Importance of predicting the dose temporal profile for large solar energetic particle events

It has been proposed that proton intensities on the stream-limited plateau present a minimal radiation hazard to astronauts, and hazardous intensities occur upon coronal-mass-ejection-driven shock arrival at the spacecraft. Forecasts then need only predict the arrival time and severity of the shock prior to its arrival at the spacecraft. We evaluated dose to the bone marrow, eye, and skin for the largest events since 1996, in terms of the ≥10 MeV proton flux, prior to and after shock arrival. Doses at shock arrival were compared to the space-permissible exposure limits (SPELs), and for five of the seven events, limits were exceeded for aluminum shielding thicknesses up to 3 g cm^-2 prior to the arrival of the shocks. The 30-d SPELs have been exceeded as early as 9 h after event initiation, but more generally, sometime between 20 and 50 h after event initiation. These results suggest that one cannot simply predict the arrival time and intensity of a shock if exceeding the 30-d SPELs is to be avoided. Similarly, waiting for the shock to arrive at a sentinel spacecraft such as the Advanced Composition Explorer to provide a 1-h warning of shock intensity may not prevent one from exceeding the 30-d SPELs. In order to predict when 30-d SPELs will be exceeded for a given event, one must forecast the dose temporal profile associated with that event.