Origin of very high and ultra high energy cosmic rays

Understand

While there is some level of consensus on a Galactic origin of cosmic rays up to the knee ($E_{k}\sim 3\times 10^{15}$ eV) and on an extragalactic origin of cosmic rays with energy above $\sim 10^{19}$ eV, the debate on the genesis of cosmic rays in the intermediate energy region has received much less attention, mainly because of the ambiguity intrinsic in defining such a region.

• The energy range between $10^{17}$ eV and $\sim 10^{19}$ eV is likely to be the place where the transition from Galactic to extragalactic cosmic rays takes place.
• Hence the origin of these particles, though being of the highest importance from the physics point of view, it is also one of the most difficult aspects to investigate.
• Here I will illustrate some ideas concerning the sites of acceleration of these particles and the questions that their investigation may help answer, including the origin of \underline{ultra} high energy cosmic rays.