Justification of the treatment of the normal field component. Each panel shows a 2D distribution of field values across the full data set (see Fig. 1). Top row: distribution of 1 min average values of B_T R² versus B_RR². Bottom row:same axes and colour scale, the distribution formed by rotating the normal component into the R-T plane via the substitution B_R = |B|cos(α), B_T = |B|sin(α). The three columns show PSP, STA, and Wind data, respectively.