Solar-cycle variations of radiation environment for ISS

Measurement data on radiation doses at the location of an orbital station (OS) Mir in a period, 1991-1996 were described and a dynamic trapped radiation model for altitudes of piloted OS was proposed in this article. Based on the experimental data, it was concluded that there are solar variations in the trapped radiation dose. This was first suggested from the data obtained using R-16 on board OS Mir and the evidence was confirmed by the data from other instruments, REM, TEPC and LIVIN on board the same OS. Then, dose dependence on the altitude of OS Mir orbit was examined and distinct dependency was indicated. Furthermore, it was shown that there exists a solar-cycle variation of trapped energetic protons at altitudes of nearly 1000 km using the TIROS/NOAA satellite. From all available experiment data and model representations it was assumed that long term variations of RB at orbital station flight altitudes are caused by the existence of secular variations of the geomagnetic field and time variation of solar activity. The absolute value of the radiation dose during solar minimum years from 1995 to 1997 on OS Mir reached 100 mrad/day and decreased thereafter. The next increase of radiation will appear in 2005 to 2008, which will be most hazardous for manned missions on ISS. (M.N.)