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The aim of this study is to clarify if the assumption of ionizationequilibrium and a Maxwellian electron energy distribution is valid in flaringsolar plasmas. We analyze the 2014 December 20 X1.8 flare, in which the\ion{Fe}{xxi} 187~\AA, \ion{Fe}{xxii} 253~\AA, \ion{Fe}{xxiii} 263~\AA\ and\ion{Fe}{xxiv} 255~\AA\ emission lines were simultaneously observed by the EUVImaging Spectrometer onboard the Hinode satellite. Intensity ratios among thesehigh temperature Fe lines are compared and departures from isothermalconditions and ionization equilibrium examined. Temperatures derived fromintensity ratios involving these four lines show significant discrepancies atthe flare footpoints in the impulsive phase, and at the looptop in the gradualphase. Among these, the temperature derived from the\ion{Fe}{xxii}/\ion{Fe}{xxiv} intensity ratio is the lowest, which cannot beexplained if we assume a Maxwellian electron distribution and ionizationequilibrium, even in the case of a multi-thermal structure. This resultsuggests that the assumption of ionization equilibrium and/or a Maxwellianelectron energy distribution can be violated in evaporating solar plasma around10~MK.

Departure of high temperature iron lines from the equilibrium state in flaring solar plasmas