  The Pierre Auger Collaboration intends to extend the energy range of its
southern observatory in Argentina for high quality data from 0.1 to 3 EeV. The
extensions, described in accompanying papers, include three additional
fluorescence telescopes with a more elevated field of view (HEAT) and a nested
surface array with 750 and 433 m spacing respectively and additional muon
detection capabilities (AMIGA). The enhancement of the detector will allow
measurement of cosmic rays, using the same techniques, from below the second
knee up to the highest energies observed. The evolution of the spectrum through
the second knee and ankle, and corresponding predicted changes in composition,
are crucial to the understanding of the end of Galactic confinement and the
effects of propagation on the lower energy portion of the extragalactic flux.
The latter is strongly related to the cosmological distribution of sources and
to the composition of the injected spectrum. We discuss the science motivation
behind these enhancements as well as the impact of combined HEAT and AMIGA
information on the assessment of shower simulations and reconstruction
techniques.
