Acceleration of Galactic Cosmic Rays

Galactic cosmic rays have two main components, GCRI (up to ∼ 3x10¹⁵eV/Q) and GCRII (∼ 3x10¹⁵--10¹⁸eV/Q). The `knee' at 3x10<sup>15</sup>eV/Q appears to be the result of finite acceleration time in the SN shocks presumed to be responsible on the basis of the total power of GCR escape from the galaxy. The source spectrum of GCRI is a power law with index ∼ 2.1-2.2 and can be accounted for by the net effects of acceleration and adiabatic expansion in an evolving SN shell. The source composition of GCRI can be accounted for if the acceleration takes place mainly in the outer regions of SN `bubbles' where the interstellar medium is `contaminated' with the debris of previous SN occurring in the same OB association responsible for the bubble. This scenario can simultaneously account for the source spectrum and composition and offers the possibility for GCRII to result from a second stage of acceleration by secondary shocks within the bubble.