An empirical modeling of spatial distribution of trapped protons from solar cell degradation of the Akebono satellite

Output current of silicon solar cells of Akebono satellite orbiting in the inner magnetosphere decreased from 13 A in 1989 to about 7 A in 2009, due to accumulated damage by energetic particles. Annual decrease from the same month in the previous year shows a clear oscillation due to orbit precession correlated with trapped energetic proton flux up to 1996. Although phase of the oscillation in annual variation shows a clear correlation, the amplitude tends to be larger than that expected from a degradation model based on energetic proton distribution of the NASA's AP8 model. We have worked on modeling of the L-shell distribution of trapped energetic protons which provides best-fit for the degradation of solar cells. The results by assuming Gaussian distribution of proton flux for L value are as follows. (1) If we assume a steady state before 1996, the best-fitted distribution has a peak around L = 1.6 and a width of dL = 0.1 (i.e., half width of 1/e decrease from the peak). (2) If we employ a dynamic model with temporal variation and introduce a different distribution between April 1991 and June 1992, RMSE is further improved from the static model. Modeled distributions have its center around L = 1.9, suggesting outward shift or expansion of the proton radiation belt during the interval. Our modeling gives more confined distribution than given by the AP8 and AP9 models, but is more consistent with the CRRESPRO model based on the observation before November 1991.