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The Chandra X-ray Observatory (CXO), launched in July of 1999,
contains two focal-plane imaging detectors and two transmission-grating spectrometers. Maintaining an optimal performance level for the observatory is the job of the Chandra X-ray Center (CXC),
located in Cambridge, MA. One very important aspect of the
observatory's performance is the science observing efficiency. The
single largest factor which reduces the observing efficiency of the
observatory is the interruption of observations due to passage through the Earth's radiation belts approximately every 2 2/3 days. During radiation belt passages, observations are suspended on average for over 15 hours and the Advanced CCD Imaging Spectrometer (ACIS) is moved out of the focus of the telescope to minimize damage from low-energy (100-200 keV) protons. The CXC has been using the National Space Science Data Center's "near Earth" AE-8/AP-8 radiation belt model to predict the entry and exit from the radiation belts. However, it was discovered early in the mission that the AE-8/AP-8 model predictions were inadequate for science scheduling purposes and a 10ks "pad time" was introduced on ingress and egress of perigee to ensure protection from radiation damage. This pad time, totaling 20 ks per orbit, has recently been the
subject of much analysis to determine if it can be reduced to maximize science observing efficiency. A recent analysis evaluating a possible correlation between the Chandra Radiation Model (CRM) and the Electron Proton Helium Instrument (EPHIN) found a greatest lower bound (GLB) in lieu of a correlation for the ingress and egress of each perigee. The GLB is a limit imposed on the CRM such that when the CRM exceeds this limit on ingress, this defines the new safing time and similarly for egress. We have shown that using this method we can regain a significant amount of lost science time at the expense of minimal radiation exposure. The GLB analysis also found that different GLB's produce varied results and hint that there
could be a time dependence associated with the GLB, possibly
related to the orientation of the Observatory's orbit. Utilizing CRM
V2.3, we present the search for a seasonal dependence on the value of
the GLB; we find a seasonal effect that appears to depend on the orientation of Chandra's orbit with respect to the Earth's magnetic field.
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