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Spherical Couette flow (flow between concentric rotating spheres) is one offlows under consideration for the laboratory magnetic dynamos. Recentexperiments have shown that such flows may excite Coriolis restored inertialmodes. The present work aims to better understand the properties of theobserved modes and the nature of their excitation. Using numerical solutionsdescribing forced inertial modes of a uniformly rotating fluid inside aspherical shell, we first identify the observed oscillations of the Couetteflow with non-axisymmetric, retrograde, equatorially anti-symmetric inertialmodes, confirming first attempts using a full sphere model. Although the modelhas no differential rotation, identification is possible because a largefraction of the fluid in a spherical Couette flow rotates rigidly. From theobserved sequence of the excited modes appearing when the inner sphere isslowed down by step, we identify a critical Rossby number associated with agiven mode and below which it is excited. The matching between this criticalnumber and the one derived from the phase velocity of the numerically computedmodes shows that these modes are excited by an instability likely driven by thecritical layer that develops in the shear layer staying along the tangentcylinder of the inner sphere.

On the excitation of inertial modes in an experimental spherical Couette flow