2
Compelling Science

This chapter summarizes highlights of workshop discussions of major scientific challenges facing the solar-terrestrial research community and indications of what crucial observations are needed to elucidate the workings of the coupled Sun-Earth system. The focus at the workshop was on those observations that can be acquired from the ground and that are not provided by current and proposed space-based missions.

A brief description follows of a number of fundamental research areas to be supported by distributed arrays of small instruments. These represent several of the key topical areas discussed at the workshop, and they provide an overview and a framework for the broad science programs that can be supported by DASI observations. The strength of the DASI approach is in providing continuous, real-time, wide spatial coverage of a variety of interrelated parameters with the spatial and temporal resolution needed to resolve space physics phenomena. By monitoring the layers of the atmosphere and the electromagnetic fields above an observer, DASI instruments can provide new eyes with which to visualize the coupled processes of geospace. Science drivers discussed at the workshop span the coupled system from Earth’s atmosphere, through the complex interactions of the magnetosphere with both the lower regions and the interplanetary environment, to the ultimate drivers of space weather in solar activity and variability.

MAGNETOSPHERE-IONOSPHERE

What Is the Configuration of the M-I-T System That Is Most Vulnerable to Space Weather?

The outer reaches of geospace, including the magnetosphere and ionosphere, form a buffer between the perturbations of the solar wind and the near-Earth environment in which humans live and work. Space weather is generated within this magnetized plasma environment, but the severity of a geospace disturbance in response to solar drivers is variable and is not understood. The configuration of the magnetosphere-ionosphere-thermosphere (M-I-T) system that is the most vulnerable to space weather remains a key outstanding research question. Current knowledge is not adequate to address this complex question with any confidence; moreover, each type of space weather impact could potentially be most adverse under different M-I configurations.

Role of DASI

A theme that emerged during the workshop was that answering fundamental questions, such as that regarding the configuration of the M-I-T most vulnerable to space weather, will be possible only when the entire coupled system from the Sun to the lower atmosphere is monitored and understood. Workshop participants noted frequently that widely distributed, continuous observations are required to



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Distributed Arrays of Small Instruments for Solar-Terrestrial Research: Report of a Workshop 2 Compelling Science This chapter summarizes highlights of workshop discussions of major scientific challenges facing the solar-terrestrial research community and indications of what crucial observations are needed to elucidate the workings of the coupled Sun-Earth system. The focus at the workshop was on those observations that can be acquired from the ground and that are not provided by current and proposed space-based missions. A brief description follows of a number of fundamental research areas to be supported by distributed arrays of small instruments. These represent several of the key topical areas discussed at the workshop, and they provide an overview and a framework for the broad science programs that can be supported by DASI observations. The strength of the DASI approach is in providing continuous, real-time, wide spatial coverage of a variety of interrelated parameters with the spatial and temporal resolution needed to resolve space physics phenomena. By monitoring the layers of the atmosphere and the electromagnetic fields above an observer, DASI instruments can provide new eyes with which to visualize the coupled processes of geospace. Science drivers discussed at the workshop span the coupled system from Earth’s atmosphere, through the complex interactions of the magnetosphere with both the lower regions and the interplanetary environment, to the ultimate drivers of space weather in solar activity and variability. MAGNETOSPHERE-IONOSPHERE What Is the Configuration of the M-I-T System That Is Most Vulnerable to Space Weather? The outer reaches of geospace, including the magnetosphere and ionosphere, form a buffer between the perturbations of the solar wind and the near-Earth environment in which humans live and work. Space weather is generated within this magnetized plasma environment, but the severity of a geospace disturbance in response to solar drivers is variable and is not understood. The configuration of the magnetosphere-ionosphere-thermosphere (M-I-T) system that is the most vulnerable to space weather remains a key outstanding research question. Current knowledge is not adequate to address this complex question with any confidence; moreover, each type of space weather impact could potentially be most adverse under different M-I configurations. Role of DASI A theme that emerged during the workshop was that answering fundamental questions, such as that regarding the configuration of the M-I-T most vulnerable to space weather, will be possible only when the entire coupled system from the Sun to the lower atmosphere is monitored and understood. Workshop participants noted frequently that widely distributed, continuous observations are required to

