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16 years 9 months ago #19830 by Tommy
Replied by Tommy on topic Reply from Thomas Mandel
SKomewhere above I suggested that the extreme temperature of the corona is due to a magnetic field snapping like a whip. Well, today
in an article about extreme x-ray jets found in great abundance


science.nasa.gov/headlines/y2007/06dec_xrayjets.htm?list734132

The Sun is Bristling with X-ray Jets
12.06.2007


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Dec. 06, 2007: Astronomers using Japan's Hinode spacecraft have discovered that the sun is bristling with powerful "X-ray jets." They spray out of the sun's surface hundreds of times a day, launching blobs of hot gas as wide as North America at a top speed of two million miles per hour. These jets add significant mass to the solar wind and they may help explain a long-standing mystery of astrophysics: the superheating of the sun's corona.

"This is awesome and very much unexpected," says Jonathan Cirtain of the Marshall Space Flight Center who was a key figure in the discovery. He recalls how it happened: "We found them a year ago in Nov. 2006. Hinode had just been launched and its instruments were coming online." To calibrate the spacecraft's X-ray Telescope, mission controllers in Japan pointed the telescope at a dark hole in the sun's atmosphere--a "coronal hole." Cirtain analyzed the data and "there they were!"



Above: An X-ray jet recorded by the Hinode spacecraft on Jan. 10, 2007. Quicktime movies: three jets (2.4 MB); many jets in low resolution (4 MB); many jets in high resolution (26 MB).

"After the shock wore off, I ran around dragging other scientists into my office to show them the movie." He likens the appearance of the jets erupting within a coronal hole to "the twinkle of Christmas lights, randomly oriented. It's very pretty."

Cirtain notes that X-ray jets have been seen before, but never in such abundance. The first jets were recorded by a 1st-generation X-ray telescope onboard Skylab in the 1970s. They were called x-ray jets for the simple reason that they were bright at x-ray wavelengths. The phenomenon was later confirmed by a Naval Research Lab ultraviolet telescope that flew aboard the space shuttle in the 1980s as well as by Japan's Yohkoh X-ray Telescope in the 1990s. "All those instruments saw very few jets--typically one or two per day," says Cirtain. X-ray jets were thus regarded as a curiosity of little importance.


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Hinode has changed all that. The spacecraft's advanced X-Ray Telescope can take pictures rapidly enough to catch these fast-moving eruptions. "We now see that jets happen all the time, as often as 240 times a day. They appear at all latitudes, within coronal holes, inside sunspot groups, out in the middle of nowhere--in short, wherever we look on the sun we find these jets. They are a major form of solar activity," says Cirtain.

Each jet is triggered by a magnetic eruption or "reconnection event"--essentially the same process that powers solar flares albeit on a much smaller scale. "The energy in a typical jet is about a thousand times less than the energy of an M-class (medium sized) solar flare," says Cirtain. Individually, jets are weak; en masse, however, they pack quite a punch. "If we add up all the energy jets deposit into the sun's atmosphere, the daily total is on par with solar flares."

Indeed, the jets may contribute significantly to the solar wind. Every day a hot, relentless wind of solar protons and electrons blows against Earth, deflected just before it can reach the atmosphere by our planet’s global magnetic field. Gusts in solar wind can cause bright auroras, power outages and other effects collectively known as "space weather." What drives this wind away from the sun? It's a question that has puzzled physicists for decades. Jets provide at least part of the answer:

"We've added up the mass flowing in these jets and it amounts to between 10% and 25% of the solar wind. That's a significant fraction," he says.

X-ray jets may also contribute to the mysterious heating of the sun's outer atmosphere, the ghostly "corona" seen during solar eclipses.

Right: The sun's outer atmosphere or "corona". Credit & Copyright: Koen van Gorp.

<b>The mystery is this: If you stuck a thermometer in the surface of the sun, it would read about 6000o C. Yet above the surface of the sun, in the corona where intuition says things should be cooler, the temperature rises to millions of degrees. What heats the corona to such extreme temperatures?

X-ray jets seem to help. Cirtain and colleagues have examined four jets in great detail and found that they launch magnetic waves into the sun's upper atmosphere. These waves, called Alfven waves, propagate into the corona where they *crack* like a whip, heating the gas where the crack occurs. (Note: When a whip is cracked on Earth, the sharp sound we hear is a result of energy being transferred from the fast-moving tip of the whip to the air around it. The same basic process is at work with Alfven waves cracking in the corona.) Cirtain doesn't believe jets can wholly explain the super-heating of the corona, but "they make an important contribution."</b>

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16 years 9 months ago #20776 by JMB
Replied by JMB on topic Reply from Jacques Moret-Bailly
I am not a specialist of the Sun, although I wrote a paper on far UV radiated by the Sun (arxiv:physics/0507141).
The problem with your thermometer is that the surface of the Sun is not at equilibrium, so that there is a different temperature for all transitions of all atoms, for all light beams. 6000 K is only an average temperature of the "surface" of the Sun, maybe made mainly of ions H- and H+, which could appear as close to a blackbody in the visible.
I recall that the temperature of a transition is given writing that the ratio of the populations in involved levels is exp(-E/kT), and that for a monochromatic light beam, it is given by Planck's law.

For the Sun, the thin sheet of H- making its surface does not absorb much lines in far UV or X rays. Sharp, very hot lines of Fe XII, Mg X, Ne VIII, He I, C II, .... may cross the sheet of H- and heat resonantly atoms of the corona. The corresponding transfer of energy may be low.

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16 years 9 months ago #20588 by Tommy
Replied by Tommy on topic Reply from Thomas Mandel
That may very well be, BUT, it is a fact that magnetic fields also exist, extending far into the corona. I would tend to think that along with these magnetic fields, a large current folw of ions also occurs. If this is so, then what happens when the magnetic field "breaks"? Well the current flow continues, just like it does in the ignition coil of a car. So in a car we go from a 12 volt spark to a 40,000 volt spark. I am sure that some of this is in fact going on, it is elementary electronics, or is it plasmaonics. What they are saying in the article is that there are x-ray jets, magnetically induced, and then they use the crack of a whip as a model. Well, not a perfect metaphor, but close.

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16 years 9 months ago #20779 by JMB
Replied by JMB on topic Reply from Jacques Moret-Bailly
It is sure that variations of magnetic fields induce electric fields able to excite plasma. Is this excitation large enough ?

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16 years 9 months ago #20482 by Tommy
Replied by Tommy on topic Reply from Thomas Mandel
THe idea is that when we have magnetic fields which move and ions which can also move, we have a current flow. But when the magnetic field "breaks" the collaspe induces a continuing current flow without a path and that results in a spark. My understanding is that the surface of the sun is covered with these small fields or shall we say micro solar flares. It is clear that this "solar sparking" occurs, I just don't know how much.

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16 years 9 months ago #18144 by Jim
Replied by Jim on topic Reply from
The temperature of the solar corona and plasma is not the same of the temperature of liquids solids and gases. Its too bad this duel use of a property is poorly understood and there is a need to fix this so as to be less puzzled less distracted by false issues like the corona being millions of degrees and the solar surface 6,000. If you just discard the concept and use frequency you then only have a high frequency plasma in the corona and not the puzzle of how can be so.

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