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'Coronal Heating' Could Be Explained by Solar Gravitation
In message , Thomas Smid
writes Are you questioning that the sun is a self-gravitating body? (now *that* would be a case for crank.net). The virial theorem applies to the sun like for any other structure held together by gravitation. Even the corona is bound to the sun as you can calculate for yourself from its temperature and the solar gravitation. Only some high energy particles escape as the solar wind. Isn't the solar wind just part of the corona? And what's the average speed of a hydrogen ion at 10^6K? -- Save the Hubble Space Telescope! |
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'Coronal Heating' Could Be Explained by Solar Gravitation
Thomas Smid wrote:
Ralph Hartley wrote in message ... the Corona has a temperature higher than its binding energy, so it escapes as the solar wind. The photosphere does not. On average, the corona actually still has a temperature less than the (gravitational) binding energy, but there is a relatively large number of particles with a high enough energy to escape. No, it does not. It has been known since the 1950s that in order to have a stable corona you would need an external confining pressure exceeding the pressure of the interstellar medium by many orders of magnitude. This failure to construct a realistic model of a hydrostatic corona led Parker to construct his solar wind model. You are asking why the photosphere, with hotter layers above and below, stays cool? The corona is sparse and transparent, so the photosphere can radiate heat into space. The emission of radiation is not the cause of the cooling but merely the consequence. Just imagine that all the atoms in a volume of gas are in an excited atomic state. The subsequent decay of the electrons to a lower level would only lead to an emission of radiation but leave the kinetic energy of the atom unchanged. For the latter to be reduced one needs inelastic collision processes of the atoms (or protons in the solar case). This description is inaccurate. It is the inelastic collisions that excite the atoms in the first place, and the energy is then lost from the plasma for good when the atoms deexcite by emitting radiation. Without these one would not see the sun as a sharply defined disk but as a fuzzy ball (the latter would still radiate though because of recombination of protons and electrons, which however does not affect the overall temperature) And this is plainly wrong. The reason that we see the Sun as a sharply defined disk is that the opacity of the solar plasma drops drastically at the photospheric temperature. Therefore over a short interval in temperature, or equivalently radius, the plasma goes from being opaque to being completely transparent. Ulf Torkelsson . |
#13
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'Coronal Heating' Could Be Explained by Solar Gravitation
Thomas Smid wrote:
Ralph Hartley wrote in message ... the Corona has a temperature higher than its binding energy, so it escapes as the solar wind. The photosphere does not. On average, the corona actually still has a temperature less than the (gravitational) binding energy, but there is a relatively large number of particles with a high enough energy to escape. No, it does not. It has been known since the 1950s that in order to have a stable corona you would need an external confining pressure exceeding the pressure of the interstellar medium by many orders of magnitude. This failure to construct a realistic model of a hydrostatic corona led Parker to construct his solar wind model. You are asking why the photosphere, with hotter layers above and below, stays cool? The corona is sparse and transparent, so the photosphere can radiate heat into space. The emission of radiation is not the cause of the cooling but merely the consequence. Just imagine that all the atoms in a volume of gas are in an excited atomic state. The subsequent decay of the electrons to a lower level would only lead to an emission of radiation but leave the kinetic energy of the atom unchanged. For the latter to be reduced one needs inelastic collision processes of the atoms (or protons in the solar case). This description is inaccurate. It is the inelastic collisions that excite the atoms in the first place, and the energy is then lost from the plasma for good when the atoms deexcite by emitting radiation. Without these one would not see the sun as a sharply defined disk but as a fuzzy ball (the latter would still radiate though because of recombination of protons and electrons, which however does not affect the overall temperature) And this is plainly wrong. The reason that we see the Sun as a sharply defined disk is that the opacity of the solar plasma drops drastically at the photospheric temperature. Therefore over a short interval in temperature, or equivalently radius, the plasma goes from being opaque to being completely transparent. Ulf Torkelsson . |
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'Coronal Heating' Could Be Explained by Solar Gravitation
Thomas Smid wrote:
