The physics of the solar lower transition region

We discuss quiet-Sun intensities of spectral lines of O IV lambda 1401.16 a nd Si IV lambda 1402.77, their ratio, and their relation to expected proper ties of the lower solar transition region. The data consist of simultaneous measurements of the two line intensities for 16,988 pixels with spatial di mensions of 1 " square obtained by the Solar Ultraviolet Measurement of Emi tted Radiation spectrometer on the Solar and Heliospheric Observatory space craft. In ionization equilibrium, the electron temperatures of maximum conc entration of the two ions are 1.6x10(5) and 6.3x10(4) K, respectively. Assu ming ionization equilibrium, the line intensity ratio is directly proportio nal to the emission measure ratio between plasma at these temperatures. Thu s, the observations represent 16,988 snapshots of two temperature regions o f the differential emission measure at arcsecond spatial scales in the quie t Sun. We derive an average quiet-Sun pi 1401.16/lambda 1402.77 ratio, acqu ired from observations over 3 hr and a 30 " x 290 " spatial region, of 0.26 7 +/-0.050. The 1 sigma deviation of this ratio is only 20%, but it is abou t 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 t he spectra. From analysis of another ratio, i.e., O IV lambda 1399.77/O IV lambda 1401.16, we argue that this decrease could be due to a higher electr on density in the intense features relative to the lower intensity features . We discuss the implications of these observations for understanding the m orphology of the transition region.