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Distributed Arrays of Small Instruments for Solar-Terrestrial Research: Report of a Workshop FIGURE 2.1 An example of a magnetometer array. The Mid-continent Magnetoseismic Chain is a National Science Foundation project that conducts research in magnetospheric sounding using ground magnetic field observations. Nine new magnetometers (locations shown in bright purple) are being added to parts of existing chains (locations shown in blue and black) to provide new research capabilities. SOURCE: Courtesy of University of California, Los Angeles. Available at <spc.igpp.ucla.edu/mcmac>. characterize these coupled regions of geospace. As an example, they noted that the effectiveness of ground-based instruments in monitoring plasma regions and boundaries is being demonstrated by the use of arrays of magnetometers to track the position of the plasmapause (Figure 2.1) and by the current arrays of GPS receivers, which produce total electron content maps of coupled M-I disturbances (see Box 2.1).

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Distributed Arrays of Small Instruments for Solar-Terrestrial Research: Report of a Workshop BOX 2.1 Global Redistribution of Solar-produced Low-latitude Ionospheric Plasma Distributed arrays of ground-based Global Positioning System (GPS) receivers have been used to identify solar-produced low- latitude ionospheric plasma forms as a strong source of the plasmaspheric erosion plumes that couple the inner and outer magnetosphere. (See Figure 2.1.1.) Distributed GPS observations suggest that this enhanced total electron content (TEC) results from a rapid poleward redistribution of post-noon-sector low-latitude thermal plasma during the early stages of geomagnetic disturbances. Eastward electric fields near dusk produce a poleward displacement of the equatorial anomalies and enhancements of TEC in the post-noon plasmasphere and mid-latitude ionosphere. Strong magnetospheric electric fields are generated as storm-injected energetic particles fill the enhanced ring current. These subauroral electric fields erode the plasmasphere boundary layer, producing plasmaspheric drainage plumes that carry the high-altitude material toward the dayside magnetopause. The near-Earth footprint of the plasmaspheric erosion events is seen as the mid-latitude streams of storm-enhanced density that sweep poleward across the North American continent. These processes produce storm fronts of dense thermal plasma that extend continuously from low latitudes into and across the polar regions. FIGURE 2.1.1 A snapshot of ionosphere total electron content over the northern polar region derived from distributed GPS TEC receivers. Superimposed on the TEC image is the instantaneous pattern of a high-latitude electric field, which is derived from distributed SuperDARN radar observations. Also shown (and included in the analysis) are in situ observations from space (Defense Meteorological Satellite Program driftmeter observations along the satellite trajectory) and the positions of the distributed large incoherent-scatter radars that provided detailed altitude profiles through the major space weather feature (polar tongue of ionization) during the event. SOURCE: Foster, J.C., A.J. Coster, P.J. Erickson, J.M. Holt, F.D. Lind, W. Rideout, M. McCready, A. van Eyken, R.J. Barnes, R.A. Greenwald, and F.J. Rich. 2005. “Multiradar Observations of the Polar Tongue of Ionization,” J. Geophys. Res. 110(A9): A09S31, doi:10.1029/2004JA010928.

