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  Solar prominences and coronal holesJul 27, 2011 2:59 AM PDT | url
 
Added 1 new A* page:Thanks to AnsuGisalas for linking to A* in a discussion thread on this TechRepublic article; it was something of a non sequitur, but I'll take it. :D
 
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The recent string of posts on telescopes and the 1893 World's Fair and all that actually started, my notes just reminded me, with a post from mid-last-week on the then-mysterious event in May 1999 dubbed The Day the Solar Wind Disappeared. In that post I mentioned that later research suggested the almost complete shut-off of the solar wind in Earth's direction was due to active regions on the Sun's surface choking off the coronal hole through which the solar wind would otherwise have been issuing, and went on to explain that "active regions" are just areas on the Sun's surface where something is happening, aside from just the usual solar currents of heat and magnetism, and gave an example of sunspots as one such type of region.
 
But there are other types of active regions--and ones that make for much cooler pictures, too. For instance, a solar prominence is an extension of relatively cool plasma above the Sun's surface (and those "filaments" in the sunspot picture in the earlier article were types of solar prominences too, just viewed top-down so you couldn't really tell they were above the surface); some of them get really big, like this one seen from Skylab in 1973, which was called one of the largest ever recorded:
 
Image
image by NASA (source)
 
Here's a somewhat smaller one seen by the Solar Dynamics Observatory in 2010, with Earth and Jupiter superimposed inside it to give some idea of the planet-eating sizes of these ribbons of plasma:
 
Image
image by NASA (source)
 
And a coronal hole is more or less what it sounds like, I guess; these cooler, depressed areas of the surface are filled with much lower density plasma, and the solar wind comes tearing out of them along open magnetic field lines--"open" because they head out into space without looping back to the Sun's surface, which is why the solar wind can escape from those spots. Here's half of a STEREO image of one (that dark spot in the middle-ish; there are also some at the poles, but this isn't a great view of them), taken with its Extreme Ultraviolet Imager:
 
Image
image by NASA (source)
 
The twin STEREO probes and the SDO are nifty space instruments; I've talked about them before here (with more big Sun images). Here's a massive coronal hole stretching across the top half of the Sun, as seen on February 1-3 of this year by SOHO in extreme ultraviolet:
 
video on Youtube
 
Not to be left out, there's also the ESA/NASA SOHO probe, which has taken some nifty extreme ultraviolet photos of solar-wind-borne plumes that extend out of large coronal holes that usually inhabit the Sun's poles:
 
Image
image by NASA/ESA (source)
 
 
 
 
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