The Quiet Sun The sun is currently being studied by several spacecraft Ulysses, SOHO, STEREO, and ACE. Messenger also contains instruments that can do some solar studies. http://www.stereo.gsfc.nasa.gov http://sohowww.nascom.nasa.gov/ http://www.srl.caltech.edu/ace/ace_mission.html http://ulysses.jpl.nasa.gov/ Mar 19 10:45 AM Sep 18 1:44 PM The sun is the star nearest the earth. It is the ultimate energy source of most life on earth. The sun's "surface" is called the photosphere and is the part of the sun that we actually see. The photosphere behaves like a blackbody its color is determined by its temperature (5800 K). It is the lowest layer of the sun's atmosphere the two outer layers are transparent. The photosphere shows limb darkening toward its edge. This happens because the light from the limb must travel through more of the sun's atmosphere than light from the center. Mar 19 8:23 AM Mar 19 8:31 AM The sun's photosphere also has granulation, which is caused by convection cells (like in a pot of boiling fudge). http://www.kis.uni freiburg.de/~pnb/movies/gr10m_44_2.mpg Directly above the photosphere is the chromosphere, which is not normally visible. It has a very spikey appearance due to the presence of spicules, which rise and fall. The spicules are long fingers of luminous gas. Mar 19 10:25 AM Mar 19 10:30 AM 1
The sun's outermost layer is the corona, which is above the chromosphere. The corona has a temperature of several million K, due to interactions with the sun's magnetic field. Only visible during a total solar eclipse or by using a coronagraph. Mar 19 10:37 AM http://solar heliospheric.engin.umich.edu/hjenning/corona.html The solar wind is formed as the hot gases of the corona escape from the sun and flow outward into space. They travel at a speed of several hundred miles per second. Mar 19 10:41 AM The active sun is powered by the sun's magnetic field. The solar magnetic field produces several features: Mar 19 10:44 AM 1) Sunspots Mar 20 9:32 AM Sunspot activity varies from low to high and back again over an 11 year period. The last solar maximum (Solar Max) was in 2000, so we are currently in a period of relatively low activity. Appear darker because they are cooler than their surroundings, usually occur in groups, and they are depressed below their surroundings. Mar 20 9:35 AM Sunspots can be used to determine the sun's rate of rotation (25 days at equator, 35 days at the poles). http://www.astr.ua.edu/ay102/lab7/lab_7_rotation.html Mar 20 9:40 AM 2
From 1645 1715, there were almost no sunspots (Maunder Minimum). Sunspots occur where the sun's magnetic field penetrates the surface. The winters were very cold during this time ("little Ice Age"). Why do we think that Christmas should be white? This inhibits convection and so the region is cooler and darker than the surrounding area. The magnetic field lines exit through one sunspot and re enter through another. Every 11 years, the sun's polarity reverses (22 year solar cycle). Mar 20 9:53 AM Most sunspots last for only a few days and then disappear a few can persist much longer, though. The intense magnetic field of a sunspot produces a split spectrum (Zeeman line) of the sunspot (pg. 255). The sun's magnetic field is twisted around by the sun's rotation making it very complex. Mar 20 9:56 AM Plages are light areas that show up just before a sunspot appears. They occur as the magnetic field lines push toward the surface, making it hotter. Filaments are composed of large amounts of glowing gases that are carried upward along the magnetic field lines. They are actually large loops called prominences that are seen from above. Mar 20 10:00 AM Mar 20 10:07 AM Mar 20 10:21 AM Mar 20 10:17 AM 3
Coronal holes are places where the corona is almost absent, allowing an escape path into space. This helps to produce the solar wind. Mar 20 10:19 AM Coronal mass ejections throw large amounts of matter out into space if they reach the earth, a magnetic storm can be the result. Mar 20 10:23 AM What powers the Sun? http://hesperia.gsfc.nasa.gov/sftheory/cme.htm Long ago, it was felt that the sun was simply a large lump of burning coal. A solar flare occurs when built up energy is suddenly released into space this energy is spread across the entire EM spectrum. The problem was, the sun would only burn for about 5000 years if that were true. Once it became apparent that the Sun was much older than that, another explaination had to be found. Mar 20 10:29 AM The next proposal was that the sun was powered by gravitational collapse. This meant that it was shrinking and that its energy was produced as it contracted and heated. Not a bad idea, but the age problem shows back up again the sun is very old and so it would have already gone out if this was the correct explaination. Mar 21 9:32 AM Finally, people realised that the sun was powered by nuclear fusion. In 1905, Albert Einstein gave us the equation: E = mc2 where, E= energy m=mass c=speed of light (m/s) This equation means that mass and energy can be converted into one another. Mar 21 9:34 AM Mar 21 9:36 AM 4
By the 1930's, it was realised that conditions inside the sun's core might allow some of the sun's mass to be converted into energy according to Einstein's equation (Hans Bethe). We call this process thermonuclear fusion. In the sun, hydrogen is fused into helium (600 millions tons per second). 4H > He + neutrinos + energy Mar 21 10:02 AM Mar 21 9:43 AM The mass of the helium is less than the mass of the four hydrogens this is the matter that was converted into energy. Gravity is continually trying to make the Sun collapse the energy produced in the core resists this eternal pull. Total solar output (luminosity) is about 3.9 x 1026 watts. The energy produced by the core is carried through the next layer of the sun by radiation (radiative transport). The earth's surface intercepts about 1400 w/m2 (solar flux). The next layer is opaque so the energy now travels toward the sun's surface by convection. This produces the convection cells that we see on the surface. Mar 21 9:52 AM Mar 21 9:55 AM The sun should be producing about 1038 neutrinos per second in its core. 100 billion of them should be striking each cm2 of the earth per second. Neutrinos only occasionally interact with ordinary matter so they are difficult to detect. Two large detectors have been built to study them one in Japan and one in South Dakota. Mar 21 10:09 AM Mar 22 9:57 AM 5
The detectors are filled with ultrapure water or with ultrapure perchloroethylene (dry cleaning fluid). Occasionally a neutrino will react with a neutron and produce a flash of light called Cerenkov radiation. Fewer neutrinos reach the earth from the sun than theory predicts (solar neutrino problem). For many years, it was thought that neutrinos had no mass. It has now been found that neutrinos do have mass (but not much) and so they can change as they travel to the earth. This does account for the difference between the observed and detected rates. Mar 22 10:03 AM Mar 21 9:58 AM Helioseismology is the study of how the sun vibrates and the information that its vibration provides. http://gong.nso.edu/ Since much of the sun's interior is very densely packed, it takes a very long time for photons to reach the outside about 170,000 years! Far out beyond Pluto, the solar wind ceases it is canceled out by the stellar winds of the surrounding stars. This boundary is known as the heliopause the two Voyagers are nearing it. A star the mass of the sun has a life span of about 10 billion years the sun is halfway through its life, so it is about 5 billion years old. Mar 21 10:11 AM Mar 22 10:08 AM http://voyager.jpl.nasa.gov Mar 22 10:11 AM 6