Origin of Plasmas in the Earth's Neighborhood
NEXT MOVE IN GEOSPACE
What is the origin of turbulent forces that sweep by Earth, becoming visible as auroral displays, confounding radio broadcasts, civilian and military electronics, power distribution, and satellite systems? Invisible magnetic storms can induce currents in long, power-transmission lines, affect power station transformers, and even cause power blackouts.
Disturbances in nearby space are consequences of the reactions in Earth's neighborhood to the violent behavior of the Sun. The past 20 years of space observations have led to a quiet but remarkable revolution in concepts of the Sun—Earth system. No longer is Earth visualized as simply a body traveling through a vacuum around the Sun and being warmed to life by its visible emissions. Rather, the data from spacecraft show that Earth on its course is plowing through an energy-laden flood from the Sun — magnetic fields, energetic charged particles, radiations throughout the electromagnetic spectrum, and electrified gases (called plasmas) flowing at speeds up to 1000 kilometers per second (2.2 million mph). Collisions with Earth's magnetic envelope create a dynamic environment surrounding Earth that reaches far beyond previous environmental concepts. This region, called geospace, extends to millions of kilometers around Earth and contains the atmosphere, the ionosphere, the magnetosphere and the near-Earth interplanetary medium.
Geospace is an integrated, strongly interactive system throughout which energy is continuously being generated, stored, transformed and released. Not only do geospace processes affect Earth's atmosphere and ground systems, but geospace is now known to be representative of plasma systems commonly found throughout the universe.
Data from single spacecraft have led scientists through evolutionary stages — first discovery, then exploration sufficient to provide a schematic representation of the solar—terrestrial environment like the one to the right. It is now time to proceed to a comprehensive understanding of geospace dynamics with a multi—spacecraft mission that can return coordinated, simultaneous observations.
Recent advances in geospace knowledge and satellite technology make it possible for the National Aeronautics and Space Administration to propose, for the mid-1980's, a four-satellite mission called Origins of Plasma in the Earth's Neighborhood, or the OPEN Program. Hundreds of investigators in the United States and in more than a dozen other nations are poised to participate in the first systematic, quantitative study of the flow of plasma and energy through geospace.
DISCOVERY OF GEOSPACE
Before the 1950's, little was known about the region outside the thin cocoon formed by Earth's atmosphere and ionosphere. The existence of a magnetic field had been known for centuries. It was believed to extend into the void of space with a shape much like the field of a simple bar magnet. There was evidence for some cause-and-effect association between such episodic events on the Sun as solar flares and events at Earth such as brilliant enhancement of auroral lights at high latitudes and transient disturbances in Earth's magnetic field. But neither the terrestrial phenomena nor their apparent connection with the active Sun was understood. The successful launching of artificial satellites, begun in 1957, dramatically changed this situation. The in situ observations ushered in a series of new and exciting discoveries about Earth's space environment.
Using simple radiation counters on Explorer—1, Professor James Van Allen and his colleagues were startled to discover very energetic charged particles trapped in belts around the Earth far above the upper atmosphere.
Later, spacecraft traveling to other planets discovered that essentially all of interplanetary space is filled with a hot, tenuous plasma continuously flowing away from the Sun behaving as though it were an electrified wind. When this solar wind strikes Earth's magnetic field, a shock wave forms upstream of the Earth. The shape of the magnetic field is compressed on the dayside and blown into an extended tear-drop on the nightside. Early satellite measurements also indicated that the magnetic field carved out a cavity in which large amounts of energy could be stored and whose flow was linked to the Earth by terrestrial magnetic field lines.
These discoveries during the 1950's and early 1960's opened the way for an era of intensive exploration. Scientists began to sketch the first map of geospace, define its boundaries and determine its dimensions. As the range of exploratory measurements became more complete, the scientists’ questions advanced from "What is in geospace?” and ”Where is it?” to "How is geospace formed?” and “How does it behave?"
GEOSPACE CONCEPT, 1980
The decade of the 1970's markedly advanced the exploratory phase of studies and opened a new era of quantitative physical investigation. Much of geospace, except for the distant geomagnetic tail, has now been mapped — at least superficially. It has proven to be a remarkably complex system indeed.
The different regions of geospace are populated by distinctly different plasma regimes. On the dayside of Earth at high latitudes, some magnetic lines of force part to form a funnel in the magnetosphere (the polar cusp region) that opens a channel down to Earth's surface. Through this funnel, energetic solar particles may plunge directly into the ionosphere and atmosphere.
Boundary layer plasmas, apparently composed of a mixture of cooling solar wind plasma that has penetrated the magnetospheric cavity and other particles that are escaping from sources or storage reservoirs inside the magnetosphere, flow around the entire periphery of the magnetosphere.
High energy electrons and ions fill the radiation belts that encircle Earth in the equatorial regions. And yet another component of hot ionized gas is found in a thin sheet along the mid—plane of Earth's invisible, comet—like magnetic tail. The energy stored by these groups of particles ultimately is responsible for Earth's magnetic storms.