Ulf Torkelsson wrote in message ... Thomas Smid wrote: On average, the corona actually still has a temperature less than the (gravitational) binding energy, but there is a relatively large number of particles with a high enough energy to escape. No, it does not. It has been known since the 1950s that in order to have a stable corona you would need an external confining pressure exceeding the pressure of the interstellar medium by many orders of magnitude. This failure to construct a realistic model of a hydrostatic corona led Parker to construct his solar wind model. The kinetic energy required to escape the solar gravitational field corresponds to temperatures in excess of 10^7 K. The coronal temperature is well below this. OK, yes you can construct an isothermal hydrostatic model of the solar corona, but such a model has a finite pressure at infinity, that is higher than the pressure of the interstellar medium surrounding the Sun, therefore this cannot be the correct description of the solar corona, and it rather turns out that you get a good description of the entire heliosphere by abandoning the assumption that the corona is hydrostatic, and instead use a solar wind model. Of course, for a gas in thermodynamic equilibrium there will always be particles with an energy high enough to escape the gravitational field. However, the loss of particles is so insignificant that you can consider the corona (as well as the solar wind) as quasi-stable. Well such models do not manage to generate a solar wind, which is as fast and as massive as is observed, so that model is out, and has been known to wrong since the 1960s, when the solar wind was observed. The emission of radiation is not the cause of the cooling but merely the consequence. Just imagine that all the atoms in a volume of gas are in an excited atomic state. The subsequent decay of the electrons to a lower level would only lead to an emission of radiation but leave the kinetic energy of the atom unchanged. For the latter to be reduced one needs inelastic collision processes of the atoms (or protons in the solar case). This description is inaccurate. It is the inelastic collisions that excite the atoms in the first place, and the energy is then lost from the plasma for good when the atoms deexcite by emitting radiation. The kinetic energy of the protons is lost when they collisionally excite neutral atoms (of which there a few in the photosphere but still some). It is irrelevant for the gas temperature what happens to the excited atoms afterwards, i.e. if the radiation leaves the volume or not). No, it is not irrelevant at all. There are, in general, two ways in which an excited atom can lose its extra energy, by radiative deexcitation, in which case the photon carries away the energy from the plasma for good if the plasma is optically thin, or by collisional deexcitation, in which case it is transferred as kinetic energy to a new atom. Without these one would not see the sun as a sharply defined disk but as a fuzzy ball (the latter would still radiate though because of recombination of protons and electrons, which however does not affect the overall temperature) And this is plainly wrong. The reason that we see the Sun as a sharply defined disk is that the opacity of the solar plasma drops drastically at the photospheric temperature. Therefore over a short interval in temperature, or equivalently radius, the plasma goes from being opaque to being completely transparent. No, the reason we see the sun as a sharply defined disk is the sharp drop of gas density in and above the photosphere. This sharp density drop is caused by the cooling due to inelastic collisions. As a result, there is nothing left above the photosphere that you could see. Opacity alone could not produce the same effect. It would limit the visibility to a certain depth but would not change anything about the 'fuzziness' above this height if there would not be sharp drop of density. I am sorry, but your are simply wrong here. The sharply defined solar disc is due to the sudden drop off of the opacity, which depends on both density and temperature. The temperature dependence of the opacity is even easy to observe. The temperature in the solar photosphere is lower above a sunspot, thus the opacity is smaller there, and we are able to see to a deeper level inside the sunspot. This results in that from our point of view the umbra, the central part of the sunspot, is displaced towards the centre of the sun as seen from the Earth as the sunspot approaches the solar limb. This Wilson effect was described already in the 18th century. Ulf Torkelsson |
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'Coronal Heating' Could Be Explained by Solar Gravitation
Thomas Smid wrote:
Ulf Torkelsson wrote in message ... Thomas Smid wrote: On average, the corona actually still has a temperature less than the (gravitational) binding energy, but there is a relatively large number of particles with a high enough energy to escape. No, it does not. It has been known since the 1950s that in order to have a stable corona you would need an external confining pressure exceeding the pressure of the interstellar medium by many orders of magnitude. This failure to construct a realistic model of a hydrostatic corona led Parker to construct his solar wind model. The kinetic energy required to escape the solar gravitational field corresponds to temperatures in excess of 10^7 K. The coronal temperature is well below this. OK, yes you can construct an isothermal hydrostatic model of the solar corona, but such a model has a finite pressure at infinity, that is higher than the pressure of the interstellar medium surrounding the Sun, therefore this cannot be the correct description of the solar corona, and it rather turns out that you get a good description of the entire heliosphere by abandoning the assumption