The Temperature and Density of a Solar Flare Kernel Measured from Extreme-ultraviolet Lines of O iv

Previously unexplored diagnostics of O iv in the extreme-ultraviolet region 260–280 Å are used to derive a temperature and density for a solar flare kernel observed on 2012 March 9 with the Extreme-ultraviolet Imaging Spectrometer on the Hinode satellite. Seven lines from the 2s2p 2–2s2p3s transition array between 271.99 and 272.31 Å are both temperature- and density-sensitive relative to the line at 279.93 Å. The temperature, T, is constrained with the λ268.02/λ279.93 ratio, giving a value of log(T/K)=5.10±0.03 . The ratio λ272.13/λ279.93 then yields an electron number density, N e, of log(Ne/cm−3)=12.52 with a lower limit of 11.90 and an upper limit of 14.40. The O iv emitting volume is estimated to be 0.″4 (300 km) across. Additional O iv lines at 196, 207, and 260 Å are consistent with the derived temperature and density but have larger uncertainties from the radiometric calibration and blending. Density diagnostics of O v and Mg vii from the same spectrum are consistent with a constant pressure of 1017.0 K cm−3 through the transition region. The temperature derived from O iv supports recent results that O iv is formed around 0.10 dex lower at high densities compared to standard zero-density ionization balance calculations.