Introduction and Workshop Background
SPACE PHYSICS AND SPACE WEATHER
The Sun is the primary source of energy at Earth, and the Sun’s output determines the conditions in interplanetary space at Earth and throughout the solar system. Earth’s magnetic field and associated electrical current systems are continuously reacting to changing conditions in the solar wind that are driven by processes occurring at the Sun. The characteristics of Earth’s ionosphere and neutral thermosphere are influenced both by local processes and by coupling of the ionosphere and thermosphere to the overlying regions of the geospace1 environment.
Solar-terrestrial science addresses a coupled system extending from the Sun and heliosphere to Earth’s outer magnetosphere and ionosphere to the lower layers of the atmosphere, which are connected via the thermosphere and lower ionosphere. Processes in each region can affect those in the other regions through coupling and feedback mechanisms. Knowledge of this complex system requires a broad spectrum of observations that are continuous in time and that can provide measurements spanning the broad extent of the Sun-Earth domain. Study of Earth’s magnetosphere and ionosphere formed the historical starting point for space physics research, and the two regions remain an important focus for study because they constitute the space environment in which most human activities occur and they provide important prototypes for understanding the magnetospheres and ionospheres of other planets and small solar system bodies. In addition, the basic physical phenomena of space plasmas, which occur in remote and therefore inaccessible locations in the universe, also can be studied directly in Earth’s magnetosphere.
Despite the great advances made over the past 40 years, scientific knowledge of the complex Sun-Earth system is far from complete, and fundamental questions remain unanswered. For example, researchers are not able to completely specify the physical processes that transfer energy from the solar wind to Earth’s space environment and have not yet established the nature of the global response of Earth’s magnetosphere and ionosphere to the variable solar wind drivers under all conditions. These are the most basic and important questions that can be asked about geospace, but they have not received satisfactory answers. However, the maturity of the field now allows the deployment of highly capable observing systems, interrogation of large databases, development of sophisticated quantitative models, and construction of new definitive experiments both for Earth’s space environment and for those of other solar system bodies.
Space weather describes the conditions in space that affect Earth and its technological systems. Through various complex couplings, the Sun, the solar wind, and the magnetosphere, ionosphere, and thermosphere can influence the performance and reliability of space-borne and ground-based technological systems. Solar energetic particle events and geomagnetic storms are natural hazards, just as are hurricanes and tsunamis. Severe geomagnetic storms can interfere with communications and navigation systems, disturb spacecraft orbits because of increased drag, and cause electric utility blackouts over wide areas. Ionospheric effects at equatorial, auroral, and middle latitudes constitute a major category of space weather effects that need to be better characterized and understood.
DISTRIBUTED ARRAYS OF SMALL INSTRUMENTS—THE NEXT LOGICAL STEP
Space physics proper began in the late 1950s with the launch of the first Earth-orbiting satellites. The field is distinguished from astronomy and astrophysics as well as from earlier attempts to understand our space environment by its capability of measuring in situ the plasmas that surround Earth and pervade the solar system. However, despite the distinctive and defining role played by in situ observations, the importance of ground-based investigations (carried out with radars, magnetometers, riometers, ionosondes, all-sky cameras, coronagraphs, and neutron monitors) to pre-space-age knowledge of the Sun-Earth system has been crucial. Today, nearly half a century after the dawn of the space age, remote sensing from ground-based facilities remains essential to efforts to characterize and understand Earth’ s space environment and to investigate the workings of its ultimate energy source, the Sun.
One of the most serious obstacles to progress in understanding and predicting the space physics environment is the inadequate spatial distribution of ground-based measurements. This results, in part, because of the remoteness and lack of supporting infrastructure in some important regions, such as at polar latitudes, and, in part, because measurements in populated areas have been made and treated as single-point observations, as opposed to being the output of a coupled, distributed network. Effects of disturbances in the upper atmosphere occur as part of a complex, coupled system. Continuous observations spanning time and space with adequate resolution and good data accessibility are necessary to advance understanding of such processes. A global set of observations is needed especially to drive, validate, or assimilate into global models of the geospace environment. Due to the importance of cross-scale coupling in plasma processes, there is a need to resolve both high and low spatial scales in these global models.
Modern ground-based instrumentation can provide continuous real-time observations of the upper atmosphere in fixed regions of space, removing the spatial/temporal ambiguities that can arise in analyzing in situ observations from a moving platform. Advances in miniaturization techniques and in networked communications enable the fielding of large numbers of reasonably priced sensors in coordinated ground-based arrays of small instruments whose individual fields of view can be integrated to provide the spatial coverage and resolution needed to address space physics processes and space weather effects.
