Does anyone know of some good theories on why the sun's corona is much hotter than it's surface?
827956
The leading theories have to do with magnetic reconnection, but there are people still pushing the idea that nano- or micro-flares can heat the corona.  I've done research on the nanoflare idea, and I have to say - reconnection seems more plausible.
584790
There is no one single uniform temperature in the sun. It depends which layer we are talking about ...\n\nThe Core\nThe innermost layer of the sun is the core. With a temperature of 15 million kelvins (27 million degrees Fahrenheit) \n\nSolar Envelope\nOutside of the core is the radiative envelope, which is surrounded by the convective envelope. The temperature is 4 million kelvins (7 million degrees F). The core contains 40 percent of the sun's mass in 10 percent of the volume, while the solar envelope has 60 percent of the mass in 90 percent of the volume.\n\nPhotosphere\nThe photosphere is the zone from which the sunlight we see is emitted. The photosphere is a comparatively thin layer of low pressure gasses surrounding the envelope. It is only a few hundred kilometers thick, with a temperature of 6000 K. \n\nChromosphere\nIn an eclipse, a red circle around the outside of the sun can sometimes can be seen. This is the chromosphere. Its red coloring is caused by the abundance of hydrogen.\n\nFrom the center of the sun to the chromosphere, the temperature decreases proportionally as the distance from the core increases. The chromosphere's temperature, however, is 7000 K, hotter than that of the photosphere. Temperatures continue to increase through the corona.\n\nSunspots\nSunspots are dark spots on the photosphere, typically with the same diameter as the Earth. They have cooler temperatures than the photosphere. The center of a spot, the umbra, looks dark gray if heavily filtered and is only 4500 K (as compared to the photosphere at 6000K). \n\nThe sun has enormous organized magnetic fields that reach from pole to pole. Loops of the magnetic field oppose convection in the convective envelope and stop the flow of energy to the surface. This results in cool spots at the surface which produce less light than the warmer areas. These cool, dark spots are the sunspots.\n\nCorona\nThe outermost layer of the sun is the corona. Only visible during eclipses, it is a low density cloud of plasma with higher transparency than the inner layers. The white corona is a million times less bright than the inner layers of the sun, but is many times larger.\n\nThe corona is hotter than some of the inner layers. Its average temperature is 1 million K (2 million degrees F) but in some places it can reach 3 million K (5 million degrees F).\n\nTemperatures steadily decrease as we move farther away from the core, but after the photosphere they begin to rise again. There are several theories that explain this, but none have been proven.\n\nSolar Flares\nIn the corona, above sunspots and areas of complex magnetic field patterns, are solar flares. These sparks of energy sometimes reach the size of the Earth and can last for up to several hours. Their temperature has been recorded at 11 million K (20 million degrees F). The extreme heat produces x rays that create light when they hit the gases of the corona.
170430
I think - not sure just think- the sun is much bigger?!!!!\n\nand it is made of gas, helium! \nandddd it's so hot!
813508
The temperature of the sun varies depending on proximity to the surface.  The core temperature can be more than 13 million degrees Celcius.  The corona is only about 5 million degrees Celcius.  As a large ball of plasma fusing gases together, the sun does not have an atmosphere in the usual sense of the word.
278956
This has been asked and answered many times before:\n\nhttp://answers.yahoo.com/search/search_result;_ylt=Amylv82zdJRUVkmYew.javjpy6IX?p=temperature+of+the+sun
948421
The Sun is the most prominent feature in our solar system. It is the largest object and contains approximately 98% of the total solar system mass. One hundred and nine Earths would be required to fit across the Sun's disk, and its interior could hold over 1.3 million Earths. The Sun's outer visible layer is called the photosphere and has a temperature of 6,000°C (11,000°F). This layer has a mottled appearance due to the turbulent eruptions of energy at the surface. \nSolar energy is created deep within the core of the Sun. It is here that the temperature (15,000,000° C; 27,000,000° F) and pressure (340 billion times Earth's air pressure at sea level) is so intense that nuclear reactions take place. This reaction causes four protons or hydrogen nuclei to fuse together to form one alpha particle