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Distributed Arrays of Small Instruments for Solar-Terrestrial Research: Report of a Workshop What Are the Processes and Effects Associated with Plasma Redistribution During Disturbed Conditions? The problem of plasma redistribution pertains to the various regimes that have historically been used to describe the magnetosphere, ring currents, plasmasphere, radiation belts, and ionosphere. Each region is distinguished by its plasma density and temperature. Some have sharp boundaries between them, whereas others are diffuse. The common problem is that during severe space weather events these boundaries and plasma regimes redistribute in ways that researchers can only glimpse, and yet it is these redistributions that lead to space weather effects such as (1) spacecraft charging in rarefied density regions with enhanced high-energy particles, (2) satellite-ground communication problems and high-frequency communications disruptions due to the effects of scintillations disrupting propagation paths, and (3) satellite orbital drag effects due to enhanced neutral densities. Workshop discussions focused on plasma redistribution processes in the inner M-I system. The electric fields that drive plasma redistribution arise from a number of sources. Earth’s interaction with the solar wind drives the circulation of the magnetosphere and the electric field associated with the large-scale circulation of the auroral ionosphere. Internal magnetospheric processes and feedback between the ionosphere and magnetosphere result in additional regional and temporally varying electric fields. One of the most important electric fields generated by the M-I system is created by the divergence of the asymmetric ring current. The electric field and currents resulting from the ring current are at low latitudes and are typically quite strong and variable during disturbed periods. Strong subauroral electric fields perturb the outer plasmasphere, and large, variable, magnetic perturbations give rise to ground-induced currents that have the potential for disrupting terrestrial electrical power grids. It is important to understand fully the subauroral electric and magnetic disturbances that result from the ring current. Unknowns A major present-day problem is that although they have shown how dynamic, how rapid, and even how regionally localized the effects of space weather can be, space- and ground-based resources are so isolated (outside the correlation distances and times from each other) that knowledge of or ability to specify or forecast such effects cannot meet the need either to understand the severe response mechanisms or to provide users with space weather mitigation strategies based on realistic specification of the events, let alone forecasts. Related Questions What are the sources and effects of disturbance electric fields? What is the temporal relationship between the equatorial effects of undershielded (penetration) electric fields and the onset of strong erosion plumes in the subauroral ionosphere? What are the causes of longitude effects in the geospace response to disturbances? What is the relationship of the occurrence of low-latitude (off-equatorial) scintillation to the occurrence of redistribution events? In what ways are enhanced events similar to and different from equatorial anomalies formed on quiet/normal-days? In what way does the redistribution of low-energy plasma affect the development of magnetospheric storms? Of particular significance are feedback and modification of magnetospheric processes by the redistributed cold plasma.

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Distributed Arrays of Small Instruments for Solar-Terrestrial Research: Report of a Workshop Role of DASI During the workshop participants noted that all of the above questions demand the types of coverage and multi-technique measurements afforded by distributed, continuously operating instrument arrays that can resolve temporally changing structures. Arrays of instruments attuned to boundary region phenomena and processes are required (see Box 2.2). DASI will be able to interrelate the variety of parameters associated with these regions as such disturbances evolve, and with a resolution that will enable identifying and following their boundaries. Extension of the needed monitoring capability into hard-to-access regions constitutes a major goal of DASI. BOX 2.2 Boundary Region Processes Transport of magnetic flux and plasma across the open-closed field line boundary is one of the most important processes in the magnetospheric system, and it plays a central role in both the supply of plasma to the central plasma sheet, and the supply of energy to the overall convection process. CADI, the Canadian Advanced Digital Ionosonde project, is a good example of the use of distributed instrumentation to provide continuous, real-time monitoring of an important boundary region. The high-latitude SuperDARN high-frequency radars (Figure 2.2.1) monitor the electric fields and plasma redistribution pattern across this region. FIGURE 2.2.1 Graphic from the SuperDARN home page on the World Wide Web that shows the fields of view of the northern SuperDARN radars. SuperDARN sites are also present in the Southern Hemisphere. See <superdarn.jhuapl.edu/>. SOURCE: Johns Hopkins University Applied Physics Laboratory.

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Distributed Arrays of Small Instruments for Solar-Terrestrial Research: Report of a Workshop What Is the Role of the Ionosphere-Thermosphere System in the Processes Associated with Particle Energization? Some of the most severe ramifications of space weather are associated with particle energization during geospace disturbances. The acceleration processes themselves are not fully understood, but their adverse geoeffectiveness is beyond doubt. They lead to extremely high-energy electrons and ions that create the ring currents and radiation belts that have a direct impact on space travelers and hardware. Energetic particles can damage satellite components, circuitry, and systems via spacecraft surface charging, as well as deep dielectric charging caused by the penetration of MeV-energy electrons. Large fluxes of MeV-energy electrons are also of concern with respect to doses of radiation received by crew on the International Space Station, by space shuttle astronauts undertaking space walks, and even by airplane crews who regularly fly high-altitude polar routes. Whereas the acceleration of many of the high-energy-particle populations results from heliospheric or magnetospheric processes, the ionosphere-thermosphere system contributes to auroral acceleration associated with intense field-aligned currents that close in the ionosphere. The locations and strengths of these acceleration regions, their energy sources, and consequent precipitation of particles can be inferred occasionally via detection of mid- to low-latitude auroras or magnetic perturbations. Ionospheric conductance is modified by particle precipitation, and in turn, the characteristics of the magnetospheric acceleration mechanism can depend on ionospheric feedback. Unknowns The processes involved with, and the effects of, particle energization are manifested in coupled, global variations in density, optical emissions, currents, and waves. Simultaneous observations of these features can reveal the evolution of magnetospheric particle populations. The topic of plasma redistribution (noted in the preceding section) is tied to this issue in that the high-altitude extension of plasma erosion features influences the development of energetic particle distributions in the magnetosphere. The interrelationship of cold plasma redistribution (see above) and M-I coupling and control of auroral and subauroral fields and currents in the system-wide particle energization needs to be understood. Related Questions What is the low-altitude mapping of the current-closure regions and boundaries of the magnetosphere-ionosphere system? What processes are involved in the formation of new energetic particle belts in the inner magnetosphere during storms? What is the role of ionospheric polarization electric fields in modulating magnetospheric processes? Role of DASI Workshop participants considered the application of distributed instrument arrays, including monitors of the aurora and associated currents, ionospheric density and conductivity, electric fields, and thermospheric winds, to address the questions listed above. Such arrays could include an extension of THEMIS instrumentation (see Box 2.3.) to lower latitudes, especially in optics and magnetometers. However, high-frequency, ultralow-frequency, and very-high-frequency radio propagation techniques could all be brought to bear on this problem.

