We examine the hypothesis that cool loops dominate emission from solar transition region plasma below temperatures of 2 × 10~5 K. We compare published VAULT images of H Lyα, a lower transition region line, with nearly contemporaneous magnetograms from Kitt Peak, obtained during the second flight (VAULT-2) on 2002 June 14. The measured surface fields and potential extrapolations suggest that there are too few short loops and that Lya emission is associated with the base regions of longer, coronal loops. VAULT-2 data of network boundaries have an asymmetry on scales larger than supergranules, also indicating an association with long loops. We complement the Kitt Peak data with very sensitive vector polarimetric data from the spectropolarimeter on board Hinode to determine the influence of very small magnetic concentrations on our analysis. From these data, two classes of behavior are found. Within the cores of strong magnetic flux concentrations (> 5 × 10~(18) Mx) associated with active network and plage, small-scale mixed fields are absent, and any short loops can connect just the peripheries of the flux to cell interiors. Core fields return to the surface via longer, most likely coronal, loops. In weaker concentrations, short loops can connect concentrations and produce mixed fields within network boundaries, as suggested by Dowdy and colleagues. The VAULT-2 data that we examined are associated with strong concentrations. We conclude that the cool-loop model applies only to a small fraction of the VAULT-2 emission, but we cannot discount a significant role for cool loops in quieter regions. We suggest a physical picture for how network Lyα emission may occur through the cross-field diffusion of neutral atoms from chromospheric into coronal plasma.
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