The solar wind is a stream of ions emitted from the sun. Are there more positive than negative charges released? That is, is there any imbalance in the charge emitted from the sun in the solar wind?
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asked May 22, 2016 at 22:09
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$\begingroup$ I am sure you have read this Wikipedia article, en.wikipedia.org/wiki/Solar_wind but it does not give a clear, to me anyway, definitive answer. I would guess an overall negative charge, (based on the aurora being mostly produced by electrons) but hopefully someone with far more knowledge than I will answer. $\endgroup$– user108787Commented May 22, 2016 at 22:23
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1$\begingroup$ The solar wind, on average, has to be neutral, otherwise the sun will get charged, which will change the electric field in such a way that the charges that are in excess get shed slower until equilibrium is achieved, again. $\endgroup$– CuriousOneCommented May 22, 2016 at 23:14
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$\begingroup$ My guess would be that, if the solar wind carries away unbanlanced charge, the resulting accumulating charge of the sun will soon stop any excess being blown away, because the electromagnetic force is so much stronger than gravity. $\endgroup$– hdhondtCommented May 22, 2016 at 23:16
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1$\begingroup$ @CuriousOne Glad we agree. Your comment got there as I was typing mine... $\endgroup$– hdhondtCommented May 22, 2016 at 23:17
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1$\begingroup$ @hdhondt: This one is fairly straight forward, but that shouldn't make one believe that the solar wind is a simple physical system. Far from it... its interactions with the galactic gas are still not fully understood, as the recent Voyager data has shown. The responsible scientists were waffling for years about the question when the probes had actually reached the shock front and what it would look like. $\endgroup$– CuriousOneCommented May 22, 2016 at 23:20
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2 Answers
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Are there more positive than negative charges released?
In general no, and this statement by itself would imply you were asking about whether the sun charges up, not about current.
I wrote a detailed answer at https://physics.stackexchange.com/a/253491/59023 that explains why stars remain mostly neutral and that most observations [e.g., Bale et al., 2013; Pulupa et al., 2014] support the idea that the solar wind, on the macroscopic scale, has no net current flowing out from the sun.
That is, is there any imbalance in the charge emitted from the sun in the solar wind?
Since plasmas are made of roughly equal positive and negative charges and that electric fields typically act to eliminate themselves, the short and simple answer is no. If the sun were to charge up one could imagine the resulting net electric field would act to stop the solar wind from flowing. Indeed it is the case that such static models predict solar breezes instead of the observed supersonic solar wind.
In summary, the most recently accepted models (which are supported by observations), called exospheric models, argue in favor of no net macroscopic current and insignificant net charging of the star.
Update
A recent paper by Wilson et al. [2019] directly calculates the net current density (normalized to the core electron thermal current density) of the electrons in the ion rest frame finding most time periods are within a few percent of zero. Note that $\mathbf{j}_{e} = -e \ \sum_{s} \ n_{s} \ \mathbf{v}_{s} = 0$ was not a constraint imposed during the fitting process of the velocity distribution components. That is, the small magnitude of the current density in the ion rest frame is a consequence of the solar wind actually maintaining $\lvert \mathbf{j}_{e} \rvert$ ~ 0 in the ion rest frame for most intervals. The deviations from this are associated with changes in the magnetic field magnitude and direction, as one would expect from Ampere's law, e.g., at the shock ramps examined in this work.
Similar results were much more recently observed using data from the Parker Solar Probe mission [e.g., Halekas et al., 2019 (arXiv eprint 1912.02216v1, to be published in ApJ in Feb. of 2020)]. The data shows that this constraint appears to hold even at smaller solar radial distances, down to below ~0.3 AU.
answered May 23, 2016 at 14:58
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1$\begingroup$ Thanks for the update and new references! I see, zero net current (or a very tiny current that time-averages to zero) certainly makes sense since the Sun is old; anything else would diverge to a huge integrated charge. I'll read these today to see if they directly address the question of a small equilibrium net-charge on the Sun that would not require any current to maintain. $\endgroup$– uhohCommented Dec 31, 2019 at 1:27
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The sun is a ball of plasma. It is essentially a group of electrons and nuclei whizzing around. These electrons and nuclei correspond to no net charge. The get flung out into space still in the form of plasma. There is still no net charge. The individual particles have charge but no net charge.
