe model for fast magnetic reconnection onset,” Physical Review Letters 95, 235002 (2005); P. A. Cassak, J. F. Drake, and M. A. Shay, “A model for spontaneous onset of fast magnetic reconnection,” Astrophysical Journal 644, L145 (2006).

[2] J. F. Drake, M. Swisdak, H. Che, and M. A. Shay, “Electron acceleration from contracting magnetic islands during reconnection,” Nature 443, 553 (2006).