New Low-Altitude Dose Measurements

The Defense Meteorological Satellite Program (DMSP) F7 satellite, launched in November, 1983, carries a dosimeter that measures radiation dose behind four hemispherical aluminum domes of different thicknesses and distinguishes low (electron) and high (proton) thresholds of energy deposition. The dosimeter also returns accurate, high-time-resolution dose measurements. Short-term measurements of dose from three sources, inner radiation belt protons, outer radiation belt electrons and solar flares, are presented for periods in 1984 and 1985. Empirical models of dose rate are constructed for the 840 km altitude of the DMSP orbit and compared to predictions of the NASA models. The NASA model values for proton dose in the South Atlantic Anomaly (SAA) are approximately 50% higher than the DMSP average values. The NASA outer zone electron model prediction values are too high by an average factor of 6, and are not reliable for short-term predictions. Included in the analysis are two of the largest solar proton events of 1984 and 1985 that occurred on 16 February, 1984, and on 26 April, 1984. The February event was relatively short-lived and produced a hard energy spectrum. The April event was much softer but gave a total dose behind .55 gm/cm2 of aluminum shielding in excess of 25 rad(Si) for the first three days of the event.