To optimize our ability to avoid radiation risks to humans in space, a system should be developed that can provide "timely, accurate, and reliable space environment observations, specifications, and forecasts."1 At present, there are significant gaps in our understanding that diminish the ability of space weather models to perform this function. Nonetheless, the science of space physics has matured to the point where it is able to describe and model many aspects of the complex links between the Sun, the interplanetary medium, and Earth. These capabilities are essential for predicting space weather in general and the radiation environment at low Earth orbit in particular. Recent advances in numerical technology and computer architectures have meant a rapid growth in our ability to model space weather, in particular the propagation of solar eruptions through the heliosphere. The last few years have also seen rapid advances in our capabilities for representing the present state of the magnetosphere and providing short-term forecasts, and progress is likely to accelerate in the next few years.
The presently available empirical and semiempirical models that have the potential for providing predictions that will be useful during the ISS construction period should be adapted for operational use. CSSP/CSTR notes at the outset one criterion bearing on the likely usefulness of a model. From a flight director's perspective, false alarms are worse than missed events. Models with low false alarm rates are therefore the ones most likely to give predictions that flight directors would trust enough to act upon.
CMEs and flares are the first links in the chain of efficient causes that connects eruptions on the Sun to space storms at Earth, including SPEs. To understand the solar origins of space weather, therefore, we must understand how CMEs and flares are initiated. The following discussion will focus primarily on fast CMEs, the predominant source of SPEs and geomagnetic storms. At present, we know the following:
The Sun's magnetic field is the most likely source of the substantial energy needed to launch and maintain a CME.
Most CMEs come from the streamer belt or from the boundary between the polar coronal holes and adjacent active regions.