September 26, 2000
Scientists discovered an important clue while observing immense coils of hot, electrified gas, known as coronal loops. These fiery, arching fountains now appear in unprecedented detail with NASA's Transition Region and Coronal Explorer (TRACE) spacecraft.
Scientists are interested in the corona, which appears as a halo of light seen by the unaided eye during a total solar eclipse, because eruptive events in this region can disrupt high-technology systems on Earth. Astronomers also hope to use the solar corona studies to better understand other stars.
"The mysterious energy source that makes the Sun's atmosphere so incredibly hot has been an enigma for more than 70 years, and before we discover what it is, we needed to learn where it is," said Dr. Markus Aschwanden of the Lockheed-Martin Solar and Astrophysics Laboratory (LMSAL), in Palo Alto, CA.
Aschwanden is lead author of a paper describing this research to be published in the Astrophysical Journal. "Locating the source of coronal heating is a key piece of this puzzle, and we are excited that solar observatories like TRACE are allowing us to resolve the hidden events occurring in the atmospheres of stars."
The new observations reveal the location of the unidentified energy source, showing that most of the heating occurs low in the corona, within about 10,000 miles from the Sun's visible surface. The gas fountains form arches, hundreds of thousands of miles high, capable of surrounding 30 Earths. As gas emerges from the solar surface, it's heated and rises, then cools and crashes back to the surface at more than 60 miles per second.
Millions of different-sized coronal loops comprise the corona, and a 30-year-old theory assumes the loops are heated evenly throughout their height. The TRACE observations show that instead, most of the heating must occur at the base of the loops, near where they emerge from and return to the solar surface.
The old theory of uniform heating predicted that the loops would be substantially hotter at their tops because gas at the top of the loops is thinner, and does not radiate heat away as efficiently as the dense gas near the bottom. If the loop were heated evenly over its entire height, the top, which can't lose heat as well, would become hotter than the rest. Earlier, less- detailed observations of the coronal loops could not confirm nor invalidate the uniform heating theory because they could not reveal that the loop tops were really about the same temperature as the bases.
However, the high-resolution TRACE pictures show that, just as a thick piece of rope consists of many thin fibers, what was thought to be one coronal loop is actually a bundle of thin, individual loops. Although some thin loops in the bundle are hotter than other spirals, precise measurements by TRACE show that, over its height, each separate, thin loop varies much less than the uniform heating theory predicts.
"Since a loop loses heat most rapidly from its bases, most of the heat must also be going in at the bases for the loop to be at a uniform temperature," said Dr. Karel Schrijver, a member of the research team, also of LMSAL. "If this were not so, the lower parts would have been much cooler than the tops, which do not lose heat as quickly."
NASA Administrator Daniel S. Goldin unveiled the new TRACE images today, along with Ellen Futter, president of the American Museum of Natural History, at the museum's Rose Center for Earth and Space, New York.
TRACE, launched in April 1998, is training its powerful telescope on the "transition region" of the Sun's atmosphere, a dynamic region between the relatively cool surface and lower atmosphere regions of the Sun, about 10,000 degrees Fahrenheit, and the extremely hot upper atmosphere, which burns up to 3 million degrees Fahrenheit.
University of St Andrews, Scotland.
Tuesday 9th June 1998
For more that half a century, scientists have known that the surface of the Sun is barely 6,000 degrees, but that the overlying atmosphere scorches up to several million degrees. The outer part of this atmosphere forms a halo of eerie light around the Sun during a solar eclipse and consists of giant, super-hot loops that extend high up above the Sun's surface. Using the Soft X-ray Telescope on a Japanese/US/UK space satellite called Yohkoh, which means 'sunbeam', the research team, led by Professor Eric Priest of St Andrews University, has for the first time been able to measure how the temperature varies along such giant loops.
Professor Priest says "Once we had measured the temperature profile, it was exciting to compare the observations with predictions from the three main theoretical models that had been previously put forward. Some felt that the heat should be dumped at the feet of the loop and then conducted (like the flow of heat along a red-hot poker) to the rest of the loop. Others felt that the heat should be deposited at the summit of the loop, while a third camp predicted a uniform release of heat along the loop."
