  We use the feedback loop of McKeon \& Sharma (2010), where the nonlinear term
in the Navier-Stokes equations is treated as an intrinsic forcing of the linear
resolvent operator, to educe the structure of fluctuations in the range of
scales (wavenumbers) where linear mechanisms are not active. In this region,
the absence of dominant linear mechanisms is reflected in the lack of low-rank
characteristics of the resolvent and in the disagreement between the structure
of resolvent modes and actual flow features. To demonstrate the procedure, we
choose low Reynolds number cylinder flow and the Couette equilibrium solution
EQ1, which are representative of very low-rank flows dominated by one linear
mechanism. The former is evolving in time, allowing us to compare resolvent
modes with Dynamic Mode Decomposition (DMD) modes at the first and second
harmonics of the shedding frequency. There is a match between the modes at the
first harmonic but not at the second harmonic where there is no separation of
the resolvent operator's singular values. We compute the self-interaction of
the resolvent mode at the shedding frequency and illustrate its similarity to
the nonlinear forcing of the second harmonic. When it is run through the
resolvent operator, the `forced' resolvent mode shows better agreement with the
DMD mode. A similar phenomenon is observed for the fundamental streamwise
wavenumber of the EQ1 solution and its second harmonic. The importance of
parasitic modes, labeled as such since they are driven by the amplified
frequencies, is their contribution to the nonlinear forcing of the main
amplification mechanisms as shown for the shedding mode, which has subtle
discrepancies with its DMD counterpart.
