Example of an angular structure function. Panel (a) shows hypothetical turbulent (triple-dot-broken curve) and ordered (dot-broken curve) contributions to the total angular structure function ΔΦ ²(ℓ) plotted in panel (b) (solid curve). Panel (c) presents the same information as panel (b) but displayed as a function of ℓ ² instead of ℓ. The total angular structure function is subtracted from a Taylor series fit obtained from its values at ℓ ⩾ 6'' (dot-broken curve in panels (b) and (c)) to get the turbulent autocorrelation function shown in panel (d). The correlation length characterizing turbulence is represented by δ.