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

Abstract

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 $\mathrm{log}(T/{\rm{K}})=5.10\pm 0.03$ . The ratio λ272.13/λ279.93 then yields an electron number density, N _e, of $\mathrm{log}({N}_{{\rm{e}}}/{\mathrm{cm}}^{-3})=12.52$ 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 10^17.0 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.