Comparison of SASI activity and shock evolution in the 2D and 3D simulations of the 27  M _☉ model. (a) Average shock radius ( r _sh = a ₀) and mass accretion rate of the collapsing stellar core at 400 km; (b) components of the SASI ℓ = 1 amplitude vector; (c) rms shock deformation σ; and (d) kinetic energy of non-radial mass motions in the gain layer.