Pre-Test SAA Contours for WFC3 (2002)

This ISR documents the preliminary selection of SAA contours for the WFC3. At the time of this writing, documentation on WFC3 instrument shielding is unavailable as are ground or in-flight testing. However, assumptions are made based on the experience of past HST instruments, ACS design, and on the current understanding of the SAA environs and its evolution over time (i.e., are the models applicable, does the SAA change with time, etc.) Based on the information briefly reviewed here, the model defined by WFPC2 empirically and used by ACS is appropriate for both channels of the WFC3. At the writing of this memo, there are no obvious operational SAA restrictions associated with instrument reconfigurations, other than the limiting of observing in the WFC3 SAA contour. This assumption will be further tested as information from ground testing becomes available and of course verified in orbit. Introduction HST’s orbit intersects the “South Atlantic Anomaly (SAA)”, a region of high density and high intensity particle radiation. For HST instruments and all other low-earth-orbit satellites, the SAA threatens both instrument lifetime and exposure quality. Images are compromised by cosmic rays and high dark and background levels. Detectors exhibit both short and long term effects as a result of SAA interactions. Charge Transfer Efficiency (CTE) problems accelerate as an instrument’s exposure to radiation increases. Residual glow and high dark counts are measured after an SAA passage and the number of hot pixels may increase. The deposition of high energies into a material by a single particle can Copyright© 1999 The Association of Universities for Research in Astronomy, Inc. All Rights Reserved. Instrument Science Report WFC3 ISR 2002 -01 damage electronics, flip memory bits, compromise instrument operations and sometimes alter material properties. Therefore, the SAA contours and the associated operational procedures are both clearly and uniquely defined for each instrument onboard the HST. This TIR documents the selection of preliminary SAA contours for the WFC3 and the operational procedure with respect to the SAA. The Servicing Mission Orbital Verification (SMOV) timeline for WFC3 will include tests that demonstrate the impact of the radiation environment. As the ground system is being developed however, the SAA selection is based on indirect but high fidelity inputs such as: • the history of previous science instruments (WFPC2, STIS, NICMOS); • relevant ACS studies (many WFC3 systems are similar); • knowledge of the geometry and temporal variability of the SAA. This TIR reviews the SAA geometry and composition, relevant design specifications for the WFC3, the SAA impact on current HST instruments, the SAA variability with solar cycle, the applicability of SAA models and finally explains the selection of WFC3 SAA contours. The SAA: Geometry and Composition SAA Geometry and Particle Spectrum The earth’s magnetosphere is offset and tilted with respect to the geographic axis, as seen in Figure 1. The South Atlantic Anomaly is the region where the Van Allen belts intersect the earth’s atmosphere closest to the earth. Low Earth Orbiting (LEO) satellites with altitudes < 800km and inclinations < 40 degrees will pass through regions of the SAA during a percentage of their orbits. Sensitive instruments like those aboard HST are protected in various ways against the adverse effects of particle radiation, e.g., by turning off electronics, precluding data takes during the SAA, or by scheduling brighter targets in SAA impacted orbits. As Biretta and Baggett (WFPC2 ISR98-04) point out, the SAA reduces the amount of HST time available by several hundred orbits per year. Available on average are 7-9 orbits of SAA free passage per day and ~ 6-8 SAA-impacted orbits. The actual numbers depend on the size and shape of the SAA as described uniquely for a particular instrument. One of the critical scheduling trades involves the relationship between SAA passage and the HST earth-shadow passages because the faintest target observations require the long exposures, dark skies and low background. High energy protons and electrons trapped in the magnetosphere are the most effective “disrupters” for HST SIs. Heavier ions and other sources of radiation, e.g., solar flares, galactic and extragalactic sources, also make up the radiation environment of HST however natural magnetic shielding at the low orbital altitudes protect against many of these