In this study we examine the occurrence probability of intense X ray emissions (>2 keV) from the auroral ionosphere in daylight and in darkness. The Polar Ionospheric X ray Imaging Experiment (PIXIE) on board the NASA/GGS Polar spacecraft remotely images the Earth's ionosphere from high altitude (apogee of about 9 Earth radii). This instrument is capable of observing the entire northern auroral region at once, and is normally unaffected by scattered sunlight; resulting in an unbiased study of differences in auroral emissions between daylight and darkness. The differences in the occurrence probability between daylight and darkness are interpreted as being due to differences in the local ionospheric conductivity, as described in previous studies using in situ particle populations and lower energy ionospheric emissions. There is also a clear dependence upon season which is in accordance with the Russell-McPherron effect. Under similar geomagnetic conditions, comparisons of occurrence probability separated by whether each region is in daylight or in darkness illustrate that intense auroral emissions are consistently more likely when a given region is in darkness than when the same region is in daylight. ¿ 2000 American Geophysical Union