<p>Comments below <a href="https://astronomy.stackexchange.com/a/33716/7982">this answer</a> to <em>Why does the Solar Wind consist of charged particles?</em> have led me to ask the following:</p>

<p><strong>Question:</strong> Does plasma recombination in the solar wind happen to any significant or measurable degree?</p>

CC BY-SA 4.0
2019-10-20T12:57:42.067Z
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.
PM 2Ring
CC BY-SA 4.0
2019-10-20T13:05:24.300Z
@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?
uhoh
CC BY-SA 4.0
2019-10-20T13:32:40.473Z
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.
PM 2Ring
CC BY-SA 4.0
2019-10-20T15:20:04.447Z
@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.
uhoh
CC BY-SA 4.0
2019-10-24T11:05:05.870Z
You might be interested in the discussion in https://physics.stackexchange.com/questions/48620/special-conditions-at-layer-f2-ionosphere
AtmosphericPrisonEscape
CC BY-SA 4.0
2019-10-25T00:50:00.240Z
@AtmosphericPrisonEscape yes, plenty of "plasma goodies" there, thanks!
uhoh
<solar-system><plasma-physics>
Does plasma recombination in the solar wind happen to any significant or measurable degree?
uhoh
# Does plasma recombination in the solar wind happen to any significant or measurable degree?

(Asked by: username_0 on 2019-10-20)

Comments below [this answer](https://astronomy.stackexchange.com/a/33716/7982) 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?

**Tags:** solar-system, plasma-physics
> **Comments:**
> **username_1**: 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.  
> **username_0**: @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?  
> **username_1**: 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.  
> **username_0**: @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.  
> **username_2**: You might be interested in the discussion in https://physics.stackexchange.com/questions/48620/special-conditions-at-layer-f2-ionosphere  
> **username_0**: @AtmosphericPrisonEscape yes, plenty of "plasma goodies" there, thanks!  

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