Using an extended quasi-linear theory that takes account of the distri
bution of medium-scale background interplanetary magnetic field, we ca
lculate the pitch angle diffusion coefficients of interplanetary energ
etic ions, for a slab turbulence model consisting of a superposition o
f parallel and antiparallel transverse R and L waves. Our results show
that the background variation broadens the resonance function signifi
cantly at pitch angles near 90 degrees, so that protons down to approx
imate to 25 keV interact with hydromagnetic waves at all pitch angles,
and the steepening of the magnetic field power spectrum in the dissip
ation range does not result in a resonance gap or infinite scattering
mean free path.