or helium nucleus. The alpha particle is about .7 percent less massive than the four protons. The difference in mass is expelled as energy and is carried to the surface of the Sun, through a process known as convection, where it is released as light and heat. Energy generated in the Sun's core takes a million years to reach its surface. Every second 700 million tons of hydrogen are converted into helium ashes. In the process 5 million tons of pure energy is released; therefore, as time goes on the Sun is becoming lighter. \n\n \n\nThe chromosphere is above the photosphere. Solar energy passes through this region on its way out from the center of the Sun. Faculae and flares arise in the chromosphere. Faculae are bright luminous hydrogen clouds which form above regions where sunspots are about to form. Flares are bright filaments of hot gas emerging from sunspot regions. Sunspots are dark depressions on the photosphere with a typical temperature of 4,000°C (7,000°F). \n\nThe corona is the outer part of the Sun's atmosphere. It is in this region that prominences appears. Prominences are immense clouds of glowing gas that erupt from the upper chromosphere. The outer region of the corona stretches far into space and consists of particles traveling slowly away from the Sun. The corona can only be seen during total solar eclipses. (See Solar Eclipse Image). \n\nThe Sun appears to have been active for 4.6 billion years and has enough fuel to go on for another five billion years or so. At the end of its life, the Sun will start to fuse helium into heavier elements and begin to swell up, ultimately growing so large that it will swallow the Earth. After a billion years as a red giant, it will suddenly collapse into a white dwarf -- the final end product of a star like ours. It may take a trillion years to cool off completely. \n\n\nSun Statistics \nMass (kg) 1.989e+30 \nMass (Earth = 1) 332,830 \nEquatorial radius (km) 695,000 \nEquatorial radius (Earth = 1) 108.97 \nMean density (gm/cm^3) 1.410 \nRotational period (days) 25-36* \nEscape velocity (km/sec) 618.02 \nLuminosity (ergs/sec) 3.827e33 \nMagnitude (Vo) -26.8 \nMean surface temperature 6,000°C \nAge (billion years) 4.5 \nPrincipal chemistry \nHydrogen\nHelium\nOxygen \nCarbon\nNitrogen\nNeon\nIron\nSilicon\nMagnesium\nSulfur\nAll others  \n92.1%\n7.8%\n0.061%\n0.030%\n0.0084%\n0.0076%\n0.0037%\n0.0031%\n0.0024%\n0.0015%\n0.0015% \n\n* The Sun's period of rotation at the surface varies from approximately 25 days at the equator to 36 days at the poles. Deep down, below the convective zone, everything appears to rotate with a period of 27 days. \n\n\nEclipse & Sun Movies\n\nSOHO - Halo Mass Ejection Event. \nSOHO - Halo Mass Ejection Event - Zoomed Out. \nSOHO - Explosive Coronal Mass Ejections from the Sun. \nSOHO-6 Comet at 2-6 solar radii from the Sun. \nSolar Magnetic Field Map \nMan's Fascination of the Sun. \nThe Sun's interior, photosphere, and corona. \nSun's magnetic field, prominences, solar wind, aurora. \nCorona Animation of Solar Prominences. \n1993-1994 animation of the Sun as seen in soft x-rays. \n1994 Eclipse. \nA QuickTime movie (15
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Nasa - The Solar Wind\nhttp://www-istp.gsfc.nasa.gov/Education/wsolwind.html\n\nSolar wind is a constant flow of gases from the sun.  It results from the expansion of gases in the corona.  Billions and billions of particles are produced by the sun each second it burns, giving off heat and light.  A vast majority of these particles have an electric charge (a plus or minus), and in turn these particles are affected by the sun’s magnetic field.  These small particles are then blasted into the upper part of the sun’s atmosphere by intense radiation from the sun’s core.  The upper region of the sun’s atmosphere that these particles are blasted into is known as the corona.  When electrons are caught in the corona they become heated to temperatures exceeding four million degrees F.  This causes the particles to move very rapidly, and they collide into each other and lose their electrons and become electrically charged particles.  These particles are known as ions and they make up the solar wind.\n\nhttp://www.kelso.wednet.edu/coweeman/contents/Mars_Project/solar_winds.htm
14003
The sun and lightning may seem to be equal, but it appears that lightning is 5 times hotter than the sun. The sun's surface is 1,000°F. Times that by 5, and you have the temp of a lightning bolt.
126240
A better quetsion is why you think it would it be.\n\nThe sun is a huge fusion reaction separated from Earth by millions of miles of vacuum. By what possible mechanism could the two bodies ever reach thermal equilibrium?
