How is the corona hotter than the surface?

Ceritus · 11th August 2005, 02:30 AM

This bugs the heck out of me! Why is the corona of the sun hotter than its surface? Considering the corona is formed by the engery being transmitted from its core which eventually passes through the surface.

Dr. Fendetestas · 11th August 2005, 03:33 AM

The Sun has several layers: nucleus, convective zone, photosphere, cromosphere, transition layer and corona. In the photosphere there is a negative temperature gradient (i.e. is colder farther from the centre). In the cromosphere the change is reversed and temperature begins to increase, from 6000 K to a million K in the corona. The reason for this is that part of the energy is transmited as a wave, from the convective zone. This wave moves with a certain speed, initially smaller than the speed of sound. But the density of the Sun is not constant, it decreases with the distance to the centre. And the speed of sound depends on the density of the medium. So, eventually, the speed of the wave gets bigger than he speed of sound and a shockwave is created. This shockwave will be responsible for the dramatic increase in temperature.

Hellbound · 11th August 2005, 05:51 AM

I'd always heard it explained that the effect occurs because the sun itself is somewhat opaque to light and infrared energy. Inside the sun, most of the energy emitted gets reabsorbed by surrounding particles and re-emitted again. Essentially, the energy that hits you is actually just from a small range around you. The Corona is much less dense, and much more transparant to energy...so you get energy not just from the area around you, but from the entire visible surface of the sun as well.

Is there any validity to that explanation, or am I way off base? Can't recall where I heard it from, but it seems to make sense.

Ziggurat · 11th August 2005, 09:54 AM

One thing to keep in mind is that the sun is a highly non-equilibrium system. There's enormous energy flows moving through the whole thing, which makes intuitive guesses about temperature unreliable. And temperature is not equivalent to energy density either - depending on the heat capacity, you can have hot materials with less thermal energy than cold materials. Part of the discrepency comes from the fact that temperature is not actually as intuitive a quantity as we often think. Temperature is a measure of how much the entropy of a system changes with changes in energy.

One particularly bizarre example of this is negative temperatures. Yes, negative temperatures are possible, but all that means is that entropy increases as you decrease the energy of the system. This actually only occurs in certain kinds of systems at very HIGH energies. Negative temperatures are, on an intuitive level, hotter than positive temperatures. They're also highly unstable, and will almost always decay down to lower-energy, positive-temperature states unless you do something to maintain them. One example is nuclear spins in a magnetic field. If you align all the spins in the direction of the field, then reverse the field rapidly, they'll all be anti-aligned with the field. This is a negative temperature situation, since if a few of the spins start to flip, the entropy of the system increases, but the energy decreases. As it cools, the temperature actually goes MORE negative, and finally diverges when half the spins are pointed in the direction of the field and half pointed in the opposite direction. As it cools more, the temperature drops to finite values from the positive infinity direction. But although the temperature diverges, the energy involved is very much finite (the heat capacity drops to zero when spins are randomly oriented). If you just try heating up the system in a magnetic field, you can never make it go to negative temperatures, because thermal energy alone can't ever make the majority of spins point opposite to the field. You need a non-random process to do that, which means aligning the spins first and THEN flipping the field.

Sort of a tangent, I know, but the main point is that temperature isn't equivalent to thermal energy, and that the temperature gradients around the sun may have more to do with changes in the degrees of freedom (ie, the heat capacity, or how much entropy changes with energy) for the various layers than with how much thermal energy is there.

LucyR · 11th August 2005, 02:02 PM

I've heard that a likely source of energy is the presence of strong magnetic fields in the corona which then presumably excite charged particles. The fields terminate at or just under the surface. Theyâ€™re known to exist but their origin is not well understood.

Also, I think Ziggurat's points are good and should be borne in mind.

11th August 2005, 02:30 AM	#1
Ceritus Suspended Join Date: Jul 2005 Location: I am the mind that you call home. Posts: 650	How is the corona hotter than the surface? This bugs the heck out of me! Why is the corona of the sun hotter than its surface? Considering the corona is formed by the engery being transmitted from its core which eventually passes through the surface.

