Main: the spherical harmonic power spectrum, C(ℓ) (Equation (12)), for the fluctuations in the radial displacement vector, δξ_r (Equation (8)). δξ_r directly probes the corrugations of the contact discontinuity. Each spectrum corresponds to an SNR from Figure 4, as indicated in the legend. Inset: C(ℓ) compensated by the (a) spectrum, C^(a)(ℓ), to show which harmonics are growing with respect to the youngest SNRs in Figure 4. The C(ℓ) admit to a power law C(ℓ) ∝ ℓ^−8/3, which I show is consistent with 2D R. H. Kraichnan (1967) turbulence, Equation (13), that is self-generated on the thin shell, as discussed in Section 4.1.