key science goals. However, in times of constrained budgets, it may not be possible to enact all of these imperatives, and the panel has, therefore, undertaken a prioritization process to help identify which of these are the most important. The panel’s prioritization—which informed the survey committee but does not carry its imprimatur—is based on the overarching objective to identify initiatives that will most cost-effectively enable the science of the future. Accordingly, the SWMI panel adopted the following criteria:
• General considerations for prioritization
—Focus on elements that are directly relevant to the enumerated SWMI science goals.
—Consider historical evidence about contributions of various elements to scientific progress.
—Consider the cost of an imperative versus its likely return.
• For mission-related imperatives
—How well does the mission address the SWMI science goals?
—Is it highly focused or does it address a broader range of the goals?
—How feasible is it?
—Will it fit within the expected budget?
—What is the science return for the cost?
—Does it require technology development?
—What is the broader impact (for example, for the science objectives of the other panels and for the development of a forecasting system for space weather)?
• For other capabilities
—How central is the capability to the accomplishment of the SWMI science goals?
—Is it currently in danger?
—Could it make a much bigger contribution with a modest enhancement?
Using these criteria, the SWMI panel prioritized its imperatives not only within but also across the three categories discussed above—missions, DRIVE initiative, and space weather—in order to identify the most cost-effective approach to accomplishing its science goals. The full set of prioritized SWMI imperatives, with their mapping to the three categories, is presented in Table 9.4.
It is clear from the foregoing discussion that the variable solar wind is the dominant driver of magnetospheric dynamics. To fully understand the ways in which long-term solar variations as well as short-term eruptive events such as solar flares or coronal mass ejections can produce dramatic effects of importance to humans at Earth, solar wind measurements upstream of Earth’s magnetosphere are essential. At present a nearly continuous record of measurements of the solar wind flow velocity, density, and temperature is available from the very early days of the space program up to the present time. Any interruption in the continuity of these measurements would have serious consequences for the ability to study the effects that solar variations have on Earth’s magnetosphere, ionosphere, and atmosphere, and for the ability to forecast significant societal impacts.