astrophysics - Is it known what causes the "knee" in the observed Cosmic Ray spectrum?

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?