Temperature Measurements in the Solar Transition Region Using N III Line Intensity Ratios

Abstract

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 (2s2p²-2p³)/(2s²2p-2s2p²) are very sensitive to the electron temperature. In addition, the lines involving the ratios fall within a range of only ~12 Å in N III the lines fall in the 980-992 Å range, and in O IV in the 780-791 Å 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 flown 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 ~30% than those predicted in the typical quiet Sun. Assuming an isothermal plasma, most of the measured ratios correspond to temperatures in the range 5.7×10⁴-6.7×10⁴ K. This range is considerably lower than the calculated temperature of maximum abundance of N III, which is ~7.6×10⁴ K. Detailed analysis of the spectra further indicates that the measured ratios are probably somewhat overestimated because of resonant scattering effects in the 2s²2p-2s2p² lines and small blends in the 2s2p²-2p³ 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 (2s2p²-2p³ transitions) to a strong Ne VIII line, sufficiently accurate ratio measurements cannot be obtained.