  Gravitational coupling between planets and protoplanetary discs is
responsible for many important phenomena such as planet migration and gap
formation. The key quantitative characteristics of this coupling is the
excitation torque density -- the torque (per unit radius) imparted on the disc
by planetary gravity. Recent global simulations and linear calculations found
an intricate pattern of low-amplitude, quasi-periodic oscillations in the
global radial distribution of torque density in the outer disc, which we call
torque wiggles. Here we show that torque wiggles are a robust outcome of global
disc-planet interaction and exist despite the variation of disc parameters and
thermodynamic assumptions (including $\beta$-cooling). They result from
coupling of the planetary potential to the planet-driven density wave freely
propagating in the disc. We developed analytical theory of this phenomenon
based on approximate self-similarity of the planet-driven density waves in the
outer disc. We used it, together with linear calculations and simulations, to
show that (a) the radial periodicity of the wiggles is determined by the global
shape of the planet-driven density wave (its wrapping in the disc) and (b) the
sharp features in the torque density distribution result from constructive
interference of different azimuthal (Fourier) torque contributions at radii
where the planetary wake crosses the star-planet line. In the linear regime the
torque wiggles represent a weak effect, affecting the total (integrated) torque
by only a few per cent. However, their significance should increase in the
non-linear regime, when a gap (or a cavity) forms around the perturber's orbit.
