Sunspot Temperatures?

[just_some_guy]

We know that sunspots are cooler than the surrounding medium because they are darker (thought to be ~2000 deg C cooler). Is there any other scientific proof the sunspots are indeed cooler?

 

[Saiph]

The luminosity is a big clue actually, as there are only three ways to make a light more intense (that I know of). you can focus it more (like a laser, but not a valid approach here), make the radiating surface bigger, or make it hotter.<br /><br />A change in temperature makes a big shift in luminosity, and as we can compare the sunspot to similar sized patches of the sun, size isn't an issue.<br /><br /><br />Other than that, there is the blackbody spectrum. The intensity of the various wavelengths of light trace out a distinct curve called a blackbody curve. This curve gives low values for very high, and very low energy light, and peaks somewhere in the middle. It's shape is determined entirely by the temperature of the object (the height is based on total luminosity, basically size). The hotter the object, the more towards the blue colors (or higher energy to be more accurate) the curve peaks at. <br /><br />A sunspot has a spectrum (when you look at just the sunspot) that peaks at a longer wavelength of light (lower energy) indicating a lower temperature. <div class="Discussion_UserSignature"> <p align="center"><font color="#c0c0c0"><br /></font></p><p align="center"><font color="#999999"><em><font size="1">--------</font></em></font><font color="#999999"><em><font size="1">--------</font></em></font><font color="#999999"><em><font size="1">----</font></em></font><font color="#666699">SaiphMOD@gmail.com </font><font color="#999999"><em><font size="1">-------------------</font></em></font></p><p><font color="#999999"><em><font size="1">"This is my Timey Wimey Detector.  Goes "bing" when there's stuff.  It also fries eggs at 30 paces, wether you want it to or not actually.  I've learned to stay away from hens: It's not pretty when they blow" -- </font></em></font><font size="1" color="#999999">The Tenth Doctor, "Blink"</font></p> </div>

 

[just_some_guy]

OK, thanks - the black body spectrum info is the response I was after. <br /><br />I had wondered whether it was possible to shift the curve so far blue (hotter) that the visible light part of the spectrum actually appeared darker than the main body of the Sun, even though in this scenario the emission is hotter. Like how a very hot star's black body spectrum can be mostly in the ultraviolet. I guess, when this is the case the small part of the spectrum in the visible light range is still considerably brighter than a cooler star because of the much higher energies involved.

 

[tfwthom]

Answers to http://seds.lpl.arizona.edu/nineplanets/nineplanets/nineplanets.html<br /><br />The surface of the Sun, called the photosphere, is at a temperature of about 5800 K. Sunspots are "cool" regions, only 3800 K (they look dark only by comparison with the surrounding regions). Sunspots can be very large, as much as 50,000 km in diameter. Sunspots are caused by complicated and not very well understood interactions with the Sun's magnetic field <div class="Discussion_UserSignature"> <font size="1" color="#3366ff">www.siriuslookers.org</font> </div>

 

[vogon13]

And a point rarely made about black body curves is that a curve that peaks at a shorter wavelength than another, is everywhere along its' length <b><i>higher</i></b> than the other curve. <div class="Discussion_UserSignature"> <p><font color="#ff0000"><strong>TPTB went to Dallas and all I got was Plucked !!</strong></font></p><p><font color="#339966"><strong>So many people, so few recipes !!</strong></font></p><p><font color="#0000ff"><strong>Let's clean up this stinkhole !!</strong></font> </p> </div>

 

[Saiph]

To elaborate on Vogon's statement:<br /><br />All else being equal (basically size here) a hotter object radiates more light in <i>every</i> color than a cooler object. It also peaks higher.<br /><br />http://scienceworld.wolfram.com/physics/PlanckLaw.html<br /><br />This link illustrates my point. Notice how each curve is higher at every point than the last, cooler, curve. No two lines will cross.<br /><br />Now, if you vary the size, that can cause some shifts in overall luminosity. However, per square meter (basically fixing the size again) the hotter objects always emit more light, in all colors.<br /><br />Geesh, i repeated that alot. <div class="Discussion_UserSignature"> <p align="center"><font color="#c0c0c0"><br /></font></p><p align="center"><font color="#999999"><em><font size="1">--------</font></em></font><font color="#999999"><em><font size="1">--------</font></em></font><font color="#999999"><em><font size="1">----</font></em></font><font color="#666699">SaiphMOD@gmail.com </font><font color="#999999"><em><font size="1">-------------------</font></em></font></p><p><font color="#999999"><em><font size="1">"This is my Timey Wimey Detector.  Goes "bing" when there's stuff.  It also fries eggs at 30 paces, wether you want it to or not actually.  I've learned to stay away from hens: It's not pretty when they blow" -- </font></em></font><font size="1" color="#999999">The Tenth Doctor, "Blink"</font></p> </div>

 

[just_some_guy]

OK, good I understand black body radiation curves better now.<br /><br />So, how does the Sun's atmosphere above sunspots get so hot (far hotter than surrounding atmosphere)? I've heard its got to do with magnetic flux tubes getting twisted up and breaking causing flares etc. How does magnetic reconnection work (without the MHD formulas)?

 

[Saiph]

I don't think it's any hotter directly above the sunspots than anywhere else.<br /><br />The corona in general (the outermost layer) is hotter, it is believed to be due to the magnetic fields stirring the tenous gas of the corona. And despite the high temperature, there isn't much heat (real energy) due to the overall low density.<br /><br />The sunspots themselves are cooler, because the magnetic fields mess with the convection currents that supply that particular area. <div class="Discussion_UserSignature"> <p align="center"><font color="#c0c0c0"><br /></font></p><p align="center"><font color="#999999"><em><font size="1">--------</font></em></font><font color="#999999"><em><font size="1">--------</font></em></font><font color="#999999"><em><font size="1">----</font></em></font><font color="#666699">SaiphMOD@gmail.com </font><font color="#999999"><em><font size="1">-------------------</font></em></font></p><p><font color="#999999"><em><font size="1">"This is my Timey Wimey Detector.  Goes "bing" when there's stuff.  It also fries eggs at 30 paces, wether you want it to or not actually.  I've learned to stay away from hens: It's not pretty when they blow" -- </font></em></font><font size="1" color="#999999">The Tenth Doctor, "Blink"</font></p> </div>

 

[just_some_guy]

Check out these pics from SOHO - the top left pic shows the photosphere with dark sunspots in H alpha. The bottom two pics show the corona and chromosphere in two different UV wavelengths. Its quite clear that there is a lot of energy being generated above the sunspots, some of which is thermal energy. The loops you can see above the sunspots are thought to be plasma flowing along magnetic flux tubes, which penetrate the photosphere at the sunspots.

 

[newtonian]

Saiph - Hi! How are you?<br /><br />Yes, the corona is way much hotter - actually quite incredible, as is the temperature of the IGM.<br /><br />I forget off hand how they measured the temperature of the corona.<br /><br />And there is my old question: have we measured the temperature of a corona for any other star?<br /><br />And have we determined if the ultimate source of the magnetic dynamo heating the corona goes all the way to the core of the sun?<br /><br />If I remember correctly, there was evidence of floating magnetic fields from the core to the surface.