It is by now very common knowledge that the vast majority of (known) cosmic rays are protons, alpha particles and such, yet I cannot find in my recent reading any discussion of why this might be, or what may have happened to the electrons, etc...

You would think, naively, that the lighter electrons would be easier to accelerate to such incredible speeds, but..

Are lighter-weight electrons more easily deflected by our magnetosphere? Are they captured in our Van Allen belts?

Is there ongoing research/experiments in this area?

P.S.: Yes, as stated on SE physics, lighter-weight electrons are more readily deflected, but.... Aren't they moving at higher speeds to begin with? Wouldn't this counteract that? Sort of like how objects of different masses fall at the same speed, due to the equivalence principle....

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    $\begingroup$ There's an answer over on Physics $\endgroup$
    – hegel5000
    Oct 26, 2022 at 0:06
  • $\begingroup$ Yes, but.... Doesn't some sort of 'Equivalence Principle' cancel out the lightness of the electrons? I realize we are talking about Electromagnetism, not Gravity, but still.... $\endgroup$
    – Kurt Hikes
    Aug 17, 2023 at 0:10

1 Answer 1


The most abundant subatomic particle in cosmic rays, is the proton (positively charged nucleon, baryon) not electron has 3 reason.

There are 2-3 major reasons for this:

  1. Any electron would be bent by the magnetic field, just like protons. Except it is just like a non-newtonian fluid, the faster the velocity, the stronger the force, as the velocity and kinetic energy is more it will act more on the particle deflecting it off the path.

  2. According to Newton's 3rd law, F=MA, If the mass decreases the force required to deflect also decreases and since it is 1,800 times less than a proton, the force required to stop it is very less as compared to protons.

  3. The incoming shockwave from a supernova (the most common way of producing Galactic Cosmic Rays), majorly accelerates protons, as the nebulae and gas surrounding it is majorly composed of protons because the eventually when the star explodes, the supernova remnant, is after all the cooled down plasma (which is stripped of it's electrons.), therefore it is ionized and since the electrons are not bound with the protons, it is more likely for them to diffuse into the interstellar medium. Also few of the nuclei capture electrons, sending them on an ballistic trajectory independent of the magnetic field and become electrically neutral atoms (Thank you, Uhoh!)

Perhaps, few of them, who managed to penetrate the magnetosphere, would be trapped in the Van Allen Belts. But before that it has to enter inside which is prevented by the above reason, few who penetrated are trapped here

Note: Also some sort of equivalence principle also works, in case of the electron canceling it's lightness, because it is faster it's gravitational field too will increase (though gravitational and acceleration are two different terms, they have similar effects) increasing it's weight, and it is proportional to force required to do work, which makes it slow down.


  • $\begingroup$ as an aside, protons and helium nuclei do occasionally pick up electrons in transit and become neutral cosmic rays then traveling on a ballistic trajectory independent of magnetic fields. There's a whole field of astronomy dedicated to these uncharged baryonic cosmic rays. Strangely, Wikipedia's Cosmic ray doesn't mention the uncharged component. Aha! Energetic neutral atom and Interstellar Boundary Explorer (IBEX) $\endgroup$
    – uhoh
    Aug 18, 2023 at 2:02
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    $\begingroup$ @uhoh Thank you! I will add it to my post $\endgroup$
    – Arjun
    Aug 18, 2023 at 11:00

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