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{The \ion{Na}{x} X-ray lines between 10.9 and 11.2~\AA\ have attracted littleattention but are of interest since they enable an estimate of the coronalabundance of Na to be made. This is of great interest in the continuing debateon the nature of the FIP (first ionization potential) effect. } {Observationsof the \ion{Na}{x} lines with the Solar Maximum Mission Flat CrystalSpectrometer and a rocket-borne X-ray spectrometer are used to measure theNa/Ne abundance ratio, i.e. the ratio of an element with very low FIP to onewith high FIP.} {New atomic data are used to generate synthetic spectra whichare compared with the observations, with temperature and the Na/Ne abundanceratio as free parameters.} {Temperature estimates from the observationsindicate that the line emission is principally from non-flaring active regions,and that the Na/Ne abundance ratio is $0.07 \pm 50$\%.} {The Na/Ne abundanceratio is close to a coronal value for which the abundances of low-FIP elements(FIP $< 10$~eV) are enhanced by a factor of 3 to 4 over those found in thephotosphere. For low-temperature ($T_e \leqslant 1.5$~MK) spectra, the presenceof \ion{Fe}{xvii} lines requires that either a higher-temperature component ispresent or a revision of ionization or recombination rates is needed. }

Highly Ionized Sodium X-ray Line Emission from the Solar Corona and the Abundance of Sodium