  The observed energy spectra of accelerated particles at interplanetary shocks
often do not match the diffusive shock acceleration (DSA) theory predictions.
In some cases, the particle flux forms a plateau over a wide range of energies,
extending upstream of the shock for up to seven flux's e-folds before
submerging into the background spectrum. Remarkably, at and behind the shock
that we have studied in detail, the flux falls off in energy as
$\epsilon^{-1}$, consistent with the DSA prediction for a strong shock. The
upstream plateau suggests a different particle transport mechanism than those
traditionally employed in DSA models. We show that a standard (linear) DSA
solution based on a widely accepted diffusive particle transport with an
underlying resonant wave-particle interaction is inconsistent with the plateau
in the particle flux. To resolve this contradiction, we modify the DSA theory
in two ways. First, we include a dependence of the particle diffusivity
$\kappa$ on the particle flux $F$ (nonlinear particle transport). Second, we
invoke short-scale magnetic perturbations that are self-consistently generated
by, but not resonant with, accelerated particles. They lead to the particle
diffusivity increasing with the particle energy as $\propto\epsilon^{3/2}$ that
simultaneously decreases with the particle flux as $1/F$. The combination of
these two trends results in the flat spectrum upstream.
