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Measurements of the temperature and density structure of the solar coronaprovide critical constraints on theories of coronal heating. Unfortunately, thecomplexity of the solar atmosphere, observational uncertainties, and thelimitations of current atomic calculations, particularly those for Fe, allconspire to make this task very difficult. A critical assessment of plasmadiagnostics in the corona is essential to making progress on the coronalheating problem. In this paper we present an analysis of temperature anddensity measurements above the limb in the quiet corona using new observationsfrom the EUV Imaging Spectrometer (EIS) on \textit{Hinode}. By comparing the Siand Fe emission observed with EIS we are able to identify emission lines thatyield consistent emission measure distributions. With these data we find thatthe distribution of temperatures in the quiet corona above the limb is stronglypeaked near 1 MK, consistent with previous studies. We also find, however, thatthere is a tail in the emission measure distribution that extends to highertemperatures. EIS density measurements from several density sensitive lineratios are found to be generally consistent with each other and with previousmeasurements in the quiet corona. Our analysis, however, also indicates that asignificant fraction of the weaker emission lines observed in the EISwavelength ranges cannot be understood with current atomic data.

The Temperature and Density Structure of the Solar Corona. I. Observations of the Quiet Sun with the EUV Imaging Spectrometer (EIS) on Hinode