The comparison of observations and theoretical models showed clearly that the heat is deposited uniformly. The most likely mechanism at present is a clash of magnetic field lines. They tangle like spaghetti in the solar atmosphere and break, causing dozens of explosions that release energy along the loop. These turbulent explosions occur in tiny regions of intense electric current that heat the atmosphere in the same way as an electric current in a light blub or an electric fire.
Images can be found at:
http://www-solar.dcs.st-andrews.ac.uk/~eric/nature.html
Goddard Space Flight Center, Greenbelt, MD
May 28, 1998
Whole Sun Month began as a series of coordinated observations by solar researchers Dr. Sarah Gibson of the Catholic University, Washington, D.C., and Dr. Douglas Biesecker of Space Applications Corporation, Vienna, Va., both located at NASA's Goddard Space Flight Center, Greenbelt, Md., and rapidly grew into an international cooperative project as scientists recognized the benefits of simultaneous observations. The research is scheduled to be published this fall in the Journal of Geophysical Research.
"The corona is partially transparent, like thin clouds. When you look at features in the Sun's atmosphere, especially on the limb (edge) of the Sun, sometimes you can't tell if the feature is in front or behind another structure. A three dimensional view lets you move around in the image so you can correlate the atmospheric structure both with the surface features below it, and structures in the solar wind above it. This will help us better understand how coronal structures are ultimately connected to the Earth," said Gibson.
"Building this view of the corona was similar to taking a CAT scan of a person. We took one picture per day over the entire twenty eight day rotation period of the Sun. We then used a computer to assemble the images into a three dimensional view," said Biesecker.
The tomograph images were developed using the Ultraviolet Coronograph Spectrometer onboard the European Space Agency/NASA Solar and Heliospheric Observatory (SOHO) spacecraft. The corona is made up of hot, electrically charged gas called plasma. It is only visible from Earth's surface during a total solar eclipse, when it appears as a shimmering, white veil around the moon. This plasma becomes the solar wind as it streams away from the Sun at speeds of one million to one and a half million miles per hour. Periodically, energetic events on the Sun's surface send massive amounts of plasma hurtling toward the Earth. Because the plasma is electrically charged and contains magnetic fields, its impact compresses the Earth's magnetic field. A severe impact can cause geomagnetic storms capable of disrupting communication and power systems.
Whole Sun month occurred in two phases. Phase one, the data collection period, began on Aug. 10, 1996 and ended Sept. 8, 1996. Phase two was the data analysis and theoretical modeling period. The more difficult of the two periods, it continues to the present day.
"We had an ideal situation in 1996 when we collected our data. That was during a relatively calm period in the 11 year cycle of solar activity known as solar minimum. There were enough coronal structures to make things interesting, but not so many that their connections to solar wind behavior at the Earth could not be distinguished. We were able to learn new things without being overwhelmed by complexity," said Gibson.
"Whole Sun Month was so successful that we are planning to repeat it later this year in August. With the next maximum in solar activity predicted sometime around 2000, we are now in a relatively active solar period. The payoff from the first Whole Sun Month has been the ability to refine our models of the corona. This will really help us understand what is happening in the corona during this more active period," said Biesecker.
Whole Sun Month includes measurements of the corona out to approximately four times the distance between the Earth and the Sun. Participating spacecraft include the Solar and Heliospheric Observatory (SOHO) spacecraft, a joint European Space Agency (ESA) and NASA project; YOHKOH, a Japanese X-ray observatory; Ulysses, a joint ESA/NASA interplanetary probe to explore the solar poles; and Wind, a NASA spacecraft designed to study the solar wind near Earth.