The expanding network of Global Positioning System (GPS) total electron content (TEC) receivers in the U.S. sector has provided an important example of the use of distributed small-instrument ground-based arrays to capture the extent and dynamics of ionospheric space weather disturbances (see Figure 1.1). In this instance, the effects of the dynamic atmosphere-space interface, namely the ionospheric plasma variability, present the single largest source of error in global positioning when using affordable single-frequency GPS receivers. A network of more-costly dual-frequency GPS receivers has been deployed globally to support high-precision geodetic and geophysical measurements, particularly for the study of earthquake hazards, tectonic plate motion, plate boundary deformation, and meteorological processes. These same receivers can be utilized to measure the ionospheric TEC. Using World Wide Web-accessible data from these GPS arrays, the ionospheric research community has identified space weather storm fronts that develop on a continental scale and, for the first time, has been able to describe the extent and severity of stormtime ionospheric variability.
Advances in technology have created opportunities for the development of new instruments to observe parts of the Sun-Earth system with unprecedented resolution and to measure crucial, heretofore undetectable, properties. Similarly, rapid-paced developments in computing and information technology will support the ability to merge and analyze large amounts of data from distributed arrays of space physics sensors in near-real time. Closely coordinated and coupled arrays of ground-based instruments, providing a variety of analyzed data products in real time to the research, education, and applications communities, are now feasible. The deployment of distributed arrays of small instruments (DASI) represents the next logical step in the development of space physics instrumentation.
THE DECADAL SURVEY FOR SOLAR AND SPACE PHYSICS
The NRC decadal survey for solar and space physics, The Sun to the Earth—and Beyond: A Decadal Research Strategy in Solar and Space Physics,2 identified broad scientific challenges that defined the focus and direction of solar and space physics research for the decade 2003 through 2013 and recommended priorities for theoretical, ground-based, and space-based research programs of NASA and the National Science Foundation (NSF) as well as for complementary operational programs of other agencies such as the National Oceanic and Atmospheric Administration (NOAA), the Department of Defense (DOD), and the Department of Energy (DOE).
To provide continuous real-time observations with the resolution needed to resolve mesoscale phenomena and their dynamic evolution and to support the next generation of space weather data-assimilation models, the survey report recommended that the next major ground-based instrumentation initiative be the deployment of widely distributed arrays of small space physics research instruments. Analogous to the meteorological arrays that support terrestrial weather research, modeling, and predictions, space weather arrays would provide continuous real-time observations of geospace with the resolution needed to resolve mesoscale phenomena and their dynamic evolution. Ground-based arrays would address the need for observations to support the next generation of space weather data-assimilation models and would advance to a new level understanding of the physical processes that interconnect the spheres of geospace.
The solar and space physics decadal survey report’s recommendation for a Small Instrument Distributed Ground-Based Network is being developed further through an ongoing assessment of the DASI concept. DASI is a potential NSF ground-based initiative with emphasis on large-scale and system-wide programs offering an opportunity to address the Earth-ward boundaries and extent of the space weather connection. The DASI concept combines state-of-the-art instrumentation with real-time communications technology to provide both broad coverage and fine-scale spatial and temporal resolution of upper atmospheric processes crucial to understanding the coupled atmosphere-ionosphere-magnetosphere system. When implemented, data from the DASI instruments will provide the simultaneous real-time measurements that are needed for assimilation into physics-based models. Data from DASI is also vital to improve understanding of space weather processes and effects in the upper atmosphere. A complement of instruments, including GPS receivers and magnetometers, placed at educational institutions is meant to provide a rich hands-on environment for students, and instrument clusters at remote locations will contribute important global coverage. These detailed, distributed measurements will complement the capabilities of the larger ground-based facilities.
In response to a request from the NSF, an ad hoc committee was formed under the Space Studies Board’s Committee on Solar and Space Physics to organize a workshop to explore the scientific rationale for such arrays, the infrastructure needed to support and utilize them, and proposals for an implementation plan for their deployment (see Appendix A). This report summarizes the workshop discussions. It does not provide consensus findings or recommendations.
The 1.5-day workshop was held in June 2004 at the National Academies’ Jonnson Center in Woods Hole, Massachusetts. Workshop participants included representatives of the thermosphere, ionosphere, magnetosphere, and solar-heliosphere research communities. Agency representatives from NSF, NASA, NOAA, the Air Force Research Laboratory, and the Office of Naval Research attended and addressed the relevance of distributed instruments in their future program plans. The workshop agenda and a list of workshop participants are presented in Appendix B.
