  The perturbation of a medium field (a particle wake) determines the Stokes
drag on the particle itself. Besides, it affects the motion of other particles,
inducing non-equilibrium correlations between them. We study the induced
non-equilibrium correlations and forces, acting on constituent particles of the
bunch or cluster of impurities exposed to a gas stream. Such induced
correlations exhibit many striking features that are determined by the
properties of particle bunch collective scattering in a medium. The
characteristics of collective scattering are determined by the structure of the
bunch itself, that is associated with scattering on inhomogeneities, i.e., on
fluctuations of the number of particles (scatterers) in a correlation volume.
This is well-known, e.g., in optics and have been shown for gas scattering in
our recent work [1]. In particular, a random cluster of impurities experiences
much stronger Stokes drag than a regular one. Inhomogeneity of the cluster also
determines the presence of giant local fluctuations of the scattered field
inside a cluster, that was shown for gas stream scattering in [1]. This, in
turn, should lead to strong local fluctuations of induced gradient forces
inside the cluster, which can determine its stability. Moreover, the
description of a cluster in terms of effective parameters (penetration index,
effective diffusion coefficient, etc.) brakes down due to the presence of such
fluctuations.
