The sun's atmosphere is threaded with magnetic fields (yellow lines). Areas with closed magnetic fields give rise to slow, dense solar wind (short, dashed, red arrows), while areas with open magnetic fields -- so-called "coronal holes" -- yield fast, less dense solar wind streams (longer, solid, red arrows).
Constantly buffeting the Earth and its satellites, the solar wind can gust from 750,000 to 1.5 million mph. Astronomers would like to predict the dramatic changes in this constant barrage of charged particles so as to better protect the craft that orbit Earth.
Recent research shows that the solar speeds can be accurately measured by observing a relatively deep layer of the Sun's atmosphere - far beneath where the winds are thought to originate.
In what could be a boon to space weather forecasts, scientists discovered a relation between solar wind speed and fluctuations in the chromosphere - a region a few thousand miles thick on the outer surface of the Sun.
The findings were a surprise because the solar wind appears to blow out of the lower portions of the hot, wispy corona, which extends for millions of miles above the chromosphere.
"It's like discovering that the source of the river Nile is another 500 miles inland," said Scott McIntosh of the Southwest Research Institute.
The solar wind is composed of electrically charged particles that are partly accelerated by magnetic fields that permeate the corona.
The magnetic field is called "closed" when it curls back onto the Sun's surface. The solar wind is slower and denser coming out of a closed region.
But the fastest wind particles are channeled down "coronal holes" - places where the magnetic field lines point out into space, in a configuration called "open." Coronal holes can be seen as large, dark spots in X-ray images of the corona, which generally emits copious amounts of X-rays because it is millions of degrees Fahrenheit.
The chromosphere is much cooler than the corona and can only be seen during an eclipse. The other times it is overwhelmed by the bright photosphere - the layer just below the chromosphere, where majority of sunlight comes from.
Using NASA's Transition Region and Coronal Explorer (TRACE) spacecraft, McIntosh and his collaborators measured stellar sound waves traveling through the chromosphere and determined that this layer was stretched thin just below coronal holes.
"This kind of flummoxed us," McIntosh told Space.com. "We thought the chromosphere shouldn't know anything about the coronal hole above it."
To further investigate this correlation, the scientists compared the chromosphere structure to the speed of the corresponding wind as it whisked by the Earth. It turned out the solar wind velocity could be estimated accurately just by measuring the chromosphere below which it originated.
The discovery is expected to improve space weather forecasts. This is vital to communication satellites, which are vulnerable to space storms. In the future, if humans venture back to the Moon and on to Mars, broader and better cosmic storm predictions will be necessary.
The worst radiation events arise out of coronal mass ejections (CMEs) - when the Sun expels billions of tons of solar wind particles in one blast. A CME can accelerate particles as it plows through the solar wind in front of it. By knowing the speed and density of solar wind "fronts" around the Sun, scientists may predict how severe the eventual storm will be.
"Just as knowing more details about the atmosphere helps to predict the intensity of a hurricane, knowing the speed of the solar wind helps to determine the intensity of space radiation storms from CMEs," said Robert Leamon of NASA's Goddard Space Flight Center.
McIntosh and Leamon are the authors of a paper describing the research in the May 10 issue of the Astrophysical Journal.
Until now, much of our information about the solar wind and storms has come from relatively nearby spacecraft like ACE, WIND, and SOHO.
"This spacecraft fleet was placed along the Earth-Sun line because we need to know about the space weather coming our way," said Joe Gurman from Goddard. "However, compared to the size of our solar system, this is a very narrow range; it's like looking through a soda straw."
But with the correlation between the solar wind and the chromosphere, scientists will be able to use the data from TRACE to forecast the weather in half of the solar system, they said.
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