  During stellar core collapse, which eventually leads to a supernovae
explosion, the stalled shock is unstable due to the standing accretion shock
instability (SASI). This instability induces large-scale non spherical
oscillations of the shock, which have crucial consequences on the dynamics and
the geometry of the explosion. While the existence of this instability has been
firmly established, its physical origin remains somewhat uncertain. Two
mechanisms have indeed been proposed to explain its linear growth. The first is
an advective-acoustic cycle, where the instability results from the interplay
between advected perturbations (entropy and vorticity) and an acoustic wave.
The second mechanism is purely acoustic and assumes that the shock is able to
amplify trapped acoustic waves. Several arguments favouring the
advective-acoustic cycle have already been proposed, however none was entirely
conclusive for realistic flow parameters. In this article we give two new
arguments which unambiguously show that the instability is not purely acoustic,
and should be attributed to the advective-acoustic cycle. First, we extract a
radial propagation timescale by comparing the frequencies of several unstable
harmonics that differ only by their radial structure. The extracted time
matches the advective-acoustic time but strongly disagrees with a purely
acoustic interpretation. Second, we present a method to compute purely acoustic
modes, by artificially removing advected perturbations below the shock. All
these purely acoustic modes are found to be stable, showing that the advected
wave is essential to the instability mechanism.
