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Researchers Uncover Piece to Puzzle of Magnetic Explosions

James Drake illustrates how two oppositely directed magnetic fields annihilate each other when they come close together, producing an explosion  in a process called magnetic reconnection.
Media Credit: Cynthia Mitchel
James Drake illustrates how two oppositely directed magnetic fields annihilate each other when they come close together, producing an explosion in a process called magnetic reconnection.
Magnetic reconnection on the sun´s surface.
Magnetic reconnection on the sunīs surface.
A team of scientists led by Maryland physics professor James Drake has found what may be one of the final pieces to a puzzle scientists have been trying to solve for almost 40 years: how magnetic fields produce the explosive releases of energy seen in solar flares, in magnetic storms at the edge of Earth's atmosphere and in many other powerful cosmic events throughout the universe.

Magnetic field, or force, lines act much like giant rubber bands. Physicists have long been convinced that the primary mechanism for release of magnetic energy is a process called magnetic reconnection that occurs when oppositely directed magnetic field lines come in contact.

During this process, parallel magnetic field lines break and reconnect, forming back-to-back slingshots that release their energy by exploding outwards in opposite directions. Since charged particles are trapped on magnetic field lines, most of the energy in the field is converted to the flow of ionized particles (plasma) that is pulled along by the expanding field lines.

However, classic magnetic reconnection theory has one major problem: it incorrectly predicts a gradual release of energy. For example, theoretical calculations generally predicted that a solar flare should take years or even decades to release energy, while observations have shown it takes only minutes.

In the Feb. 7 edition of the journal Science, Drake, along with university colleagues Michael Shay and Marc Swisdak, released findings that for the first time indicate at least some of this explosive energy happens as the result of plasma turbulence generated during reconnection. Using large-scale computer simulations developed at Maryland, together with data from NASA's Polar satellite, the team found that intense currents of electrons are generated during magnetic reconnection.

These intense currents drive strong turbulence that takes the form of "electron holes," three-dimensional regions where the electron density is depleted. The satellite data from Polar indicate that the magnetosphere is riddled with these holes, which have diameters of up to a mile and travel at speeds in excess of 1,000 miles per second. According to the researchers, the intense electric field associated with these electron holes causes electron scattering that is sufficiently strong to sustain fast reconnection.

"Electron scattering by the electron holes also strongly heats electrons and may therefore ultimately [explain] the surprisingly large amount of energy that is transferred to electrons during reconnection events in the solar corona and the Earth's magnetosphere," said Drake, a professor in physics and in the university's Institute for Research in Electronics and Applied Physics.

Drake led a team of scientists in 2000 that published a widely acclaimed study indicating that during the magnetic reconnection process, a two-layer flow of particles is created that speeds the release of energy. In this laminar flow theory, "whistler waves" cause the plasma that is pulled along by the slinging field lines to divide into two streams, one of electrons and the other of ionized atoms.

"Based on these latest findings, I think the correct conceptual framework for understanding the explosive release of magnetic energy is a combination of laminar and turbulent mechanisms rather than one or the other alone," Drake said.

"Whistler waves provided a good explanation for every part of this puzzle but one, and that was the observation that during reco events like solar flares there is a huge amount of energy going into energetic electrons. Our latest findings indicate turbulence may be that missing piece."

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