GLAST LAT Project Face-to-Face IDT Meeting, March 19–20, 2001

Session 1 1

Face-to-Face IDT Meeting Session 3Face-to-Face IDT Meeting Session 3Characterization of the South Atlantic Characterization of the South Atlantic

Anomaly for the LAT Anomaly for the LAT

S. W. DigelS. W. Digel

March 19, 2002

SLAC

GLAST LAT Project Face-to-Face IDT Meeting, March 19–20, 2001

Session 1 2

Origin of the South Atlantic AnomalyOrigin 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

GLAST LAT Project Face-to-Face IDT Meeting, March 19–20, 2001

Session 1 3

General Issues for Operation in the SAAGeneral 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.

GLAST LAT Project Face-to-Face IDT Meeting, March 19–20, 2001

Session 1 4

Options for Coping with the SAAOptions for Coping with the SAA

• SAA task force met last May• Options defined

1. GPS and a map of boundary to have LAT turn itself (i.e., ACD high voltage) off and on

2. Stored (uploaded) commands to turn off and on

3. ACD rate (possibly in a time) to define turn off/on times

4. Dedicated SAA detector to do the same thing

5. 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]

GLAST LAT Project Face-to-Face IDT Meeting, March 19–20, 2001

Session 1 5

More OptionsMore Options

• Fly under (or around) the SAA

AP8MIN, >1 proton cm-2 s-1 (>1 MeV)

6°

30°

Inclination of orbit

18°Nominal altitude, inclination of GLAST

Rep

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GLAST LAT Project Face-to-Face IDT Meeting, March 19–20, 2001

Session 1 6

Radiation Environment of the SAARadiation Environment of the SAA

• Modeling the SAA – AP8 & AE8 models

• Fluxes are really anisotropic

• SAA changes size and is moving (~0.3°/yr to the west)

Trapped Protons

Trapped Electrons

Differential Intensities at Peak