Comparison between the theoretical and observed correlation lengths corresponding to energy in forward propagating modes (A1/B1), energy in backward propagating modes (A2/B2), residual energy (A3/B3), fluctuating kinetic energy (A4/B4), and fluctuating magnetic energy (A5/B4) as a function of the heliocentric distance. The solid curve show the correlation length corresponding to 2D turbulence. The dashed curve shows the correlation length of slab turbulence. The scatter plot with an error bar shows the observed correlation length. (a) Slow solar wind. (b) Fast solar wind.