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Distributed Arrays of Small Instruments for Solar-Terrestrial Research: Report of a Workshop BOX 2.3 THEMIS The THEMIS ground array provides a current example of the synergy of space- and ground-based coordinated studies to address significant auroral-latitude processes (substorms). (See Figure 2.3.1.) Carefully planned arrays of auroral optical imagers and magnetometers provide real-time coverage of the auroral region across North America. The major THEMIS science objective is to locate and time the substorm onset as seen at ground level. At onset, the aurora intensifies and expands, and the magnetic field caused by the ionospheric current intensifies. FIGURE 2.3.1 The rapid evolution of the aurora across the midnight sector (see illustration at <pluto.space.swri.edu/image/glossary/local_time.html>) provides a near-Earth image of the development of magnetospheric substorms. A distributed array of ground-based white-light auroral imagers is being deployed across North America as an essential part of the NASA THEMIS MIDEX mission. The imager array will provide high-resolution observations of auroral characteristics in the North American sector, with the specific objective of characterizing the spatio-temporal evolution of the electron aurora during expansive phase onset. Shown here is a composite figure that displays the combined field of view of the ground-based THEMIS auroral imager array (bottom) with an auroral snapshot by the ultraviolet imager on the Polar spacecraft (top). SOURCE: Images courtesy of Eric Donovan, University of Calgary; Polar UVI data provided by Kan Liou, Johns Hopkins University Applied Physics Laboratory.

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Distributed Arrays of Small Instruments for Solar-Terrestrial Research: Report of a Workshop What Are the Effects of Preconditioning in the Ionosphere and Magnetosphere on the Evolution of Disturbances? Disturbances in or across key regions of geospace involve the characteristics of the external drivers (for example, the interplanetary magnetic field and solar wind) as well as the pre-existing state and structure of the regions. There are strong reasons to believe that a storm’s effectiveness does not depend on its “energy” content alone but also on the current state of the plasma environment in which it develops. The environment is, in ways not fully understood, both a plasma source and also an “elastic” topology that can store energy at levels that can be extremely effective in particle acceleration when triggered by the arrival at Earth of a solar coronal mass ejection. If the geospace system is in an unfavorable condition (either depleted of plasma or already configured in such a way that energy cannot be stored efficiently), then the resulting disturbance may be lessened. Unknowns The evolution of plasma processes and the system response throughout space weather events may depend in poorly understood ways on the initial conditions of the geospace system. Related Questions Are there pre-existing states or features of the magnetosphere-ionosphere system that are important in the development of a superstorm? What are the mechanisms involved? Is the magnitude of the geospace response to a solar driving event predictable based on the pre- existing condition of the geospace system? In what ways does the ionospheric conductivity distribution affect the development of geomagnetic storms? Role of DASI Participants at the workshop noted that global data from a wide variety of DASI instruments will have to be integrated over the hours and possibly days prior to a storm to establish the preconditioned environment in order to address the variability in storm response. Because of the currently limited predictability of coronal mass ejections and resultant storms, continuous distributed observations are needed. IONOSPHERE-THERMOSPHERE INTERACTIONS Earth’s ionosphere-thermosphere system is the site of complex electrodynamic processes that redistribute and dissipate energy delivered from the magnetosphere in the form of imposed electric fields and precipitating charged particles. Previous studies have revealed much about the composition and chemistry of this region and about its structure, energetics, and dynamics. However, a quantitative understanding has proved elusive because of the inability to distinguish between temporal and spatial variations, to resolve the variety of spatial and temporal scales on which key processes occur, and to establish the cross-scale relationships among small, intermediate, and large-scale phenomena.

