The explosions, called solar flares, are capable of releasing as much energy as a billion one-megaton nuclear bombs. The destruction of magnetic fields, called magnetic reconnection, was a leading theory to explain how solar flares could suddenly release so much energy, but there were other possibilities. The new picture from RHESSI confirms large-scale magnetic reconnection as the most likely scenario.
The orange lines show the magnetic structure and the yellow shading represents the X-ray emission observed by RHESSI. Initially, the flare X-ray emission gradually brightens while the top of the magnetic arch (parallel orange lines, left) decreases somewhat in height. The X-rays rapidly brighten and the feet of the arch become visible in X-rays when the X-ray blob above the top of the arch separates from the arch. After a 2 minute delay, the X-ray blob and the magnetic loop it is associated with (orange circle, right) speed outward at about 300 km/sec. The arch and the loop gradually grow as newly reconnected magnetic fields build up around them. Click on image for movie, or GSFC site for high res. Credit: NASA
"Many observations gave hints that magnetic reconnection over large areas was responsible for solar flares, but the new pictures from RHESSI are the first that are really convincing," said Linhui Sui of the Catholic University of America, Washington, DC. "The hunt for the energy source of flares has been like a story where villagers suspect a dragon is on the loose because something roars overhead in the middle of the night, but only something resembling the tail of a dragon is ever seen. With RHESSI, we've now seen both ends of the dragon."
Magnetic reconnection can happen in the solar atmosphere because it is hot enough to separate electrons from atoms, producing a gas of electrically charged particles called plasma. Because plasma is electrically charged, magnetic fields and plasma tend to flow together. When magnetic fields and plasma are ejected from the Sun, the ends of the magnetic fields remain attached to the surface. As a result, the magnetic fields are stretched and forced together until they break under the stress, like a rubber band pulled too far, and reconnect - snap to a new shape with less energy.
The two orange lines extending above the surface of the Sun represent oppositely directed magnetic field lines extending out into the solar corona. They gradually move together and, when they touch, they break like overstretched elastic bands (represented by the white flash). Unlike elastic bands, the broken magnetic field lines immediately reconnect above and below the break point. The U-shaped reconnected magnetic field lines are seen to snap upward and downward, away from the point at which the reconnection occurred. This process extracts energy from the magnetic field, heating gas to high temperatures and accelerating particles to high energies. Click on image for movie. Credit: NASA
The thin region where they reconnect is called the reconnection layer, and it is where oppositely directed magnetic fields come close enough to merge. Magnetic reconnection could power a solar flare by heating the Sun's atmosphere to tens of millions of degrees and accelerating electrically charged particles that comprise the plasma (electrons and ions) to almost the speed of light.
At such high temperatures, solar plasma will shine in X-rays, and RHESSI observed high-energy X-rays emitted by plasma heated to tens of millions of degrees in a flare on April 15, 2002. The hot, X-ray emitting plasma initially appeared as a blob on top of an arch of relatively cooler plasma protruding from the Sun's surface in the RHESSI images. The blob and arch structure is consistent with reconnection because the X-ray blob could be heated by reconnection and the part of the magnetic field that breaks and snaps back to the solar surface will assume an arch shape. (Magnetic fields are invisible, but RHESSI can see them indirectly. Since magnetic fields and plasma flow together, plasma can be steered by magnetic fields if the fields are strong enough. On the Sun, hot, glowing plasma flows along its invisible magnetic fields, making their shapes detectable by RHESSI.)
"This temperature gradient in the hot plasma was the clincher for me," said Dr. Gordon Holman, a Co-Investigator on RHESSI and co-author of the paper at NASA's Goddard Space Flight Center, Greenbelt, Md. "If some other process was powering the flare, the hot plasma would not appear like this."
"We estimate that 200 times the total energy consumed by humanity in the year 2000 was extracted from the magnetic field destroyed in this flare, using our RHESSI observations," said Holman.
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