  In this study we isolate the mechanisms that are separately responsible for
amplifying the two distinct sets of modes that appear in a linearized
Navier-Stokes based model for compressible turbulent boundary layer flows.
These two sets of modes are: (i) the subsonic modes that have equivalents in
the incompressible flow regime, and (ii) the supersonic modes that do not have
such equivalents in the incompressible flow regime. The resolvent analysis
framework is used to analyze the linear model, where the non-linear terms of
the equations are taken to be a forcing to the linear terms. The most sensitive
forcing and correspondingly the most amplified response are then analyzed. We
find that, when considering the subsonic response modes, only the solenoidal
component of the forcing to the momentum equations can amplify these modes.
This is consistent with observations in the literature regarding incompressible
flows. Next, when considering the supersonic response modes, we find that these
are pressure fluctuations that radiate into the freestream. Within the
freestream, these modes closely follow the trends of the inviscid Mach waves.
There are three different forcing mechanisms that are found to amplify these
modes. First, the forcing to the continuity and energy equations contribute.
Second, the dilatational component of the forcing to the momentum equations has
a significant role. Finally, although less significant for the resolvent
amplification, there is also a contribution from the solenoidal component of
the forcing. Using these identified forcing mechanisms, a decomposition of the
forcing to the compressible resolvent operator, i.e. of the non-linear terms of
the linearized Navier-Stokes equations, is proposed, that can approximately
isolate the subsonic and the supersonic modes in turbulent boundary layers over
adiabatic as well as cooled walls, and for a range of Mach numbers.
