Maps of Mg x 625 Å and O v 630 Å radiances (left-hand panels) in a logarithmic, inverted scale (brighter areas correspond to fainter line radiances) from a USUN mosaic obtained on September 2008. The corresponding bi-dimensional histograms vs. μ are shown in the upper-right and lower-right panel, respectively; the position and width of the histogram peaks are also shown (magenta dots and tee-shaped symbols). The power-law and quadratic curves fitting the peaks of the log I vs. μ histograms are shown as red and blue lines, respectively. Curves derived from plane-parallel and spherical optically thin models are shown as dashed and continuous orange lines, respectively, in both right-hand panels. Distances corresponding to μ = 0 (white light limb), 1/10, 1/3, 1/2, and 2/3 are shown as circles in the left-hand panel and as vertical lines in the right-hand panels. The rectangle corresponding to the FOV limited to | Y| < 0.85  R_⊙, which is used to compute the histograms vs. μ, is also shown.