Radially evolving distribution of B_RR². Each panel in the bottom row contains 2D histograms of B_TR² versus B_RR², which show the distribution of the field in the RT plane in a given radial bin, indicated for each column. The cyan contour depicts the 90th percentiles of the data. Top row: resulting 1D distribution of B_RR². The legend gives the number of data points (1 min averages) in each radial bin. We note the left three columns are from PSP data, while the right hand panel is the summation of two years of Wind and STEREO A data.