Extensive Air Showers Near the Knee

-γ . The spectral index γ of the differential flux has been observed to maintain a constant value of about 2.7 until a few PeV, then changes to about 3.1 for at least three more decades in energy. Such a non-thermal spectrum strongly suggests that it arises from interesting, very energetic processes whose nature is not yet known. The change in the index is known as the “knee” in the spectrum. The most popular explanation of this behavior is that primary cosmic ray particles are accelerated in the plasma shock waves from Galactic supernovae. This process, called Fermi Acceleration, naturally produces a power law spectrum with a spectral index of nearly the observed value. Moreover, the predicted value of the spectral index is largely insensitive to the details of the progenitor SN, as long as the shock is “strong”. The difficulty with this model is that the maximum energies attainable are limited by the useful lifetime of the shock, with most calculations yielding Emax near, but less than, 1 PeV. While such an energetic limit is surely near enough to the “knee” to give much credibility to the hypothesis, there is no ready explanation for the continuance of the spectrum as a power law beyond this point with only slightly steeper behavior. At this conference, direct evidence was presented for the first time for the acceleration of cosmic rays via the Fermi mechanism in the expanding shells of Galactic supernovae. The HESS experiment imaged TeV gamma rays from the outer edges of SNR [1]. These gamma rays are presumed to come from interactions of higher energy charged primary particles undergoing Fermi acceleration. Beyond TeV energies, direct measurement of cosmic rays is not practical, due to the low flux. Instead, indirect study via the measurement of extensive air showers in the atmosphere is required. Confirmation of the Fermi acceleration hypothesis would rest on two observational pillars: (1) the steepening of the energy spectrum at the knee should be relatively smooth, otherwise one may be observing some new acceleration process overtaking the one which operates at lower energy, and (2) the composition of charged primary cosmic rays should become heavier through the knee region, since Emax is proportional to the charge of the primary (as would be true for any electromagnetic acceleration process). This review will summarize some of the presentations which addressed questions relating to the spectrum, composition, and nature of cosmic rays near the “knee” in the spectrum. This work is drawn from some, but not all, of the many contributions in sessions HE 1.1, HE 1.2, HE 1.5, and HE 2.1.