What conditions could create a planet that has higher amounts of antimatter in its radiation belt?

Question

In the science fiction setting I'm working on, one of the factions are supposed to be using naturally occurring antimatter because the systems they reside in have sufficient natural sources of it.

I've explored the idea of chasing positrons in gas giant storms, but found that antiprotons can be found in the radiation belts of worlds. Antiprotons have more mass and are not trapped in the hostile environment of a gas giant's atmosphere.

Unfortunately, they do not seem to be in sufficient quantities to power a fleet. From what I've read, the Van Allen radiation belt of our planet have somewhere between 100-200 nanograms of antimatter to work with. I think it came out to less energy than a quarter gallon of gasoline.

The concept seemed sound enough that I wanted to see if there were conditions that could affect the abundance of antimatter in the radiation belts of a planet. While this planet does not have to be habitable, it at least has to exist in the same star system as a habitable world.

So, I'm curious to what conditions could create a radiation belt that would produce/contain an abundance of antimatter sufficient to power starships and what properties the planet with such a belt would have, particularly its atmosphere and size.

Since this antimatter is formed during interactions with a planet's atmosphere and cosmic rays, I imagine the star(s) of a system also being a factor which could potentially make a habitable world in that system a little more difficult to have.

Answer 1

I read through the discovery paper (Adriani et al. (2011)) about antimatter in Earth's Van Allen belts, and I just want to lay out a few points before we begin:

• The antimatter in the Van Allen belts consists mostly of antiprotons, as you stated. Antineutrons may be initially produced, but free neutrons (and antineutrons) are unstable, and thus decay into protons (and antiprotons).
• The main creation mechanism for antiprotons in the Van Allen belts is the CRAND process,^[1] where galactic cosmic rays (GCRs) collide with particles to produce neutrons and antineutrons. only GCRs - not cosmic rays from other sources, like the Sun - have enough energy to do this. These neutrons and antineutrons decay, creating protons and antiprotons. On Earth, the flux should be $\sim4000\text{ m}^{-2}\text{ s}^{-1}$^[2].
• A secondary mechanism involves GCRs colliding with the interstellar medium to produce neutron-antineutron pairs. These particles then decay, sending antiprotons and other particles speeding off. The flux on Earth is $\sim3\text{ m}^{-2}\text{ s}^{-1}$^[2].
• Antiprotons are lost through interactions with the atmosphere and instabilities in the magnetic field. As with production, these losses depend on the angle of entry into the atmosphere. At high altitudes, collisions with hydrogen and helium nuclei dominate.
• The magnetosphere of a planet can shield it from cosmic rays, thus reducing the production rate of antiprotons. This is in part why Jupiter does not receive as high an antiproton flux as Earth. In a gas giant (notably Saturn), this can be mitigated by antiproton production in the rings through other mechanisms, but likely not significantly.

It seems, then, that the obvious thing to do would be to place the planet somewhere with a higher flux of galactic cosmic rays. I don't think we can decrease antiproton loss rates without affecting production via the atmospheric collision pathway. You could try to introduce a very strong cosmic ray source into the planetary system, but I don't have any ideas what that could be. It could also be detrimental to the habitability of other planets.

One problem is that we don't have a great idea of where all of the galactic cosmic rays come from. There are quite a few possibilities out there (and all of these could produce different components of the GCR flux on Earth):

• Supernovae might be a source,^[3] although some observations of the GCR spectrum dispute this. Putting your system somewhere in the vicinity of a stellar nursery could place it near quite a few supernovae, because the massive stars that are supernova progenitors have such short lives that they hardly move from their birthplaces. However, having a lot of supernovae nearby tends to make worldbuilders nervous - I think 10 parsecs is the closest you'd want to go if you want to preserve habitability.
• Active galactic nuclei^[4] are also an option, albeit one that you can't control much by moving your planetary system throughout the galaxy. These might be a source of the highest-energy GCRs, which would be quite good at penetrating magnetic fields and the atmosphere.

Now, one thing you could do is lower the activity of the central star. The Sun blocks GCRs during coronal mass ejections and similar eruptions, leading to something called a Forbush decrease,^[5] which is simply when fewer GCRs reach Earth. We also see modulations in the GCR flux during the Sun's 11 year cycle of activity. The latter leads to changes in flux of around 10-20%, at is peak. However, keep in mind that this is measured from a baseline level of activity; reduce stellar events (coronal mass ejections, stellar flares, long-term fluctuations, etc.) significantly, and you could really make an impact.

Therefore, I have two proposals to make:

1. Put your planetary system at a safe distance from a star-forming region, where you might see larger GCR fluxes.
2. Make the central star one with low stellar activity.

Perhaps you can increase the quantity of antimatter in the radiation belts by an order of magnitude or so - maybe more. I'm not sure just how strong the effects would be.