1. Face-to-Face IDT Meeting Session 3Characterization of the South Atlantic Anomaly for the LAT S. W. Digel March 19, 2002 SLAC

2. Origin of the South Atlantic Anomaly • Offset and tilt of the magnetic axis with respect to the rotation axis permits the intense trapped radiation (circulating on magnetic field lines) to reach relatively low altitudes in a large region more or less centered on the south Atlantic http://www.estec.esa.nl/wmwww/wma/rad_env.html

3. General Issues for Operation in the SAA • Radiation damage to detectors, SEUs, latchups • Charging of spacecraft, instruments – arcing, overcurrent damage to high-voltage devices • For LAT: The high rate of triggers on charged particles within the SAA is likely to saturate the data acquisition system (large deadtime), and extracting useful celestial photons may be problematic anyway.

4. Options for Coping with the SAA • SAA task force met last May • Options defined • GPS and a map of boundary to have LAT turn itself (i.e., ACD high voltage) off and on • Stored (uploaded) commands to turn off and on • ACD rate (possibly in a time) to define turn off/on times • Dedicated SAA detector to do the same thing • Self-protect the components • Option 2 is a given; ≥1 more for desired reliability [may not be clear what the impact on the instrument would be of being on during SAA passage]

5. More Options • Fly under (or around) the SAA Inclination of orbit 30° Nominal altitude, inclination of GLAST 18° 6° Representative AP8MIN, >1 proton cm-2 s-1 (>1 MeV)

6. Radiation Environment of the SAA Differential Intensities at Peak • Modeling the SAA – AP8 & AE8 models • Fluxes are really anisotropic • SAA changes size and is moving (~0.3°/yr to the west) Trapped Electrons Trapped Protons