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UM Researchers Uncover Last Major Piece to Puzzle of Massive Magnetic Explosions
COLLEGE PARK, Md. - A team of scientists led by University of 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 forty 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 magnetic 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 February 7 edition of the journal Science, Drake and his colleagues release findings that for the first time indicate that 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 1000 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 provide an explanation for 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.
Ironically, until this paper, Drake was one of the principle developers of a competing and non-turbulent explanation for the rapid release of energy. In 2000 Drake led a team of scientists 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 that 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 except one, and that was the observation that during reconnection 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."
"Formation of Electron Holes and Particle Energization During Magnetic Reconnection,"
Science, February 7, 2003, J. F. Drake, M. Swisdak and M. A. Shay,
University of Maryland;
C. Cattell, University of Minnesota; B. N. Rogers, Dartmouth College; A.
Zeiler, Max-Planck-Institute, Germany.
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