The Physics of the Solar Lower Transition Region

We discuss quiet-Sun intensities of spectral lines of O IV λ1401.16 and Si IV λ1402.77, their ratio, and their relation to expected properties of the lower solar transition region. The data consist of simultaneous measurements of the two line intensities for 16,988 pixels with spatial dimensions of 1" square obtained by the Solar Ultraviolet Measurement of Emitted Radiation spectrometer on the Solar and Heliospheric Observatory spacecraft. In ionization equilibrium, the electron temperatures of maximum concentration of the two ions are 1.6×10⁵ and 6.3×10⁴ K, respectively. Assuming ionization equilibrium, the line intensity ratio is directly proportional to the emission measure ratio between plasma at these temperatures. Thus, the observations represent 16,988 snapshots of two temperature regions of the differential emission measure at arcsecond spatial scales in the quiet Sun. We derive an average quiet-Sun λ1401.16/λ1402.77 ratio, acquired from observations over 3 hr and a 30^''×290^'' spatial region, of 0.267+/-0.050. The 1 σ deviation of this ratio is only 20%, but it is about 2.4 times greater than expected purely from counting statistics. We also find that the ratio is about 20% smaller for the most intense features in the spectra. From analysis of another ratio, i.e., O IV λ1399.77/O IV λ1401.16, we argue that this decrease could be due to a higher electron density in the intense features relative to the lower intensity features. We discuss the implications of these observations for understanding the morphology of the transition region.