Dayside/nightside auroral X ray emission differences - implications for ionospheric conductance

In this study we examine the occurrence probability of intense X ray emissi ons (>2 keV) from the auroral ionosphere in daylight and in darkness. The P olar Ionospheric X ray Imaging Experiment (PIXIE) on board the NASA/GGS Pol ar spacecraft remotely images the Earth's ionosphere from high altitude (ap ogee of about 9 Earth radii). This instrument is capable of observing the e ntire northern auroral region at once, and is normally unaffected by scatte red sunlight; resulting in an unbiased study of differences in auroral emis sions between daylight and darkness. The differences in the occurrence prob ability between daylight and darkness are interpreted as being due to diffe rences in the local ionospheric conductivity as described in previous studi es using in situ, particle populations and lower energy ionospheric emissio ns. There is also a clear dependence upon season which is in accordance wit h the Russell-McPherron effect. Under similar geomagnetic conditions, compa risons of occurrence probability separated by whether each region is in day light or in darkness illustrate that intense auroral emissions are consiste ntly more likely when a given region is in darkness than when the same regi on is in daylight.