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FEATURE ARTICLE

Reconnecting Magnetic Fields

The huge amounts of energy released from the relinking of magnetic fields in outer space are both mysterious and potentially destructive

James L. Burch, James F. Drake

Figure 1. A twisting solar prominenceClick to Enlarge ImageThe universe is wonderfully and intrinsically dynamic on many scales, especially when it comes to the forms of energy that it creates and uses. The gravitational attraction of matter with itself leads to clumping and eventually to the formation of stars, fueled by the release of nuclear energy of burning hydrogen at their cores. When high-mass stars run out of the nuclear fuel, their pressure is insufficient to balance the gravitational forces and they collapse in catastrophic supernova explosions, sending supersonic blast waves across the cosmos.

Although such gravitational and nuclear phenomena are the dominant sources of energy in the universe (along with, perhaps, dark energy), the swirling of matter as it moves in response to these sources generates another form of energy: the magnetic field. The twisting and turning of material in the core of the Earth, near the surface of the Sun and other stars, and in galaxies across the universe, amplifies magnetic fields in a process known as the dynamo. The strength of magnetic fields, measured in units of gauss, ranges from around a microgauss out in the empty expanses of our Milky Way galaxy, to 0.5 gauss at the surface of the Earth, to 1,000 gauss at the surface of the Sun. A typical refrigerator magnet is about 10 gauss. The highest magnetic fields in the universe, around 1015 gauss, are believed to exist around high-magnetic-field neutron stars, called magnetars because of their enormous fields. The power associated with such a magnetic field is tremendous—the energy in one gallon of magnetar magnetic field corresponds to that in 1018 gallons of gasoline.

Given that magnetic fields and their associated energy exist throughout the universe, it is not surprising that this energy is occasionally released, typically in the form of magnetically driven explosions. Storms in the near-Earth space environment and flares in the corona of our Sun and other stars are examples of explosions driven by the release of magnetic energy. The rates of energy release from magnetar flares dwarf those of the largest supernova explosions.

A fundamental question is, therefore, why and how do these explosions take place? The query is unfortunately not just of academic interest, as these explosions can have serious consequences for our technological society. A large fraction of the magnetic energy from solar flares is released as very high-energy particles; exposure to such particles could sicken astronauts and, in rare extreme solar events, could even prove fatal to them. It can also at least temporarily impair the instruments of spacecraft, manned and unmanned. As the Earth’s own magnetic fields are buffeted by storms from the Sun, large numbers of energetic particles are injected into the Earth’s radiation belts, creating an environment that can disrupt the operations of communications and other satellites, and in rare cases, even cause disruptions in electrical grids on the ground.

Sidebar: A Four-part MysteryClick to Enlarge ImageHow magnetic fields drive these explosions is on the one hand simple and on the other hand very complex and interesting. The simple answer is that adjacent magnetic fields pointing in opposite directions tend to annihilate each other, releasing their magnetic energy and heating the charged particles in the surrounding environment. (See the sidebar “A Four-Part Mystery” at right.) The challenge comes about because simple estimates of the time required for oppositely directed magnetic fields to annihilate one another are long—10,000 years in the case of the Sun’s corona—whereas observed energy release times from such magnetic explosions are tens to hundreds of seconds.

We now know that the mechanism for the fast release of magnetic energy requires that oppositely pointing magnetic fields be torn apart and reattached to their neighbors in a process called magnetic reconnection. This idea was proposed back in the 1950s but remains to date only partially understood, despite intense efforts of many scientists. In the following sections we discuss some of the history of magnetic reconnection, explain the basic concept, explain why the problem has been so challenging and discuss plans for addressing some of the outstanding issues with computer simulations, laboratory experiments, and both remote sensing and in situ measurements in space. The past decade and a half has witnessed noteworthy advances in our understanding, but a breakthrough requires a highly sophisticated space experiment, the NASA Magnetospheric Multiscale mission, which is now in the implementation phase and currently scheduled for launch in 2014.








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