You have to watch out for the small, cool guys. They might be the ones generating all the heat around the sun.
In their quest to find out why the sun's corona is so much hotter than the visible surface, scientists have looked at the biggest and brightest events on the surface.
Now, say three scientists at NASA's Marshall Space Flight Center, it appears that the energy source is a continuous rumble of microflares, feeding magnetic energy into the corona, heating it to 1 to 2 million Kelvin (1.8-3.6 million deg. F).
"One microflare has only about 1 percent of the energy of a large, bright loop," said Dr. Ron Moore, a solar physicist at NASA's Marshall Space Flight Center. "But you have these cool microflares constantly going off. And that's enough to heat the corona, the nearly invisible halo of gas around the sun."
Moore and solar physicists Dr. Jason Porter of NASA/Marshall and Dr. David Falconer of the University of Alabama in Huntsville, will present their findings today at the American Astronomical Society's Centennial conference in Chicago. They will report on research conducted with the Solar Vector Magnetograph (SVMG), a NASA/Marshall telescope that reveals the strength and direction of magnetic fields in the solar atmosphere, an X-ray telescope aboard Japan's Yohkoh satellite, and an ultraviolet solar telescope on Europe's Solar and Heliospheric Observatory (SOHO).
Because the sun is a plasma (gases stripped of electrons) all the way to its core, it doesn't really have a surface. What we call the visible surface is where the plasma becomes thin enough to be transparent. This reveals structures like sunspots, slightly cooler regions formed by magnetic activity, and the feet of flares, transient arches of gas looping from the surface into space and back down to the surface. These arches are so hot that they glow in X-rays.
Scientist have long puzzled over why the surface has a temperature of only 6,000 K (10,800 deg. F) and the corona - the outer atmosphere - reaches 1 to 2 million K (1.8 to 3.6 million deg. F). Since the corona cools rapidly, losing its heat as radiation and the solar wind, something has to be pumping energy up from the surface.
Large flare events and bright areas on the surface don't continuously pump enough energy, or connect well enough to the corona to sustain the high temperature throughout the entire corona.
For several years, Moore, Porter, and Falconer have believed that the source was countless microflares, miniature solar flares low in the corona and near the limit of what telescopes can see.
"Microflares are 'micro' only when compared to the rest of the sun and its activities, though," Moore cautions. Typical microflares are about the size of the Earth, and in their 5-minute lives release as much energy as 10 million H-bombs.
Porter took images from Yohkoh's Soft X-ray Telescope and overlaid images from the SVMG which shows the strength and direction of magnetic fields in the solar surface.
Porter was looking at islands of included polarity, intense north or south magnetic islands around which large, bright, persistent coronal loops are rooted. Encircling these islands are neutral lines, narrow alleys where the magnetic field switches from one polarity to another. Along these lines, the SVMG showed these magnetic fields to be under a type of stress called shear.
"These regions are sites of enhanced coronal heating and microflaring," Porter said. Brightenings in the source region at the feet of a large loop are sometimes followed by brightenings in the large loops - and sometimes not.
"A long-term comparison of the hottest emissions from the source regions and the extended loops shows that loops' brightnesses usually do not correspond with the X-ray emissions down at the feet," Porter said. "This implies that the extended loops are heated by some activity that we can't see as hot X-ray microflares."
A new study by Porter, Falconer, and Moore of magnetic islands and extended loops, observed in somewhat cooler radiation (about 1 million K) by SOHO's Extreme Ultraviolet Imaging Telescope (EIT) revealed extreme ultraviolet microflares that do correlate well with brightening in the extended loops. This indicates that the cooler microflares do drive the heating in the extended loops.
Falconer has added crucial supporting evidence. Falconer took images from SOHO's Extreme-Ultraviolet Imaging Telescope showing the sun in the glow of iron with 11 electrons stripped away (Fe XII). Although the sun is mostly hydrogen, it has traces of iron, sodium and other metals. Because these metals ionize at specific temperatures, images taken in certain wavelengths match specific temperatures.
For large regions well away from the strong magnetic fields (the area around sunspots), Falconer plotted contours between the brightest and darkest spots. He found the difference between the brightest and darkest areas was only about 50 percent. The results were the same regardless of when they were taken - a few hours apart or on six other days.
"If the bright points are what's driving the corona," Falconer asked, "then why are the dim areas - which have practically no bright points - as bright as they are?"
Studying the dim regions showed that the corona there is rooted in a magnetically controlled network that apparently contributes a significant portion of the coronal heating in these areas, Falconer said.
"The bright points - the larger magnetic bipoles in the network - don't drive most of the heating," he said. "They are not a significant contributor to the overall heating of the solar corona. The dominant energy term is the in the rest of the network, where the bipoles are smaller and the microflares are too cool to show up as coronal bright points."
To wrap up the study, Moore looked at how much energy comes from microflares around a magnetic island, and how much is needed to keep the corona hot in large bright areas rooted around the island edges.
He found that the two balance nicely.
"Each microflare withdraws a small amount from the bank of energy in the sheared region," Moore explained, "so you can have a lot of microflares go off before you deplete your store of energy." By then, more core-field stress generates and the process renews.
"Everything hangs together," Moore said. "you're not asking for more than the active region has to offer."
But it is asking a bit more than telescopes can now provide. As Porter noted, the regions supplying the heat apparently are too small to be more than barely resolved by Yohkoh EIT or by the SVMG. However, ultraviolet observations with the Transition Region and Coronal Explorer (TRACE) satellite provide much better resolution. And solar physicists are looking forward to 2004 when Japan will launch Solar B with a more powerful array of telescopes that should have even finer resolution to test their theories.