If primary cosmic rays are made mostly of protons, where are the electrons lost, and does this mean that the Earth is positively charged? Does the sun eject protons and electrons in equal number?


Great question. The electric field creates such a strong force that it would be very hard to move large amounts of just one type of charge. So astrophysical systems do generally eject equal numbers of protons and electrons. In particular, the solar wind is electrically neutral. So these cosmic rays are created in very nearly equal numbers, but by the time they get to Earth, there are far fewer electrons.

The main interaction that causes these electrons to get "lost" is inverse Compton scattering. What happens is that the electrons traveling between galaxies interact with the cosmic microwave background and lose most of their energy -- basically they bounce off of photons. This is where most of the electrons in cosmic rays are lost, which means that all the cosmic-ray electrons at earth will come from inside our galaxy. Of course, the scattering happens for protons and heavier ions, too, but it's far weaker for them. (The rate of energy loss is inversely proportional to mass, and protons and other ions are far heavier than electrons.)

As for net charges, there are still lots of open questions. But basically, if we accumulate much charge, we will soon attract a lot of the opposite type of charge, and become neutral again. So we don't think the Earth has much of a net charge, though I don't know of any precise limits.

  • $\begingroup$ So these cosmic rays are created in very nearly equal numbers, but by the time they get to Earth, there are far fewer electrons Perhaps true for solar source (not my expertise), but from galactic SNRs there is strong anisotropy in electron/proton CRs at the source (cf. J. Astrophys. Astr. (2011) 32, 427–435, PDF). $\endgroup$ – Kyle Kanos Jun 14 '18 at 11:58

Cosmic rays at the highest energies are likely protons or iron nuclei or something in between (Auger and High-Res continue to disagree on this point), but as AMS-02 has pointed out, there are lots of electrons and positrons. Since the energy of the incoming cosmic rays is irrelevant we have to consider them at all energies, including very low energies, at which point it becomes very difficult to measure rates.

  • 1
    $\begingroup$ This: "it becomes very difficult to measure rates". I.e. we don't really know whether there are net fewer electrons than protons or not, only that there are net fewer high energy electrons than high energy protons. $\endgroup$ – Ben Jul 5 '17 at 9:06

I will answer the question in the title:

What happens to the electron companions of cosmic ray protons?

They leave them behind in the plasma on which they originated.

Have a look

Plasma is the fourth state of matter, it is composed of ions and electrons, and is continually ejected by the sun. Any positive excess on the upper atmosphere will be easily neutralized by absorbing electrons from the plasma.

  • $\begingroup$ This sounds very weird. A continuous outward current cannot occur in the steady state, or the star will accumulate infinite charge, which will at some point start repelling the electrons. $\endgroup$ – Emilio Pisanty Feb 21 '14 at 10:02
  • $\begingroup$ @EmilioPisanty The high energy protons that are coming are not emitted by the star. They are coming from the plasma the way they get to be so energetic is still a matter of research. For very high energies en.wikipedia.org/wiki/… . Have a look at this for the more tame ones physicsworld.com/cws/article/news/2013/feb/14/… . The ones from the sun are house trained:). The plasma is all around the earth ( see the link in the answer) and can neutralize any imbalances. $\endgroup$ – anna v Feb 21 '14 at 12:01

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.