This chapter is a review of the background and status of several current problems of interest concerning cosmic rays of very high energy and related signals of photons and neutrinos.

Introduction

The steeply falling spectrum of cosmic rays extends over many orders of magnitude with only three notable features:

1. (a) The flattened portion below 10 GeV that varies in inverse correlation with solar activity,

2. (b) The “knee” of the spectrum between 1015 and 1016 eV, and

3. (c) the “ankle” around 1019 eV.

4. For my discussion here I will divide the spectrum into three energy regions that are related to the two high–energy features, the knee and the ankle: I: E < 1014 eV, II: 1014 < E < 1018 eV and III: > 1018 eV.

In Region I (VHE) there are detailed measurements of primary cosmic rays made from detectors carried in balloons and on spacecraft. These observations, and related theoretical work on space plasma physics, form the basis of what might be called the standard model of origin of cosmic rays. Cosmic rays are accelerated by the first order Fermi mechanism at strong shocks driven by supernova remnants (SNR) in the disk of the galaxy. The ionized, accelerated nuclei then diffuse in the turbulent, magnetized plasma of the interstellar medium, eventually escaping into intergalactic space at a rate that depends on their energy.