Science and Measurement of Global Ionospheric Storms

Recent observations are revealing surprising features of global ionospheric storms that defy simple explanation. We review new and recently published results that define new science questions regarding these storms. Geomagnetic storms have been studied for decades and possible physical mechanisms for the observed ionospheric variability have been proposed. The increased availability of data from numerous Global Positioning System receivers has revealed new qualtitative and quantitative behavior that earlier observations did not resolve. This has led to new science questions ranging from fundamental physics to collective behavior of the ionosphere as part of the larger geospace "system." Analysis of multiple storms reveals new patterns of commonality across multiple events, but also variations in response that are not easily explained. Certain aspects of the global dynamics are a manifestation of long-range electromagnetic forces at work on planetary scales. Energy and mass coupling between the ionosphere and magnetosphere are significant, suggesting that understanding geospace cannot be achieved by focusing on a single regime (such as ionosphere alone, or magnetosphere alone). The interface between the magnetosphere and the solar wind is clearly important also and influenced by ionospheric dynamics. Observations show that spatial and temporal dynamics encompass all scales (from minutes to days, from tens of meters to planetary scale). The neutral thermosphere dynamics strongly influences the storm time response, but understanding of the many possible responses is incomplete. The wide variety of phenomena exhibited during geospace storms has renewed the debate about basic physical processes that might be operating. Not surprisingly, fundamental physical mechanisms are being reexamined such as: shielding of electric fields by the conducting ionosphere, and the role of electric fields in plasma drift. Recent modeling work suggests that the role of the complex plasma-neutral interaction needs to be far better understood. It is clear that new observations are needed to resolve these significant and fundamental questions. New experimental and observational methods offer great promise for significant new insights. We will discuss recent community dialog oriented towards meeting the challenges.