  It is shown that amplification of the magnetic field in supernova remnants
(SNRs) occurs in all six objects where morphological measurements are presently
available in the hard X-ray continuum at several keV. For the three
archetypical objects (SN 1006, Cas A and Tycho's SNR) to which nonlinear
time-dependent acceleration theory has been successfully applied up to now, the
global theoretical and the local observational field strengths agree very well,
suggesting in addition that all young SNRs exhibit the amplification effect as
a result of very efficient acceleration of nuclear cosmic rays (CRs) at the
outer shock. Since this appears to be empirically the case, we may reverse the
argument and consider field amplification as a measure of nuclear CR
acceleration and it has indeed been argued that acceleration in the amplified
fields allows the CR spectrum from SNRs to reach the knee in the spectrum or,
in special objects, even beyond. The above results are furthermore used to
investigate the time evolution of field amplification in young SNRs. Although
the uncertainties in the data do not allow precise conclusions regarding this
point, they rather clearly show that the ratio of the magnetic field energy
density and the kinetic energy density of gas flow into the shock is of the
order of a few percent if the shock speed is high enough V_s > 10^3 km/s, and
this ratio remains nearly constant during the SNR evolution. The escape of the
highest energy nuclear particles from their sources becomes progressively
important with age, reducing also the cutoff in the \pi^0 -decay gamma-ray
emission spectrum with time after the end of the sweep-up phase. Simultaneously
the leptonic gamma-ray channels will gain in relative importance with
increasing age of the sources.
