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Spectroscopic diagnostics of solar coronal plasmas critically depends on theuncertainty in the measured line intensities. One of the main sources ofuncertainty is instrumental scattered light, which is potentially mostimportant in low-brightness areas. In the solar corona, such areas includepolar and equatorial coronal holes, which are the source regions of the solarwind; instrument-scattered light must thus pose a significant obstacle tostudies of the source regions of the solar wind. In this paper we investigatethe importance of instrument-scattered light on observations of equatorialcoronal holes made by the Hinode/EIS spectrometer in two different phases ofthe solar cycle. We find that the instrument-scattered light is significant atall temperatures, and in both regions it amounts to approximately 10% of theaverage intensity of the neighboring quiet Sun regions. Such contributiondominates the measured intensity for spectral lines formed at temperatureslarger than Log T = 6.15 K, and has deep implications for spectroscopicdiagnostics of equatorial coronal hole plasmas and studies of the sourceregions of a large portion of the solar wind which reaches Earth. Our resultssuggest that the high temperature tail of in the coronal hole plasmadistribution with temperature, however small, is an artifact due to thepresence of scattered light.

EUV Emission and Scattered Light Diagnostics of Equatorial Coronal Holes as Seen by Hinode/EIS