Temperature measurements in the solar transition region using NIII line intensity ratios

UV emission from B-like N and O ions offers a rather rare opportunity for recording spectral lines in a narrow wavelength range that can potentially be used to derive temperatures relevant to the solar transition region. In these ions, the line intensity ratios of the type (2s2p(2) - 2p(3))/(2s(2)2p - 2s2p2) are very sensitive to the electron temperature. In addition, the lines involving the ratios fall within a range of only similar to12Angstrom; in N III the lines fall in the 980 - 992 Angstrom range, and in O IV in the 780 - 791 Angstrom range. In this work, we explore the use of these atomic systems, primarily in N III, for temperature diagnostics of the transition region by analyzing UV spectra obtained by the Solar Ultraviolet Measurement of Emitted Radiation spectrometer. own on the Solar and Heliospheric Observatory. The N III temperature-sensitive line ratios are measured in more than 60 observations. The mean measured ratios are lower by similar to30% than those predicted in the typical quiet Sun. Assuming an isothermal plasma, most of the measured ratios correspond to temperatures in the range 5.7 x 10(4) - 6.7 x 10(4) K. This range is considerably lower than the calculated temperature of maximum abundance of N III, which is similar to7.6 x 10(4) K. Detailed analysis of the spectra further indicates that the measured ratios are probably somewhat overestimated because of resonant scattering effects in the 2s(2)2p - 2s2p(2) lines and small blends in the 2s2p(2)-2p(3) lines. Actual lower ratios would only increase the discrepancy between the ionization balance calculations and present temperature measurements based on a collisional excitation model. In the case of the O IV spectra, we determine that because of the close proximity in wavelength of the weak line (2s2p(2) - 2p(3) transitions) to a strong Ne vIII line, sufficiently accurate ratio measurements cannot be obtained.