Properties of the diffuse emission around warm loops in solar active regions

Abstract: Coronal loops in active regions are the subjects of intensive investigation, but the important diffuse 'unresolved' emission in which they are embedded has received relatively little attention. Here we measure the densities and emission measure (EM) distributions of a sample of background-foreground regions surrounding warm (2 MK) coronal loops, and introduce two new aspects to the analysis. First, we infer the EM distributions only from temperatures that contribute to the same background emission. Second, we measure the background emission co-spatially with the loops so that the results are truly representative of the immediate loop environment. The second aspect also allows us to take advantage of the presence of embedded loops to infer information about the (unresolvable) magnetic field in the background. We find that about half the regions in our sample have narrow but not quite isothermal EM distributions with a peak temperature of 1.4--2 MK. The other half have broad EM distributions (Gaussian width $>$3$\times$10$^{5}$ K), and the width of the EM appears to be correlated with peak temperature. Densities in the diffuse background are $\log$ (n/cm$^{-3}$) = 8.5-9.0. Significantly, these densities and temperatures imply that the co-spatial background is broadly comptabile with static equilibrium theory (RTV scaling laws) provided the unresolved field length is comparable to the embedded loop length. For this agreement to break down, the field length in most cases would have to be substantially longer than the loop length, a factor of 2--3 on average, which for our sample approaches the dimensions of only the largest active regions.