Comments below this answer to Why does the Solar Wind consist of charged particles? have led me to ask the following:
Question: Does plasma recombination in the solar wind happen to any significant or measurable degree?
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1$\begingroup$ Good question. My guess is that recombination would only be significant in the far reaches of the solar system, since even Neptune has a Van Allen belt. Near Earth, the solar wind is far too hot for recombination, around 100,000 K. $\endgroup$– PM 2RingOct 20 '19 at 12:57
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$\begingroup$ @PM2Ring Can it actually cool itself between Earth and Neptune or is the density so low and collisions so unlikely that the particles are doomed to ballistic trajectories? $\endgroup$– uhohOct 20 '19 at 13:05
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$\begingroup$ I'd like to hear what the actual astrophysicists have to say about this topic. The solar wind particles don't have purely ballistic trajectories. Temperature of a gas or plasma is a measure of the mean kinetic energy in the centre of momentum frame, so if you match speed with the solar wind near Earth and capture some in a box, the stuff in the box will be moving randomly consistent with a temperature of $10^5$ K. I suppose it can lose heat via radiation, to some extent, although heat emitted by one "chunk" of solar wind will mostly get absorbed by another chunk. $\endgroup$– PM 2RingOct 20 '19 at 13:32
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$\begingroup$ @PM2Ring Individual charged particles only radiate if they accelerate, usually due to collision or coulomb interaction of some kind when they scatter. If they are too far apart and scattering doesn't happen often, then while you can define a temperature mathematically, they really can't do much of anything in terms of cooling. You can define a temperature even if the distribution of relative velocities no longer has any characteristics of thermal equilibrium. $\endgroup$– uhohOct 20 '19 at 15:20
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2$\begingroup$ You might be interested in the discussion in physics.stackexchange.com/questions/48620/… $\endgroup$– AtmosphericPrisonEscapeOct 24 '19 at 11:05
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