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Distributed Arrays of Small Instruments for Solar-Terrestrial Research: Report of a Workshop What Processes Affect Ion-Neutral Coupling in the Presence of Particle Precipitation? Neutral winds in the upper atmosphere play an important role in the global response of the atmosphere to geomagnetic disturbances: Neutral winds can significantly change the chemical distribution of the thermosphere, changing heating, cooling, and ionization rates. Neutral winds advect temperature perturbations to low latitudes, such that the polar heating is spread over the whole globe. Neutral winds push ions up field lines and across field lines. The motion up (and down) field lines redistributes the plasma significantly. The dragging motion across field lines can cause changes in the ionospheric currents and may induce ground currents. By dragging ions the neutral wind can change the electric field and dynamics in the magnetosphere. During events in which the ionospheric electric field becomes large, the ions flow very quickly. The resulting friction between the thermospheric neutral winds and the ions can result in significant heating of the thermosphere. This heating causes the atmosphere to lift, increasing drag on satellites. During magnetic substorms all of the above elements are very important, since large electric fields, combined with large particle precipitation, increase the frequency of collision between the neutrals and ions. Related Questions How does the global thermosphere-ionosphere respond to geomagnetic storms? How does the global response vary with altitude? How does the global response vary with time? What are the local and global responses to solar proton events? How deep into the atmosphere do such effects penetrate? Role of DASI The effects that couple the thermosphere with geomagnetic storms occur on a large-scale, system-wide level. Properties of the interacting layers of the upper atmosphere must be sampled over a wide range of latitudes and local times before, during, and following the magnetospheric disturbance. Workshop participants described the need to obtain thermospheric measurements from an extended and relatively dense array of measurement sites over a range of latitudes. For example, deployed arrays of autonomous Fabry-Perot interferometers or Michelson interferometers would monitor thermospheric temperature, composition, and dynamics at distributed sites and within many important coupling regions. What Are the Causes of Thermosphere-Ionosphere Variability During Geomagnetically Quiescent Periods? The dynamic coupling between the lower and upper atmosphere is described in recent work that has reported variability in the principal (F2) peak of ionospheric density observed at middle latitudes in both the Northern and Southern Hemispheres during quiescent periods. That variability has been attributed to global-scale waves, including tides and planetary waves that originate in the lower

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Distributed Arrays of Small Instruments for Solar-Terrestrial Research: Report of a Workshop FIGURE 2.2 All-sky image acquired in the atomic oxygen “green line” (>557.7 nm) airglow showing a coherent short-period gravity wave. This photochemical emission originates from a well-defined layer in the upper atmosphere at an altitude of approximately 96 km. Gravity waves transport large amounts of momentum into the MLTI (Mesosphere and Lower Thermosphere/Ionosphere) region where they create profound effects on the background wind and temperature fields. SOURCE: Courtesy of Michael J. Taylor, Utah State University; data shown were obtained from Bear Lake Observatory, Utah, on June 5, 2002, using a monochromatic charge-coupled device imager as part of a coordinated measurements program with the NASA TIMED satellite. atmosphere and propagate upward into the thermosphere-ionosphere (T-I). This interpretation has been borne out by “whole atmosphere” modeling studies, which treat a combined domain that extends from the ground through the thermosphere. The quantification of T-I-ionosphere variability during quiescent periods is a fundamental research problem that has very practical consequences. It is impossible to quantify accurately the T-I impact of any space weather event unless the underlying state is known. There are identical implications for the development of any capability to provide realistic T-I space weather forecasts. Unknowns Upward-propagating lower atmospheric waves, including atmospheric tides, planetary waves, and gravity waves (see Figure 2.2), are a major source of quiescent T-I variability. The mesopause region (ca. 80 to 110 km) is the gateway between the lower atmosphere and the T-I. Even though the mesopause is well monitored by medium-frequency and meteor radars, resonance (for example, Na, K, Fe) lidars, imagers, and Fabry-Perot interferometers, significant gaps in current spatial and/or temporal sampling of the mesopause region preclude accurate determination of global-scale wave characteristics. Related Questions What is the effect of thermosphere-ionosphere variability during quiescent periods on the development of space weather disturbance events?

