tokens: The gray scale is linear from zero to erg cm −2 s −1 sr −1 Å −1 , reversed (black is the maximum intensity) to reveal fainter emission between bright regions. The right panels show the average spectra (black) superposed with the average quiet Sun from the IRIS data studied here in red. Although the R-MHD calculation is for a network region, the region is dominated in area by non-network emission; thus, we compare with the quiet IRIS data. Note that the computed line k 2 peak separations are substantially smaller than observed. Significantly, in comparison with plage observations, these k 3 and K 3 components are far deeper in the computations. This suggests that the source function is on average larger where this feature is formed than is captured in the calculations. In Figure 9 we show spatial autocorrelation data computed from the 3D and 1D intensities of the k line, together with spatially averaged intensity profiles. Solid lines show observations, while dashed lines show computations. The autocorrelation characteristic lengths show three kinds of behavior depending on the wavelengths relative to line center. In the line wings, the images have the smallest spatial scales, with FWHM values of about 750 and 610 km. In 3D, these scales steadily increase toward the line cores, peaking near the observed peaks k 2 V and k 2 R . (In 1D these lengths decrease toward the line center before showing a small increase at k 2 V wavelengths.) Within the line core, the widths computed in 3D drop dramatically to below 700 km. Figure 9. Average profiles (top panel) are shown with autocorrelation full widths for the IRIS network and quiet regions (red and gray, respectively) in the bottom panel. The same data for the 3D and 1D calculations of Sukhorukov Leenaarts ( 2017 ), are also shown as dashed lines. The comparison of 3D and 1D images in Figure 8 is instructive in ways complementary to the interesting points made by Bjørgen et al. ( 2018 ). Figure 9 shows clearly that 3D transfer effects are important within ±1 Å of line center: the FWHM length scales are larger on average by a factor of two for the k line (15%–20% for the K line) in 3D than in 1D. The 3D features all appear “fuzzier” to the eye than their 1D counterparts, on scales of a few hundred kilometers in Figure 8 . We will speculate further on what is missing in these calculations in Section 5 . The second and fourth rows of Figure 8 show how the calculated model might be observed through filter instruments, with FWHM values of 0.1, 1, and 3 Å. Wider FWHM instruments have been used in solar physics for many years to record the Ca ii lines, and other, narrower filter widths have been developed. 11 11 See, e.g., https://www.su.se/isf/ . These panels suggest that 3D smearing effects are biggest at wavelengths within the emission cores. The differences are probably lower limits, given that the source functions for strongly scattering lines are smeared by the use of the short characteristics, which
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