Has anyone done research on how hot aurora is?
I mean if it is plasma it should be hot and since it is emitting mostly green light due to nitrogen (~78%) in the air, could it then be considered that it is at least temperature at which nitrogen turns to plasma?
I have tried looking it up in Wikipedia but could not find any data of temperature.
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asked Jul 14, 2014 at 20:49
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$\begingroup$ I think you misunderstood the Wiki article. By plasma, they just mean ionized gas, which is in this case mainly achieved through strong electromagnetic fields. The auroras we observe are not actually hot. $\endgroup$– ACuriousMind ♦Commented Jul 14, 2014 at 20:54
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2$\begingroup$ I'm with @ACuriousMind here in that the auroras shouldn't be "hot" - I don't think the light we see from them is not black body radiation - but I am curious about the temperature of that plasma. $\endgroup$– Thermo's Second LawCommented Jul 14, 2014 at 21:28
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2 Answers
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A quick google search for "aurora plasma temperature" brings up several interesting results, which seem fond of reporting temperatures in electron volts. That's entirely sensible, but probably not quite what you want. While we could do some math to convert those measurements to Kelvin, Rocket measurements of plasma densities and temperatures in visual aurora by A. G. McNamara, however, conveniently reports temperatures in Kelvin already. According to that source, the temperature of the electron plasma in an aurora ranges from 500 K to 1400 K. Note, however, that the density, and therefore the total heat capacity, of the plasma is very low; the altitude at which auroras occur is practically outer space. So, if you somehow managed to fly up there and stick your hand in one, it probably wouldn't feel all that warm.
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answered Jul 14, 2014 at 23:50
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1$\begingroup$ But sticking your hand in them would probably still hurt quite a bit from being in space... $\endgroup$– tpg2114Commented Jul 14, 2014 at 23:59
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2$\begingroup$ Note also that a plasma need not have all components at the same temperature. That paper also says there is a non-thermal population of electrons, and certainly the atoms and nuclei could be at other temperatures still. $\endgroup$– user10851Commented Jul 15, 2014 at 0:03
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$\begingroup$ True. I couldn't find any reports on measurements of the temperature of the ion or ambient neutral atom populations, but they probably bring the total average temperature down quite a bit. $\endgroup$ Commented Jul 15, 2014 at 0:10
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$\begingroup$ Boltzmann's constant is useless, temperatures should be measured in units of eV. $\endgroup$ Commented Jul 15, 2014 at 2:03
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1$\begingroup$ @LIUFA As previously noted, it probably would hurt, because you're in space, just not because of the temperature. It wouldn't feel hot because the total amount of energy in the plasma is actually really small, compared to the heat capacity of your body, and its conductivity is really low. In other words, it'd take a long time to for heat to transfer from the plasma to your hand, and when it does, your hand will cool off the plasma a lot faster than the plasma heats up your hand. $\endgroup$ Commented Jul 15, 2014 at 15:18
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I don't think it makes sense to talk about a temperature with regard to Aurora effects. It's an epiphenomenon, independent of temperature.
It's rather like asking "how fast is a car engine moving down the highway?" -- wholly dependent on your frame of reference.
In this case the difficulty is using the term "temperature", which is too tied to perception to differentiate a purely physical answer.
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$\begingroup$ What about temperature makes you think it is tied to perception, or reference frames? $\endgroup$– ACuriousMind ♦Commented Jul 15, 2014 at 0:08
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$\begingroup$ "The more we learn about the world, and the deeper our learning, the more conscious, specific, and articulate will be our knowledge of what we do not know, our knowledge of our ignorance." Would you like to have a chat about what I know about the Quantum Field? $\endgroup$ Commented Jul 15, 2014 at 0:11
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4$\begingroup$ Not particulary, since it is not relevant to the case at hand. My question was intended to get you to eleborate and amend your answer. Your answer, as it stands, does just not make much sense to me, and, I am confident, not to others either. There is nothing in the definition of temperature that, at first sight, would cause problems when applied to the aurora plasma: If the aurora has components in thermal equilibrium, they have temperature. If it has not, then temperature is not an applicable concept. Where do you think perception enters? $\endgroup$– ACuriousMind ♦Commented Jul 15, 2014 at 0:31
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6$\begingroup$ The plasma emitting the light is just ionized air molecules/atoms. You may be justified in calling the light emitted an epiphenomenon, since it is just caused by electrons in the plasma returning to lower states, but the plasma, as a collection of sparse particles, is certainly a gas in the thermodynamic sense. And for thermodynamic systems, concepts like thermal distributions (A system is in equilibrium if its distribution is thermal) and temperature are indeed well-defined. $\endgroup$– ACuriousMind ♦Commented Jul 15, 2014 at 0:56
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4$\begingroup$ You might be interested that one can indeed speak of thermal and non-thermal plasmas. If you read the whole article carefully, you will also find out that a plasma is nothing more than a gas in which a significant portion of the constituents is ionized (and the electrons remain as free electrons). Improve your answer, if you like, or don't, I will not respond further here. $\endgroup$– ACuriousMind ♦Commented Jul 15, 2014 at 1:08