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It is a question that has hounded solar physicists since the 1940s: Why is the outer layer of the sun’s atmosphere, the region farthest from the heat-producing core, hotter than both the lower atmosphere and the sun’s surface?
Experts have put forth various explanations, from sound waves or magnetic waves dissipating in the upper solar atmosphere, or corona, to short bursts of energy known as nanoflares that erupt as tangled magnetic field lines in the corona reconnect. Now observations from a new generation of sun-observing spacecraft are implicating a different mechanism, one that could provide the corona with a significant portion of its heat by continually delivering hot ionized gas, or plasma, to the upper atmosphere.
Spicules, which are short-lived fountains of plasma shooting up from the sun’s chromosphere, or lower atmosphere, seem to play a role in heating the corona to searing temperatures at millions of degrees kelvins, investigators have found. Spicules, whose origins are somewhat mysterious, last just 100 seconds, rising from the chromosphere at speeds of 50 to 100 kilometers per second. As lead study author Bart De Pontieu of the Lockheed Martin Solar and Astrophysics Laboratory in Palo Alto, Calif., points out, that is fast enough to travel from San Francisco to London in minutes. De Pontieu and his colleagues reported their findings in the journal Science.
The group based its study on observations from NASA’s new Solar Dynamics Observatory, launched in 2010, and the Japanese Hinode spacecraft, which began service in 2006. Both solar observatories can take detailed images of the sun every several seconds, the kind of quick-time observation needed to identify transient or rapidly changing phenomena.
As spicules measuring in the tens of thousands of degrees kelvins rise from the chromosphere, the researchers noticed, patches of the corona above flare up at one million to two million degrees.
The researchers do not yet know what launches the chromospheric plasma at such high speeds nor what heats it to the extreme temperatures it reaches in the corona. But the link between spicules and coronal heating holds promise for closing the books on a 70-year-old mystery, says Kenneth Phillips of University College London.
Although spicules seem to be important phenomena in certain regions of the sun, time will tell whether they deliver enough hot plasma on a global scale to explain the corona’s tremendous heat, says James Klimchuk of the NASA Goddard Space Flight Center in Greenbelt, Md. Klimchuk calls the new observations “very exciting” but notes that his own preliminary calculations indicate that spicules provide only a small share of the hot plasma in the corona, leaving plenty of room for other, more conventional modes of coronal heating.
For his part, De Pontieu sounds a similar note of caution that the long-standing problem of the corona’s temperature has yet to be conclusively resolved. “I think it’s important to point out that we have not solved coronal heating, but we have provided a piece of the puzzle,” he says. “We’ll see down the line whether this proves to be a dominant process or simply a contributor.”
This article was originally published with the title Feeling the Heat.
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11 Comments
Add CommentI hate to be overly picky. However, in the interest of accurate terminology, I think it is important to point out a misuse of terms here. When referring to absolute temperature this way the text should read '[number] degrees Kelvin' not '[number] degrees kelvins'. Kelvin is a proper noun and refers to the measurement system. As such it is always singular.
Reply | Report Abuse | Link to thisAre you implying that when I refer to the temperature outside as 85 degrees farenheits or 8 degrees celciii it has been wrong all these years?
Reply | Report Abuse | Link to thisI suspect new discoveries will also bring new understanding about how solar mechanisms also affect the Earths temperature & ozone layer in ways not now understood
Reply | Report Abuse | Link to thisI hate to be overly picky, however, the kelvin is not referred to in terms of degrees. The text should read simply '[number] Kelvin'
Reply | Report Abuse | Link to thisEven more pickily, it's 'kelvin', not 'Kelvin'.
Reply | Report Abuse | Link to thisMan! You guys are seriously picky!
Reply | Report Abuse | Link to thisYou folks are missing the point with picky units, we are talking 2 million degrees in the solar atmosphere, compared to 5800K on the surface. This is an incredible difference in temperature, when all they can come up with is 100 second bursts of plasma! These people.. De Pontieu, Klimchuk and Phillips must be desperate to close this question.