611962
How hot is concentrated light, what is the temp of the sun?\n\nWELL WE HAVE A ENERGY SHORTAGE, ONLY AT SUNSET.\n\n I read about Solar collection in power plants like Solar One.  In these plants the where they make molten salt, for heat transfer for running steam generator. At the reciever of Solar One plant or concentrated light point temperature reachest approximately 1,050 Fahrenheit.\n    \n      THIS LINK IS ABOUT YOUR SUN AND THE TEMPS ARE HOT.\n         SO HOW MUCH LIGHT CAN WE CONDENTRATE MORE  \n           DEVELOPMENT IN SOLAR SAILS\n\nhttp://hyperphysics.phy-astr.gsu.edu/hbase/hframe.html \n\n Earth's Sun is a medium-sized star which lies on the main sequence with 90% of the known stars. It has a effective surface temperature is 5780 K, putting it in spectral class G2. Its mass is 1.989 x 1030 kg and its mean radius is 6.96 x 108 meters. The mass of the sun is over 99.8% of the mass of the entire known solar system, leading de Pater and Lissauer to refer lightly to the solar system as "the Sun plus some debris". \n\nThe sun radiates energy at the rate of 3.85 x1026 watts. Just outside the earth's atmosphere solar energy is received, assuming normal incidence, at the rate of 1340 watts per square meter. \n \n\nThe orbit of Earth ranges from 1.47 to 1.52 x 1011 meters from the Sun. The average light travel time to the earth is 8.3 minutes. \n\nThe radius of the sun at 696,000 km is 109 times the Earth's radius. Its surface gravity is 274 m/s2 or 28.0 times that of the Earth. Its mean density is 1410 kg/m3 or 0.255 times the mean density of Earth. \n\nThe composition of the sun is 71% hydrogen, 27.1% helium and less than 2% of all other elements. \n\nThe center temperature is modeled to be 15.5 million K. The Sun is fueled by the proton cycle of nuclear fusion.\n\nEscape velocity = 618 km/s\n\nSO
532654
Well Venus is much closer to the sun than we are so it is a lot hotter there... stuff that exists as a liquid here (water) would only be able to be a gas there due to the high temps. This also makes the atmosphere there much denser than ours. Not much sunlight would penetrate the thick atmosphere so there isn't any plant life to contribute oxygen to and remove carbon dioxide from the atmosphere the way plants do here.
300263
Our sun began forming within an immense interstellar cloud of drifting gas and dust. Inside this cloud (called a 'nebula') were regions of relatively high density where gravity began drawing together bits and pieces of the gas and dust. Over several million years, this spherical region became denser and denser which caused its central temperature to rise. Finally, some 4.5-billion years ago, the temperature was hot enough to start nuclear fusion. Our sun was born.\n\nGod should be spelled B-I-G B-A-N-G\nPBrax@netscape.com
218291
Lightning can heat nearby air up to even 10 times the temperature of the Sun's surface. Actually, the Sun's surface isn't so hot — some welding torches are hotter. ( The Sun's center, though, is 2500 times hotter: 15 million degrees Kelvin compared with 6 thousand degrees.)\n\nIt isn't the Sun's temperature, however, that accounts for the heat we feel on Earth. Rather, it's her size. The Sun has a surface area a hundred million billion times the surface area of a typical lightning bolt. We receive only 1 billionth of the Sun's energy. Even so, Earth gets much more energy from the Sun than from a lightning bolt over the same time period — a hundred million times more energy. \n\nOf course, close to the lightning strike, we can get blazing hot. Lightning striking a tree can set it afire. The distance to the target, however, must be small for us to feel the heat because lightning's radiant energy pales compared with the Sun
67886
Less atmospheric cover is the primary culprit. The atmosphere is like a thick blanket, and provides insulation to the planet's surface, trapping heat and retaining it. As you go higher, the thinner atmospheric density allows heat to escape more quickly, since it's not getting trapped in air molecules. I live at 6600 fee above sea level, and even in the summer, when the sun goes down, it cools down fast.
566076
The sun rays expand when it goes further and further, so more area to cover from the sun on the land, thats the part that is hot. go further up and the rays are thinner hence cooler.
1072113
It has been stated that the sun has a great deal of hydrogen within it due to the necessity of it being converted into helium, and the energy difference generating its heat potential.  There is a problem with this concept.  Although there is energy release in the conversion of hydrogen to helium, the interior of our sun is not what has been taught in your school.\n\nWere you to determine the acceleration value of a mass 400 miles from the very center of our sun, you would find that were a mass to be released in this location, it would accelerate to a speed greater than that of light in one second.  This means there is not a continual atomic furnace generating heat by the process of hydrogen to helium as taught.  Either there is a hollow area within our sun where mass cannot exist, or there is a new form of mass unknown to mankind.  Offhand I would reject the last.\n\nThe reason helium and/or oxygen could not burn within our sun is that the pressure surrounding the gases is too great.  Notice when you burn a piece of paper how the smoke spreads outward along with the heat energy.  Within the sun the heat energy is already beyond that of what we would experience on our planet.  There is nothing to change.
224344
it is true that at higher altitudes there is more heat provided by the sun, but there is also much less atmosphere between the surface of the earth and space.  This means that the thermal energy provided by the sun travels out to space much faster, causing a temperature drop.  In addition as previously mentioned, gasses at lower pressures tend to exist at lower temperatures as given by the ideal gas law.