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Distributed Arrays of Small Instruments for Solar-Terrestrial Research: Report of a Workshop In what ways do the characteristics and structure of the lower atmosphere influence the development of geospace space weather events? How do lower-atmosphere effects couple into the magnetosphere? Role of DASI Workshop discussions revealed that simultaneous correlative mesopause region and T-I diagnostics are needed to resolve the upward-propagating global-scale wave effects on the T-I system. Participants noted that DASI can provide multiple longitudinal instrument chains at a select series of latitudes. Optimally, these measurements should be continuous, because tides with frequencies that are harmonics of a solar day are persistent and ubiquitous, whereas traveling planetary waves are transient, with periods that range between 2 and 16 days. SOLAR: THE SUN AS A DRIVER The Sun is the primary source of energy input to the geospace environment. Its output determines the conditions in interplanetary space at Earth and throughout the solar system. Earth’s magnetic field and associated current systems are continuously reacting to changing conditions in the solar wind that are driven by processes occurring at the Sun. During periods of high solar activity, highly energized particles can be accelerated near the Sun and in the heliosphere and propagate toward Earth, endangering astronauts and satellites. Propagating solar disturbances, known as coronal mass ejections, can generate geomagnetic storms, which can damage power grids and satellites and affect GPS and other important navigation and communication systems. Despite the great advances made over the past 40 years, current knowledge is far from complete, and unanswered questions remain that are of fundamental importance to comprehension of solar and space physics, and to space weather in particular. What Are the Structure and Dynamics of the Sun’s Interior? The cyclic solar magnetic field is generated by dynamo processes occurring in a thin region at the base of the convection zone known as the tachocline. The discovery of propagating sound waves in the Sun in the 1960s and their characterization in the 1970s have led to the development of an exciting new observational technique called helioseismology, which allows the sounding of the structure and dynamics of the Sun’s interior. The discovery of the tachocline, along with the identification of large-scale meridional flows, has revolutionized dynamo models of the magnetic field. These models describe the way in which the global field is generated in a cyclical fashion, bringing researchers closer to understanding and ultimately predicting the sunspot cycle. The most dramatic new insights into solar interior dynamics in the near future will likely come from the emerging field of local helioseismology, which has already provided unprecedented insight into the structure underlying active regions and large-scale flow patterns such as meridional circulation. Imaging of magnetic activity on the farside of the Sun using a local technique called acoustic holography can provide information on the generation and evolution of active regions and up to 2 weeks’ advance notice of the formation of regions likely to produce space weather phenomena such as flares and coronal mass ejections. Here as in global helioseismology, continuous long-term monitoring is necessary in order to understand how subsurface dynamics vary over the course of a solar activity cycle. Longer-term monitoring is also necessary in order to understand and predict periodic or chaotic modulations of the solar activity cycle such as the Maunder minimum, an extended period in the 17th century in which sunspot activity largely ceased.

