Parallel compressibility (C_||, black line) and parallel compressibility (C_||S, pink line) calculated using the simulation data. (a) MESSENGER trajectory and (b) straight line indicated in Fig. 3a crossing the local maximum of the plasma pressure inside the SMS. The dashed red lines indicate the region with a putative presence of slow modes (the parallel compressibility becomes negative in the immediate vicinity of the magnetopause). The dotted blue line shows the encounter of the satellite with the compression front. Panel b also includes the normalized values of the magnetic field (red), plasma pressure (blue), and density (purple).