Forecasting and Monitoring Solar Particle Events in Support of International Space Station Operations

Solar Particle Events (SPE) are a rapid increase over the normal background in the near-earth space of charged particles, including protons and heavier ions. During SPE energetic particle fluxes from the Sun increase to thousands of times background in the space of a few hours. The largest SPE pose a hazard for crews of space missions, including the International Space Station, and operational responses to the events need to be developed. As a first-level strategy, measurements of SPE fluxes made in real time from satellites allow a continuous check of the particle populations and allow comparison to dosimeters on board spacecraft. The wide range of event sizes poses a problem in developing a strategy of avoiding high radiation doses during large events and frequent operational interruptions at times of small events that pose less hazard to space crews. Monitoring the Sun and Sun-earth space provides a means of forecasting the occurrence of SPE several hours in advance. Useful forecasts include a prediction of the intensity of a coming event and allow operations to plan alternate strategies for large and small events. A long-term forecasting record extending over the past 20 years provides a first level estimate of the value and utility of monitoring and forecasting SPE and suggests strategies for dealing in real time with the radiation increases from SPE. Most of the effort in the operational radiation support programs for space operations of the past 20 years has emphasized improved monitoring of SPE. Current scientific research into the physical process of accelerating the particles in an SPE and improvements in solar monitoring now offer the possibility of significant improvement in the short-term forecasting of SPE in event occurrence and the intensity of events. This paper provides a more detailed discussion of these topics.