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Distributed Arrays of Small Instruments for Solar-Terrestrial Research: Report of a Workshop Unknowns Despite this recent progress, much uncertainty remains regarding the physical mechanisms responsible for field generation and the precise role of the meridional circulation. More information is needed about the structure and evolution of subsurface flows and magnetic fields. Workshop participants regarded as particularly important (1) long temporal baseline studies of global oscillations and large-scale flow patterns such as meridional circulation that vary on time scales of the solar activity cycle and (2) high-temporal-cadence observations of the dynamics and evolution of localized structures such as those found beneath active regions. Time-series of Doppler-shifted light emitted at the Sun’s surface can address these needs because they can specify distinct, resonating, sound waves that, in turn, probe the Sun’s interior. Role of DASI Workshop participants highlighted the importance of continuous helioseismic observations so as to prevent periodic data gaps that are detrimental to frequency resolution. Although this can be and indeed is done in space, ground-based observations have the crucial additional benefits of (1) providing long temporal baseline observations to allow ongoing studies on time scales of solar cycles and (2) providing high-temporal-cadence observations that do not suffer from satellite telemetry constraints. Consequently, the workshop discussions emphasized continuation of the type of missions conducted by the Global Oscillations Network Group (GONG), thereby extending the long temporal baseline of observations with multiple ground stations to keep coverage continuous. GONG is a community-based program to conduct a detailed study of solar internal structure and dynamics using helioseismology. In order to exploit this new technique, GONG has developed a six-station network (Figure 2.3) of extremely sensitive and stable velocity imagers located around Earth to obtain nearly continuous observations of the Sun’s “5-minute” oscillations, or pulsations (Figure 2.4). Spaceship Earth is a successful constellation of neutron detectors. It has 11 stations, with 9 in the Northern Hemisphere and 2 in Antarctica. The station locations were chosen to give good coverage of the equatorial plane. There is very sparse directional coverage at mid-latitudes, degrading the detection capability. What Are the Causes of Solar Activity? Transient solar disturbances such as flares, coronal mass ejections (CMEs), and prominence eruptions range in frequency from one event every few days to as many as four events per day at the maximum of the sunspot cycle. These transient events are driven by magnetic energy that ultimately originates in the solar interior and that is stored and then released in an often spectacular fashion in the corona. CMEs and prominence eruptions are sudden expulsions of magnetized plasma into the solar wind. A flare is a rapid localized increase in radiative output, particularly at shorter wavelengths, generated by a process of magnetic reconnection that converts magnetic energy to heat. The various forms of solar activity do not occur in isolation as separate events, but rather tend to occur in concert because of changes in the state of the magnetic field. The sunspot cycle, which is the most well known manifestation of solar activity, is driven by variations in the magnetic field.

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Distributed Arrays of Small Instruments for Solar-Terrestrial Research: Report of a Workshop FIGURE 2.3 The Global Oscillations Network Group helioseismology network. SOURCE: Courtesy of the National Optical Astronomy Observatory, which is operated by the Association of Universities for Research in Astronomy under a cooperative agreement with the National Science Foundation. FIGURE 2.4 A computer representation of one of nearly ten million modes of sound wave oscillations of the Sun, showing receding regions in red tones and approaching regions in blue. By measuring the frequencies of many such modes and using theoretical models, solar astronomers can infer much about the internal structure and dynamics of the Sun. This technique is called helioseismology, because of its similarities to terrestrial seismology, and is at the heart of the program carried out by the Global Oscillations Network Group (GONG). SOURCE: Courtesy of the National Optical Astronomy Observatory, which is operated by the Association of Universities for Research in Astronomy under a cooperative agreement with the National Science Foundation.

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Distributed Arrays of Small Instruments for Solar-Terrestrial Research: Report of a Workshop Flares and CME-generated shocks can accelerate particles that move toward Earth at relativistic speeds and that have the potential to harm astronauts and damage satellites. Most major geomagnetic storms are the result of CMEs and CME-produced shocks, which propagate through the solar wind and impact the space environment at Earth. Strong and/or sustained southward-pointing heliospheric magnetic fields can drive geomagnetic storms, because these southward-pointing fields are favorably configured to allow magnetic reconnection to transfer energy and momentum from the solar wind to Earth’s magnetic environment. Accurate predictions of geomagnetic activity depend critically on knowledge of the magnetic structure and plasma properties of Earth-directed CMEs and the surrounding solar wind. Unknowns More information is needed about the magnetic field and plasma conditions in the solar atmosphere in order to (1) understand CME formation, (2) provide information about the magnetic structure of Earth-directed CMEs and solar wind, (3) predict the onset of large solar flares, and (4) learn how and where solar energetic particles are accelerated. Many critical aspects of solar activity are not currently being observed at enough sites around the globe to provide continuous temporal coverage. To address solar activity, continuous measurements are needed, from active regions to global scales, of plasma and magnetic field properties from the photosphere through the corona. Role of DASI Understanding the origins of solar activity and monitoring it in detail as it occurs require continuous time coverage. Some critical observations of solar activity are best done from the ground, for example because of lower costs or large telescope size. Solar activity monitors should incorporate complementary instruments that capture crucial aspects of solar activity, including photometric and polarmetric measurements over wavelengths that sample a range of heights from the deep photosphere up into the corona. How Does the Structure of the Heliosphere Modify the Solar Wind? The solar corona gives rise to the solar wind, which dominates the space environment of Earth and all the planets, forming the heliosphere. The heliosphere is a highly structured, rapidly evolving extension of the corona, and hence it reflects the evolution of the solar atmosphere on all spatial and temporal scales. Long-lasting coronal holes near solar minimum are the source of fast co-rotating interaction regions (CIRs) in the solar wind. Near solar minimum, these CIRs dominate the structure of the heliosphere near the ecliptic plane, and hence Earth’s space environment. Similarly, erupting CMEs evolve, grow through interactions with streams that are swept up, and drive extended shocks throughout the heliosphere. These shocks are the source of the vast majority of energetic particles accelerated near the Sun. The heliosphere is therefore an active participant in the generation and modulation of space weather. Unknowns There are critical obstacles to understanding of the Sun-Earth connection relating to these heliospheric processes. For example, the relationship between the closed field regions of the solar atmosphere and the open field regions, which extend into the deep heliosphere, is still not understood.

