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I've seen many versions of the figure shown below -- the famous Swordy plot. They tend to explicitly point out two features in the CR spectrum, the knee and the ankle. I know that the source of UHECR's above the ankle is currently a mystery. But I'm having a devilish time sussing out the cause of this feature at the knee.

Is it known why the slope changes?

enter image description here

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2 Answers 2

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The general consensus is that the knee represents the transition from galactic sources (supernova remnants) and extra-galactic sources (AGN, blazars, etc).

At about $10^{15}$ eV, the gyroradius of a proton is $$ r\sim3\frac{\left(mc^2/GeV\right)\left(v/c\right)}{\left(|q|/e\right)\left(B/T\right)}\simeq10^{16}\,{\rm m} $$ where we assume a background magnetic field of a microgauss ($10^{-10}$ T) and that $v\approx c$. This puts us on the same order of magnitude the size of the galactic arm we live in. Hence, particles with energies less than this should be confined to the galaxy. Particles with energies larger than $10^{15}$ eV cannot be contained within the galaxy, meaning they must be extra-galactic in origin.

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  • $\begingroup$ I have a question, which I've been trying to figure for days. How does the distance form the galactic centre will affect the energy value where knee is observed? Should it increase as we go farther from the centre since, we are allowing larger Larmor radius allowing reception of extragalctic sources at higher energies? And, shouldn't this phenomena cause deflection of lower energy cosmic rays, ruling out intergalactic sources. So, transition at knee should be from extragalactic to intergalactic? $\endgroup$
    – Gagan
    Commented Dec 19, 2022 at 7:50
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    $\begingroup$ @gaganarora Distance from the galactic center likely wouldn't impact the location of the knee as intergalactic (IG) sources are relatively uniform due to their reflections off ambient magnetic fields (recall the Fermi mechanism!). Another argument from the Hillas criterion) is that it must be that the lower-energy part of the knee is IG while the higher-energy part is extra-galactic. $\endgroup$
    – Kyle Kanos
    Commented Dec 19, 2022 at 12:44
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    $\begingroup$ But if you find that unsatisfactory, feel free to post your own question asking it so a more developed answer could be had. $\endgroup$
    – Kyle Kanos
    Commented Dec 19, 2022 at 12:45
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The knee is believed to be due to one of the following reasons:

1) the reduced efficiency of the galactic magnetic field to confine the cosmic ray particles with energies above the knee within galaxy,

2) the knee corresponds to the maximum energy that protons can have under diffusive shock acceleration in supernova remnants,

3) the contribution from a nearby source,

4) the contribution from a variety of supernovae,

5) the mass distribution of a progenitor of a cosmic ray source,

5) the cosmic ray propagation effect in the galaxy

6) unusual high energy interaction behaviour (new particle creation) at the knee energy

7) a few other models

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  • $\begingroup$ So, your answer sounds like a "no", as it, it is not known? Do you have any comment on the weight given to these possibilities in the field? $\endgroup$
    – chase
    Commented Sep 7, 2015 at 17:10

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