June 28, 2000 Observations help explain decades-old solar mystery
BOZEMAN, MT -- A decades-old mystery about the behavior of magnetic fields in solar flares may now be solved, thanks to careful observations by a pair of solar scientists.
David McKenzie, a research scientist at Montana State University-Bozeman, and Hugh Hudson of the Institute of Space and Astronautical Science in Japan, have sighted solar magnetic fields snapping back to the sun, like stretched rubber bands, during the energy release of solar flares.
This behavior, called magnetic field line shrinkage or reconnection outflow, was predicted nearly 30 years ago but never directly observed.
"This has been one of the great mysteries of solar flares," commented Dana Longcope, an assistant professor of physics at Montana State University. "This makes (the discovery) a good contribution -- something everyone's talking about in the field."
"The thing that we're excited about is that this is the kind of motion we've been looking for, and now we've been able to spot it," McKenzie said.
McKenzie will present a poster on the research June 18-22 during the Solar Physics Division meeting of the American Astronomical Society in Lake Tahoe, NV. A paper also will appear in the August issue of Solar Physics.
Solar flares and related outbursts called coronal mass ejections are immensely powerful. They can hurl clouds of plasma into space with the energy of a billion megatons of TNT, scientists say. Occasionally, the clouds are aimed at Earth at speeds approaching one million mph.
The belches result from coiled magnetic fields on the sun's surface releasing massive amounts of energy. In the wake of these outbursts, scientists theorized, those magnetic fields with feet still on the sun would arch backwards, form new connections to the sun and ultimately snap back into place.
McKenzie offered the analogy of a hot-air balloon lifting off the ground and stretching its tethers like rubber bands. Underneath the hot air balloon, the elastic tethers get pushed together and start to tangle. Broken bands are reconnected to other broken bands, so that instead of simply snapping, the tethers form new connections.
If this tangling and reconnection goes on long enough, pieces of rubber bands that are connected to the ground connect to other "grounded" segments, and subsequently snap back down to the ground. Pieces connected to the balloon get tied to other "balloon-connected" segments and are carried off.
In the case of a CME, the tethers carried away from the sun are the magnetic field embedded in the CME. The tethers with both ends connected to the sun form long rows of arches called arcades. The multi-million degree plasma they contain emits the ultraviolet and X-ray radiation observed by spacecraft like TRACE, SOHO and Yohkoh. The process of reconnection releases much of the energy that's stored in the magnetic field, giving the flare its punch.
Some scientists have used the lack of observational evidence for tethers snapping back, or reconnection outflow, to suggest that the model is incomplete or incorrect. But last year, McKenzie and Hudson finally observed some motions in Yohkoh images that were consistent with this model.
"We saw the movement of lots of discrete features, at speeds of a quarter-million to one half million mph, that were directed downwards into the top of a post-CME arcade," McKenzie said.
These initial findings, based on several "snap backs" in a single CME flare, were published last summer in The Astrophysics Journal.
McKenzie spent another year analyzing images not only from Yohkoh but also the SOHO and TRACE solar-observing satellites and from the Mauna Loa Solar Observatory in Hawaii. He's documented outflows in a total of 17 CME-related flares that occurred during the last two years.
The discovery adds to the understanding of solar magnetic fields and the sometimes violent space weather they create. That weather can disrupt modern communication, satellite, utility and other systems as well as affect space travel.
The observations also point toward future enhancements to the reconnection model. For one, the "snap back" speeds that McKenzie detected are considerably slower than predicted. For another, the "tethers" have a distinct appearance and size that will have to be explained by models of post-CME arcades.
"If we want to understand how solar flares work, we must understand how the magnetic field behaves," McKenzie said. "It will also help us to be able to predict when they happen and how big they are."
McKenzie, in fact, was part of the team responsible for last year's "S"-marks-the-spot discovery.
He, Hudson and Richard Canfield at Montana State University-Bozeman correlated the formation of a sigmoid shape on the sun with the likelihood of a coronal mass ejection occurring from that region within days.
For images, go to http://solar.physics.montana.edu/mckenzie/LDE/press.html
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