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Sept.
2, 1999: On a cold winter's day a good place to be is by
a roaring fire with a cup of hot cocoa. If the fire feels a bit
too hot -- no problem -- just move away. As with most heat sources,
the temperature in the vicinity of a fireplace falls with increasing
distance from the hearth.
A 130 year old mystery
The solution of one mystery -- the origin of the green line -- led to another more perplexing conundrum. Fe XIV ("iron fourteen") is an iron atom that has lost 13 of its 26 orbital electrons. Stripping iron of so many electrons requires an enormous amount of energy -- such an ion can only exist in a very hot gas. Careful studies of spectra obtained during eclipses show that the temperature of the corona is about two million degrees C, hundreds of times hotter than the surface of the Sun. Since the corona cools rapidly, losing its heat as radiation and the solar wind, something has to be pumping energy up from the surface. But what? That's what scientists would like to know. Lots of theories, not enough data....There are many ideas to explain the extraordinary warmth of
the Sun's corona. The leading theories fall into three categories:
miniature solar flares, atmospheric waves, and electrical dissipation.
Unfortunately, microflares are so small that they're near the limits of what current telescopes can see. The number of observed microflares doesn't quite add up to the energy budget of the corona but there could be many more very faint ones that can't yet be detected. The new TRACE satellite is providing better sensitivity and resolution than previous instruments, and solar physicists are looking forward to 2004, when Japan will launch Solar B with a more powerful array of telescopes to test these theories. Another way to heat the corona involves magnetic waves called Alfvén waves. The solar atmosphere is permeated with magnetic fields that are especially intense around sunspot groups. The sun is a rumbling, boiling, dynamic place, so the magnetic field lines are constantly shaking back and forth. These oscillations send waves of magnetized plasma (ionized gas) propagating outward into the corona. "You can think of an Alfvén wave as acting something
like a bullwhip," explains Hathaway. "A magnetic Alfvén
wave starts out in the dense atmosphere near the photosphere
and moves upward into the corona where its amplitude grows in
the tenuous gas. Eventually the wave breaks and it dumps its
energy in the form of heat. It's like cracking a whip. A small
shake near your hand (the photosphere) becomes a big crack near
the tip (the corona)."
Hathaway and colleagues have studied acoustic vibrations in the sun's atmosphere with periods slower than about 1 minute. They find that the total energy in such waves cannot deliver enough heat to the corona to keep it warm. That doesn't mean that sound waves can't do the job. The instrument that Hathaway et al used for their study was not sensitive to vibration periods faster than about 60 seconds. If there is a great deal of energy in shorter period sound waves and if they can propagate all the way out to the corona -- two very big ifs -- then acoustic waves could be a important heating source. "In principle, in situ measurements by spacecraft are great for characterizing all kinds of waves, but none of the satellites in orbit now are close enough to reveal what's really happening in the corona," continued Hathaway. "The Solar Probe Mission [scheduled for development beginning in 2001] could change all that. Solar Probe will have a special heat shield that lets it fly right through the corona within a few solar radii of the sun. With direct measurements we might finally understand what's going on in there." Meanwhile, here on Earth, impatient scientists aren't waiting for the next wave of high-tech spacecraft to solve the problem. Hundreds of astronomers and solar physicists positioned themselves along the path of totality of the August 11, 1999, solar eclipse to gain a fleeting view of the corona with state-of-the-art observing hardware. Prof. Jay Pasachoff of Williams College led an expedition of over 30 scientists and students to Romania where the eclipse lasted longest, 2 minutes and 29 seconds. Using high-time-resolution digital cameras, Pasachoff's team observed the Sun through filters sensitive to the Fe XIV green line in hopes of detecting the signatures of high-frequency Alfvén waves in the corona. Pasachoff's group was not alone. Scientists from many countries captured high resolution images of the coronal during totality in search of microflares, magnetic vibrations and other phenomena. High-caliber data obtained during the eclipse may help scientists evaluate another possible mechanism for coronal heating: electrical dissipation. It's been known since the days of Faraday and Maxwell that if you wave a magnet back and forth in the vicinity of a conducting wire, a current is induced in the wire. The same thing takes place in the sun's atmosphere. Oscillating magnetic fields generate currents that flow through the highly ionized gases above the photosphere and in the corona. How does that heat the corona? When current flows through a resistor some of the energy is dissipated as heat. A common light bulb is a good analogy. Electricity moves through a partially conducting filament, the filament glows and it also become very hot. Again, the question hinges on better observations of magnetic fields and plasmas in the corona. Scientists know that there is some resistive dissipation of energy in the corona, but they can't be sure how much. There is no shortage of ideas about what may heat the corona. Microflares, magnetic and acoustic waves, and electrical dissipation are all good candidates, but the observed energy flux into the corona from each of these mechanisms is about an order of magnitude too low to account for coronal heating. More and better data are needed to finally reveal the culprit. "My bet is that it's going to be some mixture," says Hathaway, "but only time will tell! When we do know, we'll have solved one of the big mysteries in astrophysics." |
Related Stories Surfing Magnetic Waves in the Solar Atmosphere July 8, 1999. How the Solar Wind Gets Up to Speed Solar Flares Show Their True Colors June 2, 1999. New research points to a common mechanism for spectral behavior in Solar Flares "Cool" microflares could be solar hot spots May 31, 1999. Secret of coronal heating may be multitude of tiny blasts. Finding the smoking gun before it fires March 9, 1999. Physicists discover a new tool for predicting solar eruptions. Related Sites Parents and Educators: Please visit Thursday's Classroom for lesson plans and activities related to this story. MSFC Solar Physics Home Page background information about the Sun and solar research The History of Coronium over a hundred years ago, scientists begin to understand that the solar corona is unexpectedly hot. Layers of the earth's atmosphere tutorial from the University of Michigan. Pasachoff's eclipse expedition is described at Williams College. More Space Science Headlines - NASA research on the web NASA's Office of Space Science press releases and other news related to NASA and astrophysics |
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