  We study large-distance contributions to the nucleon's parton densities in
the transverse coordinate (impact parameter) representation based on
generalized parton distributions (GPDs). Chiral dynamics generates a distinct
component of the partonic structure, located at momentum fractions x ~<
M_pi/M_N and transverse distances b ~ 1/M_pi. We calculate this component using
phenomenological pion exchange with a physical lower limit in b (the transverse
"core" radius estimated from the nucleon's axial form factor, R_core = 0.55 fm)
and demonstrate its universal character. This formulation preserves the basic
picture of the "pion cloud" model of the nucleon's sea quark distributions,
while restricting its application to the region actually governed by chiral
dynamics. It is found that (a) the large-distance component accounts for only
~1/3 of the measured antiquark flavor asymmetry dbar - ubar at x ~ 0.1; (b) the
strange sea quarks, s and sbar, are significantly more localized than the light
antiquark sea; (c) the nucleon's singlet quark size for x < 0.1 is larger than
its gluonic size, <b^2>_{q + qbar} > <b^2>_g, as suggested by the t-slopes of
deeply-virtual Compton scattering and exclusive J/psi production measured at
HERA and FNAL. We show that our approach reproduces the general N_c-scaling of
parton densities in QCD, thanks to the degeneracy of N and Delta intermediate
states in the large-N_c limit. We also comment on the role of pionic
configurations at large longitudinal distances and the limits of their
applicability at small x.
