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Sound waves drive the chromosphere


Image courtesy of the SOHO-EIT Consortium.

Researchers have found that sound waves escaping the Sun's interior create fountains of hot gas that shape and power the chromosphere, a thin region of the Sun's atmosphere. The new result helps explain a mystery dating back to the middle of the last century: why the chromosphere (and corona above) is much hotter than the visible surface of the Sun, which is closer to the fusion-powered reactor at the star's core.

The results were presented in May at the annual meeting of the American Astronomical Society in Honolulu, Hawaii.

For 20 years, helioseismologists have studied energetic sound waves as probes of the Sun's interior structure because these waves are largely trapped at the Sun's visible surface, the photosphere. The new research found that some of these waves can escape from the photosphere into the chromosphere and corona.

Using spacecraft, ground-based telescopes, and computer simulations, researchers found that the sound waves leak out from the interior of the Sun near strong knots in the Sun's magnetic fields and then propagate upward into the chromosphere and corona. The leaking waves continually propel fountains of hot gas upward into the Sun's atmosphere, which fall back to its surface a few minutes later.

"Scientists have long realized that solar magnetic fields hold the key to tapping the vast energy reservoir locked in the Sun's interior," says Paul Bellaire, program director in NSF's division of atmospheric sciences. "These researchers have found the ingenious way that the Sun uses magnetic keys to pick those locks." The research team included scientists from the University of Hawaii Institute for Astronomy; Southwest Research Institute; Lockheed Martin; and the University of Oslo.

To make the discovery, the team used observations from the SOHO and TRACE spacecrafts combined with those from the Magneto-Optical filters at Two Heights (MOTH) instrument stationed in Antarctica and the Swedish 1-meter Solar Telescope on the Canary Islands.

University of Hawaii Institute for Astronomy
University of Oslo

Chinese traffic restrictions yield quick air cleansing

Government restrictions on Chinese motorists during a three-day conference last fall cut Beijing's emissions of an important class of atmospheric pollutants by up to 40%, recent satellite observations indicate. The November restrictions are widely viewed as a dress rehearsal for efforts by the city to slash smog and airborne contaminants when China hosts the 2008 Summer Olympic Games.

The restrictions on Beijing's drivers took place during a summit meeting on Chinese-African cooperation on 4–6 November 2006, during which an estimated 800,000 of Beijing's 2.82 million vehicles were taken off the road. Researchers now have used measurements taken from space to evaluate Beijing's air quality before, during, and after the conference.

"Traffic restrictions implemented during the Sino-African Summit were remarkably successful in reducing emissions of nitrogen oxides," reports Yuxuan Wang of Harvard University and her colleagues. The research team assessed Beijing's emissions using data from the Dutch-Finnish Ozone Monitoring Instrument aboard NASA's Aura satellite.

"We expected a drop in nitrogen emissions, but not to this extent, after only a short period of time," Wang adds. She conducted the study with colleagues at Harvard and at the Royal Netherlands Meteorological Institute in De Bilt. The findings were published in Geophysical Research Letters. The research was supported by the National Science Foundation.

Past satellite studies by other researchers have detected a "weekend effect"—reduced nitrogen oxide (NOx) emissions on weekends compared to weekdays—over cities and industrial areas in Europe, Japan, and the United States.

The size of the NOx reduction in Beijing surprised Wang and her colleagues in part because it departs from recent estimates, which say that during non-heating seasons, nearly 70% of NOx emissions in the Beijing area are from vehicular emissions. Using this as a standard, the team calculated that there would need to be a 50% cut in vehicular use in Beijing to account for the observed 40% drop in NOx. In contrast, the reported traffic decrease was not quite 30%. The reason for this disparity is not yet understood.

Harvard University
Royal Netherlands Meteorological Institute


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