Participating ground based observatories include the Mauna Loa Solar Observatory, Hawaii, with the National Center for Atmospheric Research, the National Solar Observatory at Sacramento Peak, Sunspot, N.M., and the National Solar Observatory at Kitt Peak, Tucson, Ariz., both part of the National Optical Astronomy Observatories sponsored by the National Science Foundation; the Nobeyama Radio Observatory, Japan, part of the Japanese National Astronomical Observatory (NAO), and the European Incoherent Scatter Scientific Association (EISCAT), a scientific association between Germany, France, Great Britain, Finland, Norway and Sweden and Japan.
ROYAL ASTRONOMICAL SOCIETY
31st March 1998
This new work will be included by Professor Priest in his talk, 'A startling new Sun', at the National Astronomy Meeting at the University of St Andrews on Wednesday 1st April.
The surface of the Sun has a temperature of only 6000 degrees kelvin (5730 degrees C), but its outermost layers of tenuous gas - the corona, which is visible at a total solar eclipse - is surprisingly very much hotter. Its temperature is several million degrees. How the corona is heated represents one of the most important unsolved mysteries in astrophysics which has tantalized solar physicists for the past 40 years.
"But the coronal heating problem is a really tough and complex one to tackle" says Professor Priest. "The corona consists of several types of structure which may be heated by different mechanisms. There are huge magnetic loops arching high above the solar surface, tiny intense cores of emission called X-ray bright points, and dark regions, called coronal holes, where the nature of the magnetic field allows hot gas (plasma) to stream out into interplanetary space."
Two main theories have been proposed to explain the high temperature of the solar corona. One of them involves magnetic waves travelling upwards from the surface of the Sun. Like water waves, magnetic waves carry energy. The other, the 'magnetic reconnection' theory, involves the generation of intense electric currents to discharge the energy directly into the corona.
To test the wave theory, Dr Robert Walsh and Dr Jack Ireland at St Andrews used the Coronal Diagnostic Spectrometer (CDS) instrument on SOHO to search for magnetic waves with periods between 30 and 1000 seconds in an active region of the Sun's surface where the magnetic field is strong. The magnetic structure of the region was also mapped out using data from the Michelson Doppler Imager (MDI) instrument, also on SOHO. The results were startling: in the layers of gas nearest the visible surface of the Sun (the chromosphere), where the temperature is about 10,000 degrees kelvin, there are clear wave-like motions with periods of about 300 seconds and 600 seconds; further up (the transition region) where the temperature is 200,000 degrees kelvin the waves can also be seen. But by the time the one- million-degree corona is reached no such wave motions were detected. It appears that waves are travelling up some distance, but they are not getting far enough to heat the corona.
However, the St Andrews team discovered that intense coronal brightenings known as X-ray bright points are heated by magnetic reconnection. Observations from a rocket instrument called NIXT have shown that such bright points have a complex internal structure of interacting magnetic loops. This structure agrees very well with predictions made by Professor Priest, Dr Clare Parnell and Dr Sara Martin.
"Magnetic reconnection gives a unified explanation for many diverse observations from SOHO that all fall into place when viewed together" says Professor Priest. For example: - Recently, Karel Schrijver, Alan Title and colleagues at Lockheed Martin discovered from MDI observations that the solar surface consists of a "magnetic carpet", in which the magnetic structure is completely replenished every 40 hours. The mechanism for changing the magnetic connections so rapidly is the magnetic reconnection process. - With the SUMER instrument, rapid jets of plasma in explosive events have been discovered and these are also naturally produced by magnetic reconnection. - The discovery made with the CDS instrument of bright spots that have been called "blinkers" are an inevitable consequence of magnetic reconnection.
Said Professor Priest, "It is only now that we are beginning to analyse and digest the results from SOHO, but there are some amazing surprises that are revolutionising our understanding of the Sun - our closest star".
Images to support this story can be found at
http://www-solar.dcs.st-andrews.ac.uk/~robert/press.html
The general SOHO Web site is at
http://sohowww.nascom.nasa.gov
NOTE:
The SOHO satellite was launched 2 years ago as a major joint project between the European Space Agency (ESA) and NASA, with ESA as the major partner.
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