Many times I heared that the solar wind consists of protons and alpha particles. The both are positively charged, but are there electrons in solar wind?
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1$\begingroup$ +1 this is a good discussion to have. From what I understand of plasma physics, the charge balance is maintained fairly strictly throughout all of any plasma. Do some simple calculations assuming complete and total stripping of electrons in even a small amount of matter and you find that the electric potential has comparable energy to a nuclear bomb. Over large scales in space, accumulated electric and magnetic fields can matter a great deal. $\endgroup$– Alan RomingerCommented Aug 15, 2011 at 15:08
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$\begingroup$ could be estimated from what is brighter "Aurora Borealis" or "Aurora Australis"? $\endgroup$– IljaBekCommented Aug 15, 2011 at 15:09
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$\begingroup$ @troyaner I'm curious, could you elaborate on the "Aurora Borealis" / "Aurora Australis" connection? $\endgroup$– Alan RomingerCommented Aug 15, 2011 at 15:25
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1$\begingroup$ @Zassounotsukushi on the other hand it is quite easy to separate alpha and beta rays - you need only a good magnet. $\endgroup$– AnixxCommented Aug 15, 2011 at 18:28
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1$\begingroup$ @troyaner - No, there is not a preference for pole-dependent precipitation of charges. Meaning, electrons do not prefer the North over South magnetic poles when precipitating into the atmosphere due to the sign of the magnetic field. By the way, precipitating electrons are what cause auroral displays near both poles. $\endgroup$– honeste_vivereCommented Oct 31, 2014 at 19:28
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6 Answers
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Solar wind is neutral overall else the Sun will become globally very strongly charged and we don't see that happening.
It comprises Electrons/Protons and other particles.
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14$\begingroup$ The reason solar wind protons are so often mentioned is because their mass is almost three orders of magnitude greater than electrons, even though they have equal but opposite charge. $\endgroup$ Commented Aug 15, 2011 at 14:46
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$\begingroup$ This is assuming there are no currents feeding into Sol from elsewhere. It has been hypothesized that there are equivalent currents from the galactic current sheet as those entering Earth at the poles from the heliospheric current sheet, and so far experimental evidence is definitely not sufficient to disprove that hypothesis conclusively. If this were indeed the case, then Solar wind could fully well be charged without Sol becoming increasingly charged. $\endgroup$ Commented Apr 22, 2023 at 12:32
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Generally plasma may be charged in regions smaller than the Debye length. Normally it happens at boundaries. For example, fast electrons tend to fly away and are deposited on a metallic wall first. (It is like electron emission from a hot cathode.) Thus a positive charge is created within some region. The Debye length depends on plasma density and temperature.
answered Aug 15, 2011 at 15:15
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1$\begingroup$ That length is about $10 m$ for the interstellar space, could space travelers see electronic interference due to the voltage fluctuations? $\endgroup$ Commented Aug 15, 2011 at 19:11
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$\begingroup$ I cannot estimate the voltage right now, unfortunately. It should be compared with the measuring device noise amplitude. $\endgroup$ Commented Aug 15, 2011 at 20:36
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1$\begingroup$ @AlanRominger - We routinely measure electric fields with in situ spacecraft near Earth, other planets, and in the solar wind (where $\lambda_{De}$ $\gtrsim$ 10 m). We have been measuring waves with $\lambda$ $\gtrsim$ $2 \ \pi \ \lambda_{De}$ since the 1970's. So I am not sure about interference, but we can definitely measure electric fields at small scales. $\endgroup$ Commented Oct 31, 2014 at 19:25
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This is a citation about solar wind properties from:
Hargreaves, J. K. (1995). The solar-terrestrial environment. Cambridge University press
Although most ions are protons ($\mathrm{H}^+$) there is an $\alpha$-particle ($\mathrm{He}^{++}$) component typically amounting to 5% thought exceptionally up to 20% of the total. Heavier atoms total perhaps 0.5%, and, in contrast to the light ions, these are not fully ionized. The number density of positive ions varies between 3 and 10 $\mathrm{cm}^{-3} (3 \times 10^6\;\mathrm{to}\;10^7 \mathrm{m}^{-3})$, the most typical value being 5 $\mathrm{cm}^{-3}$, and there is a similar number of electrons for bulk neutrality.
So the answer (as it was posted before) is NO, the solar wind is overall neutral and YES there are enough electrons to make it so.
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The lighter electrons are thermally accelerated more than the ions which, being ~ 1800 times heavier, feel the Sun's gravity more than electrons. As Ernesto said, this causes a charge separation which is addressed by the plasma by the setting up of an ambipolar electric field, which acts to retard the electrons and accelerate the ions. This ensures that ~ as many electrons as ions leave the corona, and that an equal amount remain. It must be so, otherwise we would end up with a huge space charge in the solar wind, and the Sun would charge up to the point where it would explode due to Coulomb repulsion.
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The electrons in the solar atmosphere have a lower escape velocity because of their mass. The flux of electrons creates a positive charge in the solar atmosphere that in time creates an ambipolar electric field that then accelerate protons to space. This caused the solar wind to be essentially neutral.
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3$\begingroup$ According to classical mechanics, escape velocity doesn't depend on mass ! $\endgroup$– GsvCommented Jun 5, 2019 at 7:57
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$\begingroup$ @Gsv But energy that would need to be imparted to a particle to cause it to escape would [depend on mass], I suppose? I guess that's what Ernesto probably meant, and what would matter from a thermal perspective (which shouldn't care about speed as much as energy). Does that sound right? $\endgroup$– AzendaleCommented May 13 at 17:29
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Solar wind contains equal amounts of positive and negative charge per unit volume in any given region, but the energies and momenta of the particles do not have to agree. If they all have similar kinetic energy (which seems a good guess but I don't know) then the ones with higher mass will have higher momentum and therefore may have more noticeable effects on whatever they hit.
answered Jun 13, 2022 at 10:21