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The Wikipedia article on Solar Wind gives the following explanation:

The solar wind is a stream of charged particles released from the upper atmosphere of the Sun, called the corona. This plasma mostly consists of electrons, protons and alpha particles with kinetic energy between 0.5 and 10 keV.

If the solar wind consists of both electrons and protons, why don't they combine and give Hydrogen or any other elements? Then, why would Solar Wind be charged? Is that because of the high kinetic energy of the ejected particles? If yes, let us consider a proton and an electon. They must be of almost the same energy if so there will be no relative kinetic energy, so I think they must combine in the same way as if they were at rest or in motion at small velocities.

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You are right about the kinetic energy (ie the fast motion) of the particles being the reason, but wrong when you say

They must be of almost the same energy if so there will be no relative kinetic energy,

The individual particles are moving fast, but there is huge variation between them in just how fast and in what direction. In other words, the solar wind is very hot. The extent of disorderly motion in a gas is measured by its temperature. The wikipedia article you link gives temperatures of 100 000 to 800 000 Kelvins. So if an electron and proton did happen to collide gently enough to form an atom of hydrogen, another particle would probably smash into them before very long and knock the electron off again.

You might think they are all moving away from the Sun, but you have to allow for the effects of magnetic fields. Charged particles moving in a magnetic field curve around in all sorts of ways, and, additionally, moving charged particles create a magnetic field which affects other particles. So the movement is extremely turbulent.

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    $\begingroup$ I think you should mention that the very low density prevents the solar wind from thermalizing and eventually recombining. Put some solar wind in magnetic confinement and leave it there for a while and it will eventually cool and recombine. $\endgroup$ – uhoh Oct 20 '19 at 0:24
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    $\begingroup$ @uhoh if you know something about that please edit my answer or add a supplementary. Is it more about thermalisation it the fact that ionised hydrogen does not cool well? Asking because I don’t know $\endgroup$ – Steve Linton Oct 20 '19 at 8:26
  • $\begingroup$ High density plasmas recombine, very low density ones can't. That's all I know. en.wikipedia.org/wiki/Plasma_recombination doesn't help much, but it suggests a search, perhaps "recombination solar wind" which seems to immediately return some interesting discussions! $\endgroup$ – uhoh Oct 20 '19 at 8:28
  • $\begingroup$ Then again, I've just asked Does plasma recombination in the solar wind happen to any significant or measurable degree? $\endgroup$ – uhoh Oct 20 '19 at 8:31
  • $\begingroup$ Thanks for the edit, your answer is really good now. Sorry for the harsh words, I've just seen too many answers using heat and temperature interchangeably. $\endgroup$ – Eric Duminil Oct 21 '19 at 6:56
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Expanding on @Steve Lintons answer: In physics no quantity is just large, a quantity can only be large relative to some other quantity.

So here we want to compare the kinetic energy to something, and that must be the binding energy of i.e. a hydrogen atom. The binding energy of the hydrogen atom is 13.6 eV. And this is much smaller than the keV energies that the solar wind possesses. So the particles are much too fast w.r.t. each other, they don't get the chance to recombine.

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  • $\begingroup$ ditto $\endgroup$ – uhoh Oct 20 '19 at 0:25
  • $\begingroup$ Clarification: ev is an energy measure Kev is 1000 ev. So the kinetic energies involved are a 100's to 1000's time larger than the energy required to knock an electron away from a proton (binding energy) $\endgroup$ – TazAstroSpacial Nov 1 '19 at 1:13

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