11th August 2005, 03:33 AM	#2
Dr. Fendetestas Scholar Join Date: Jul 2005 Posts: 65	The Sun has several layers: nucleus, convective zone, photosphere, cromosphere, transition layer and corona. In the photosphere there is a negative temperature gradient (i.e. is colder farther from the centre). In the cromosphere the change is reversed and temperature begins to increase, from 6000 K to a million K in the corona. The reason for this is that part of the energy is transmited as a wave, from the convective zone. This wave moves with a certain speed, initially smaller than the speed of sound. But the density of the Sun is not constant, it decreases with the distance to the centre. And the speed of sound depends on the density of the medium. So, eventually, the speed of the wave gets bigger than he speed of sound and a shockwave is created. This shockwave will be responsible for the dramatic increase in temperature.
Dr. Fendetestas Scholar Join Date: Jul 2005 Posts: 65

11th August 2005, 09:54 AM	#4
Ziggurat Penultimate Amazing Join Date: Jun 2003 Posts: 26,195	One thing to keep in mind is that the sun is a highly non-equilibrium system. There's enormous energy flows moving through the whole thing, which makes intuitive guesses about temperature unreliable. And temperature is not equivalent to energy density either - depending on the heat capacity, you can have hot materials with less thermal energy than cold materials. Part of the discrepency comes from the fact that temperature is not actually as intuitive a quantity as we often think. Temperature is a measure of how much the entropy of a system changes with changes in energy. One particularly bizarre example of this is negative temperatures. Yes, negative temperatures are possible, but all that means is that entropy increases as you decrease the energy of the system. This actually only occurs in certain kinds of systems at very HIGH energies. Negative temperatures are, on an intuitive level, hotter than positive temperatures. They're also highly unstable, and will almost always decay down to lower-energy, positive-temperature states unless you do something to maintain them. One example is nuclear spins in a magnetic field. If you align all the spins in the direction of the field, then reverse the field rapidly, they'll all be anti-aligned with the field. This is a negative temperature situation, since if a few of the spins start to flip, the entropy of the system increases, but the energy decreases. As it cools, the temperature actually goes MORE negative, and finally diverges when half the spins are pointed in the direction of the field and half pointed in the opposite direction. As it cools more, the temperature drops to finite values from the positive infinity direction. But although the temperature diverges, the energy involved is very much finite (the heat capacity drops to zero when spins are randomly oriented). If you just try heating up the system in a magnetic field, you can never make it go to negative temperatures, because thermal energy alone can't ever make the majority of spins point opposite to the field. You need a non-random process to do that, which means aligning the spins first and THEN flipping the field. Sort of a tangent, I know, but the main point is that temperature isn't equivalent to thermal energy, and that the temperature gradients around the sun may have more to do with changes in the degrees of freedom (ie, the heat capacity, or how much entropy changes with energy) for the various layers than with how much thermal energy is there.
	__________________ "As long as it is admitted that the law may be diverted from its true purpose -- that it may violate property instead of protecting it -- then everyone will want to participate in making the law, either to protect himself against plunder or to use it for plunder. Political questions will always be prejudicial, dominant, and all-absorbing. There will be fighting at the door of the Legislative Palace, and the struggle within will be no less furious." - Bastiat, The Law

11th August 2005, 02:02 PM	#5
LucyR Graduate Poster Join Date: Feb 2003 Posts: 1,086	I've heard that a likely source of energy is the presence of strong magnetic fields in the corona which then presumably excite charged particles. The fields terminate at or just under the surface. Theyâ€™re known to exist but their origin is not well understood. Also, I think Ziggurat's points are good and should be borne in mind.
LucyR Graduate Poster Join Date: Feb 2003 Posts: 1,086

11th August 2005, 05:51 AM	#3
Hellbound Abiogenic Spongiform Join Date: Sep 2002 Location: In a handbasket Posts: 8,922	I'd always heard it explained that the effect occurs because the sun itself is somewhat opaque to light and infrared energy. Inside the sun, most of the energy emitted gets reabsorbed by surrounding particles and re-emitted again. Essentially, the energy that hits you is actually just from a small range around you. The Corona is much less dense, and much more transparant to energy...so you get energy not just from the area around you, but from the entire visible surface of the sun as well. Is there any validity to that explanation, or am I way off base? Can't recall where I heard it from, but it seems to make sense.