Participants at the workshop were asked to describe specific examples of compelling science that could be addressed by ground-based space physics instrument arrays. They were also asked to consider existing arrays and to present lessons that could be learned from their operations. As detailed below, several recurrent themes emerged during workshop discussions, specifically:
The need to address geospace as a system,
The importance of real-time observations for space weather,
The value of coordinated and continuous observations, and
The insufficiency of current observations.
To address these deficiencies, participants identified several areas for focused technology development, including development and deployment of reliable, remotely operated, ground-based ionospheric and geomagnetic measurement stations.
Geospace as a System
Understanding the Sun’s influence on Earth’s global space environment requires detailed knowledge of the atmosphere-ionosphere-magnetosphere system. This extremely complex natural system involves many different interacting elements, and Earth is the only planetary system that scientists can expect to study in detail. Today, the science of space plasma physics has matured to the level of being able both to describe many of these interactions and to model them. A major goal in solar-terrestrial science now is to unify scientific understanding so as to achieve a more comprehensive computational
framework that can predict the properties of this system—conditions known as space weather. To do this accurately, however, requires an understanding of Earth’s global behavior as it exists, rather than as it occurs in an idealized representation. Realizing such goals requires the assimilation and integration of data from disparate sources.
Geospace processes involve significant coupling across atmospheric layers and altitude boundaries, as well as coupling across multiple scale sizes from global (thousands of kilometers), to local (tens of kilometers), to micro-scale (meter-scale and smaller). Many of the associated phenomena have been studied extensively, but often at a subsystem level. It has become apparent that a systems approach—addressing geospace as a coupled whole—is needed to make significant progress in understanding the space weather environment. A combination of the multiple points of view provided by clustered DASI instruments will give greater understanding than can the individual measurements taken in isolation.
For example, continuous long-term distributed measurements are needed to advance understanding of the coupling of the neutral atmosphere and the overlying space environment. The neutral upper atmosphere cannot be considered in isolation, because the thermosphere and ionosphere are symbiotically connected. The neutral upper atmosphere is also profoundly affected by the lower atmosphere (dynamical coupling via planetary waves, tides, and gravity waves). An unanswered question is the extent to which the quiescent global variability pre-conditions the thermosphere-ionosphere response to space weather events.
Workshop participants highlighted two important roles for DASI: (1) providing the coordinated multi-technique observations needed to characterize these intercoupled processes simultaneously and (2) providing a means of digesting this information and disseminating it to a variety of users. The multiple scale sizes involved suggest that a variety of DASI instrument array configurations will be needed.
Need for Real-time Observations
The magnetosphere-ionosphere-thermosphere (M-I-T) system is a highly dynamic, nonlinear system that can vary significantly from hour to hour at any location. The coupling is particularly strong during geomagnetic storms and substorms, but there are varying time delays associated with the transfer of mass, momentum, and energy between the different domains. Also, it is now becoming clear that a significant fraction of the flow of mass, momentum, and energy in the M-I-T system occurs on relatively small spatial scales and over a wide range of temporal scales. Consequently, elucidation of the fundamental coupling processes requires continuous, coordinated, real-time measurements from a distributed array of diverse instruments as well as the perspective provided by physics-based data assimilation models. The DASI initiative will establish the required program. Because many instruments and instrument arrays can be operated in different modes to provide varying spatial, temporal, and parametric coverage, real-time observations are needed to set and coordinate the observing grids to provide optimal coverage in a dynamically evolving environment.
The ionosphere routinely causes space weather disturbances that result in satellite communications interruptions, GPS navigation errors and outages, and tracking inaccuracies that compromise the determination of the orbits of satellite and space debris. Space weather “nowcasting” involves monitoring and modeling a distributed, structured, and highly variable medium. Whereas a basic understanding of physical processes can be derived in detailed post-processing of the observational data sets, space weather forecast and nowcast requirements indicate a clear need for real-time data and the means to communicate them promptly to the user.
Insufficiency of Current Observations
Observational space physics is data-starved, leading to large gaps in the ability to both characterize and understand important phenomena. This is particularly true for space weather events, which often are fast-developing and dynamic and which extend well beyond the normal spatial coverage of current sensor arrays. A strong motivation for the DASI concept is to provide the coordinated, wide-ranging, continuous high-resolution data sets needed to guide the development of theory and models that will better describe and predict the characteristics and dynamics of Earth’s space environment. Low-cost instrumentation that is widely deployed and running continuously can provide the spatial and temporal coverage needed to capture the evolution and characteristics of the weather in geospace.