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The mid‐infrared (MIR) ratio [Ne  iii ]  15.6 μm /[Ne  ii ] 12.8 μm   is a strong diagnostic of the ionization state of emission‐line objects, due to its use of only strong neon emission lines only weakly affected by extinction. However, this ratio is not available to ground‐based telescopes as only a few spectroscopic windows are available in the MIR. To deal with this problem, we aimed to verify if there exists a conversion law between the ground‐accessible, strong MIR line ratio [S  iv ]/[Ne  ii ] and the diagnostic [Ne  iii ]/[Ne  ii ] ratio that can serve as a reference for future ground‐based observations. We collated the [S  iv ]  10.5 μm  , [Ne  ii ]  12.8 μm  , [Ne  iii ]  15.6 μm   and [S  iii ]  18.7 μm   emission‐line fluxes from a wide range of sources in the rich  Spitzer  and  Infrared Space Observatory  archives, and compared the [Ne  iii ]/[Ne  ii ], [S  iv ]/[S  iii ] and [S  iv ]/[Ne  ii ] ratios. We find a strong correlation between the [S  iv ]/[Ne  ii ] and [Ne  iii ]/[Ne  ii ] ratios, with a linear fit of log([Ne  iii ]/[Ne  ii ])  = 0.81 log ([S  iv ]/[Ne  ii ]) + 0.36, accurate to a factor of ∼2 over four orders of magnitude in the line ratios. This demonstrates clearly the ability of ground‐based infrared spectrographs to do ionization studies of nebulae.

The mid‐infrared [S iv ]/[Ne ii ] versus [Ne iii ]/[Ne ii ] correlation