that the corona is hydrostatic, and instead use a solar wind model. Of course, for a gas in thermodynamic equilibrium there will always be particles with an energy high enough to escape the gravitational field. However, the loss of particles is so insignificant that you can consider the corona (as well as the solar wind) as quasi-stable. Well such models do not manage to generate a solar wind, which is as fast and as massive as is observed, so that model is out, and has been known to wrong since the 1960s, when the solar wind was observed. The emission of radiation is not the cause of the cooling but merely the consequence. Just imagine that all the atoms in a volume of gas are in an excited atomic state. The subsequent decay of the electrons to a lower level would only lead to an emission of radiation but leave the kinetic energy of the atom unchanged. For the latter to be reduced one needs inelastic collision processes of the atoms (or protons in the solar case). This description is inaccurate. It is the inelastic collisions that excite the atoms in the first place, and the energy is then lost from the plasma for good when the atoms deexcite by emitting radiation. The kinetic energy of the protons is lost when they collisionally excite neutral atoms (of which there a few in the photosphere but still some). It is irrelevant for the gas temperature what happens to the excited atoms afterwards, i.e. if the radiation leaves the volume or not). No, it is not irrelevant at all. There are, in general, two ways in which an excited atom can lose its extra energy, by radiative deexcitation, in which case the photon carries away the energy from the plasma for good if the plasma is optically thin, or by collisional deexcitation, in which case it is transferred as kinetic energy to a new atom. Without these one would not see the sun as a sharply defined disk but as a fuzzy ball (the latter would still radiate though because of recombination of protons and electrons, which however does not affect the overall temperature) And this is plainly wrong. The reason that we see the Sun as a sharply defined disk is that the opacity of the solar plasma drops drastically at the photospheric temperature. Therefore over a short interval in temperature, or equivalently radius, the plasma goes from being opaque to being completely transparent. No, the reason we see the sun as a sharply defined disk is the sharp drop of gas density in and above the photosphere. This sharp density drop is caused by the cooling due to inelastic collisions. As a result, there is nothing left above the photosphere that you could see. Opacity alone could not produce the same effect. It would limit the visibility to a certain depth but would not change anything about the 'fuzziness' above this height if there would not be sharp drop of density. I am sorry, but your are simply wrong here. The sharply defined solar disc is due to the sudden drop off of the opacity, which depends on both density and temperature. The temperature dependence of the opacity is even easy to observe. The temperature in the solar photosphere is lower above a sunspot, thus the opacity is smaller there, and we are able to see to a deeper level inside the sunspot. This results in that from our point of view the umbra, the central part of the sunspot, is displaced towards the centre of the sun as seen from the Earth as the sunspot approaches the solar limb. This Wilson effect was described already in the 18th century. Ulf Torkelsson |
#16
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'Coronal Heating' Could Be Explained by Solar Gravitation
Thomas Smid wrote:
First of all, the corona can not be considered to be separately in a hydrostatic equilibrium as it is virtually collisionless due to to low plasma density. As explained on my page http://www.plasmaphysics.org.uk/research/sun.htm, the corona (and the solar wind) is produced by those few plasma particles that do not suffer any inelastic collisions in the photosphere and thus have the full gravitational energy of about 1keV. So you are suggesting that the corona is made up of particles that pass from the centre of the Sun to the corona without suffering any collisions. If that was the case we would also see the Sun emitting gamma-rays rather than optical light. Ulf Torkelsson |
#17
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'Coronal Heating' Could Be Explained by Solar Gravitation
Ulf Torkelsson wrote in message ...
Thomas Smid wrote: First of all, the corona can not be considered to be separately in a hydrostatic equilibrium as it is virtually collisionless due to to low plasma density. As explained on my page http://www.plasmaphysics.org.uk/research/sun.htm, the corona (and the solar wind) is produced by those few plasma particles that do not suffer any inelastic collisions in the photosphere and thus have the full gravitational energy of about 1keV. So you are suggesting that the corona is made up of particles that pass from the centre of the Sun to the corona without suffering any collisions. If that was the case we would also see the Sun emitting gamma-rays rather than optical light. Well, the corona is also emitting x-rays and for this you need obviously particle energies in the keV region. The particles come from just below the photosphere and it is only a very small fraction (less than 10^-15) that can penetrate the photosphere without being stopped by inelastic collisions. |
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