Reply | Report Abuse | Link to thisThere are so many anomalies with the current fusion idea, it sounds like serious propping up crumbling theories to me. How long can they keep on kidding us that the sun is very, very hot in the middle, cool on the surface and scalding hot on the outside? Not brilliant physics or common sense.
Spicules, you've got to be joking. You haven't even given a good, simple reason why sunspots are the coldest points (3800K) on the surface and yet they see deepest into the interior. Why does the solar wind vary so much, even stopping? Why do the ions accelerate anyway? Why does the Sun rotate more rapidly at equator than poles? Come on!
For all the picky people in evidence, none of the comments have got it right yet!
Reply | Report Abuse | Link to thisThe kelvin (not the Kelvin) is the SI unit of temperature. Like all SI units, when spelled out, it is written in lower-case; but like all SI units named after people, its symbol (K) is a capital letter. It might seem that degrees Celsius or degrees Fahrenheit violate this rule, but the unit here is "degree", in lower case. The following word is the name of the inventor, and therefore capitalized, just as we would capitalize Kelvin when speaking of Lord Kelvin himself.
Then why not degrees Kelvin? Well, that used to be the style, but kelvins is now the accepted norm. Yes, pluralization is correct, just as when one speaks of five kilograms or thirty seconds. Kelvins (capitalized there, where any word would be) are units of temperature.
Degrees, technically, are not true units but are, well, degrees. They express a degree of temperature above or below a completely arbitrary zero point. Zero degrees Celsius does not mean there is no temperature. Zero kelvins does.
The way to tell a true unit from a degree of something is to look at the zero point. Zero units of anything implies there is none of that thing. Therefore, at zero, the units become interchangeable--in fact, unnecessary. Zero length can be expressed as 0 meters, 0 inches, 0 furlongs, or 0 microns. When there isn't any, it makes no difference how you measure some.
But 0 degrees C and 0 degrees F are quite different, both from each other and from 0 kelvins. Neither of them expresses "no temperature", while 0 kelvins does. It refers to absolute zero, which would be the same point in on any such scale. So it is only right that kelvins, which are true units, should be written accordingly.
Picky? Sure. But there's method to the seeming madness.
I almost forgot what brought me here, and that was to point out that "degrees kelvins" is incorrect, and should have been caught by the editors.
Reply | Report Abuse | Link to thisAn erratum to my previous message: I said implied that the units named after people were always capital letters when abbreviated, but of course some are not single letters at all. For example, the unit of pressure, the pascal, is abbreviated Pa. Named after Blaise Pascal, it has an initial capital when abbreviated, but not when spelled out.
Thank goodness we've sorted that out, I thought for second we might get distracted. Now why does a spicule convey millions of k's into the solar atmosphere, in its brief 100 seconds life? Or is that being picky?
Reply | Report Abuse | Link to thisSpicules are not mysteries, heck there are two opposed collimated streams on each proto-star, and two on black holes, and those jet material out for millions of miles and they accomplish that in a great hurry. Their energies come from the star, and each of them operate on fusion reactions electromagnetic energy.
Reply | Report Abuse | Link to thisWhile spicules are much smaller, because they have smaller generating points that rotate around, thus shutting off the energy as the induction zone is rotated away.
Lately, solar investigators found hundreds of things in the convective zone that rotate, called tapered upper-latitude cells. Other solar investigators found in the convective zone hundreds of bright cyclonic devices that they named as small cyclonic storms at high latitudes. By theorizing those two finds to be the same items and with there being lots of them, they must be the spicules. That means they are a negative energy device.
So, as they punch through the chromosphere they attract and capture positive protons, which are released in the corona when the spicule's energy source rotates its induction zone away from that which could become a conductor of electric energy.
If someone can point out where that theory has holes in it, please comment on it.