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Distributed Arrays of Small Instruments for Solar-Terrestrial Research: Report of a Workshop Predicting how Earth-bound CMEs will interact with Earth depends strongly on the ability to predict the heliospheric interactions and processes that lead to particle acceleration, as well as the ability to determine when the evolving CME and surrounding solar wind will lead to a sustained southward-directed magnetic field. More information is needed about the structure and physical properties of the heliosphere in order to: Identify the magnetic processes that accelerate the solar wind, Provide the three-dimensional structure of the heliosphere, Understand the interactions of CMEs and solar wind streams, Predict the acceleration of energetic particles resulting from these interactions, and Understand the interactions of these particles and galactic cosmic rays (GCRs) with the heliospheric magnetic field. As a result of their high-energy interaction with the heliospheric magnetic field, GCRs can also be used as a tracer for the large-scale (~1 AU) and mesoscale (1 to 0.01 AU) structure of the heliospheric magnetic field near Earth. Role of DASI Neutrons and muons are secondary products of >1-GeV particles in Earth’s atmosphere. At the workshop, participants noted that measuring particles’ angular distribution at Earth’s surface provides direct information about the transport of these particles. Galactic and solar cosmic ray angular distributions need to be measured with complete sky coverage so that transport phenomena can be analyzed under all geometrical conditions. Due to the refracting effects of Earth’s magnetic field, multiple stations are needed to determine the full flux and angular information of the primary particles. Can Low-Frequency Interplanetary Scintillations Be Used to Make Global Determinations of Solar Wind Velocity? A key limitation in understanding the connection between solar activity and the interplanetary disturbances that eventually produce storms has been the difficulty in linking coronagraph observations of regions near the Sun with the shocks that are detected at greater distances and at later times by satellites near Earth. Planar wave fronts from compact radio sources (for example, quasars) are distorted as they pass through the solar wind, creating a moving diffraction pattern at Earth. Observations of this shifting pattern, known as interplanetary scintillation (IPS), allow properties of the solar wind, such as velocity, flow direction, and density, to be determined. By drawing on the diagnostic strengths of measurements of physical effects—for example, Faraday rotation and, at low frequencies, IPS (which is obtained with the wide instantaneous fields of view possible with a digital array)—it is possible that these ground-based radio telescopes could be used to measure magnetic field and density structures from the lower corona out to 1 AU. In addition, the antennas could be used to monitor continually for transient events, such as solar and planetary radio bursts, as well as astrophysical phenomena. For solar wind velocity measurements, the regions of different speeds can be resolved by using simultaneous observations from an array of antennas. Role of DASI At the workshop participants noted that the DASI concept for IPS velocity measurements could involve a global array of single-antenna sites or an array of multiple-antenna sites. IPS measurements

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Distributed Arrays of Small Instruments for Solar-Terrestrial Research: Report of a Workshop from multiple sites can be used to make maps that are more complete in shorter periods. Since solar disturbances are unpredictable, multiple IPS sites situated at different longitudes around Earth will increase the probability that a propagating solar disturbance will intersect the line of sight between a source and the receiver. IPS sites arranged at different longitudes around the globe will enable continuous time coverage of solar events. Multiple antennas at each site will provide more velocity measurements toward more sources. Longer baselines will provide velocities that are more accurate.