With all this detail, scientists have little more than a sketch —an approximation built on a collection of independent, single—point measurements collected at different times during the past two decades. Boundaries are in constant motion. Changes in one region invariably stimulate subsequent changes or adjustments in other regions. Thus, the picture can represent only an imperfect assemblage of localized observations that yield a blurred image of what is, in fact, a highly dynamic and changing environment.
By studying this incomplete picture, scientist have begun to appreciate the physical processes that must act to produce the interactions of various parts of geospace. Now they are using multiple spacecraft such as the three International Sun—Earth Explorers to collect data on the dynamic behavior of certain localized regions. An extended, multi—spacecraft program is necessary for the next step — to collect data on simultaneous events throughout geospace as a whole.
To understand the anatomy of the human body, students look at the structure of individual parts, one at a time. But to understand the relationships between the parts of the body -— its physiology — students must consider the living system as a whole. The whole is more complex than the simple sum of components because of the highly interactive nature of these parts. So it is with geospace. An understanding of geospace and of the interactions of its components requires a view of the entire system —— a view obtained by planned observations taken simultaneously in the key geospace regions.
The problem is akin to assembling a puzzle whose pieces vary in shape and size with time. Past geospace projects have studied units of the puzzle independently at different points in time. Consequently, when assembling these pieces, it is hard to make them fit because each is really a piece of a different puzzle. What results is only a crude approximation of the true picture that gives little knowledge of how the segments interact. The four satellites of the OPEN Program will be positioned to collect data simultaneously on key pieces of the geospace puzzle. Then for the first time, overall geospace perspectives can be assembled that are accurate at given moments of time.
The OPEN approach permits a quantitative attack on three major scientific objectives that are necessary for fulfilling the ultimate goal of understanding geospace and its impact on Earth:
- To assess the energy, mass and momentum—flow throughout geospace.
- To obtain an understanding of the interactive behavior of geospace components, and of their cause-and-effect relationships.
- To assess the impact of geospace behavior on the terrestrial environment.
Geospace is at once a large—scale environment for studying processes that are hard to reproduce in miniature in Earth—bound laboratories, and a small—sca|e environment for studying events that are common throughout the universe.
Designers of fusion reactors or advanced microwave radio transmitters are often challenged by problems in plasma physics. Plasmas must be accelerated to extreme energies. Energetic particles must be confined and transported. Particle energy must be converted into electromagnetic waves. The same phenomena also occur on a vast scale in geospace. Geospace is the only easily accessible arena for studying nature's way with energy, not only here but throughout the universe.
Most of the universe is filled with plasma. Many actions that determine the structure and development of regions from small interstellar clouds to entire galaxies depend on basic interactions between plasmas and magnetic fields.
Interplanetary spacecraft have defined geospace—like magnetospheres around Mercury, Jupiter and Saturn. Astronomers have detected similar structures around rotating neutron stars (pulsars), and even around radio-wave-emitting galaxies whose dimensions dwarf the Milky Way. The same plasma processes that are important to this wide range of astrophysical systems occur nearby in Earth's own space environment. For example, nature possesses a number of ways to accelerate charged particles to very high energies. These energetic particles abound in solar flares, in Jupiter's magnetosphere, in supernovae, in galactic nuclei — and in Earth's own magnetosphere.
Thus, a careful study of geospace and the plasma processes that control its behavior applies not only to an understanding of solar-terrestrial relations and of their effects at Earth's surface, but also to basic scientific questions about plasmas.
DISRUPTIONS ON EARTH
The satellite and ground—based auroral photographs shown on the right are spectacular visual representations of geospace energy entering the upper atmosphere. In these auroral forms, the tremendous dynamic variability of the currents that flow and of the energy dissipated causes significant effects in many ground—based systems and in the atmosphere itself.
One example of such an effect is shown in the lower left panel in which an index representing magnetic activity caused by auroral currents is plotted versus time. Horizontal bars above this plot show induced current magnitudes in power system transformers. For a particular time interval, the induced current traces are shown for three distribution stations. These induced current magnitudes are sufficient to decrease expected transformer lifetimes and, in one instance, were directly responsible for total transformer failure. These geospace perturbations have been responsible for past power blackouts in the United States and Canada.
Such examples of the present sensitivity of large engineering systems, both civilian and military, to geospace processes are numerous and span many operational fields: communications, transportation, power distribution, resource exploration and space systems. All satellites orbit within geospace. Soon, the Space Shuttle will be taking astronauts and equipment into the hostile environment of geospace on regular schedules. Knowledge of geospace is required for these missions.
Although a number of geospace effects have engineering solutions, an understanding of geospace to a level required for cause-effect predictions will greatly improve the operation of present facilities and the design and operation of future systems. This predictive capability is essential for the existing space weather service (operated by the National Oceanic and Atmospheric Administration and the Department of Defense) to satisfy anticipated requirements in the 1980's.
Another area of potential applicability of OPEN Program results is in solar effects on Earth's weather. The lower right panel shows one of many Sun-weather correlations reported in recent years: plains droughts versus the solar cycle. Similar correlations of a variety of climate and weather phenomena with geospace phenomena have been reported, involving the interplanetary magnetic field, geomagnetic storms, aurorae, particle precipitation into the atmosphere, and more. Data from the OPEN Program can be used to test theories concerning these correlations and to assess the importance of including geospace indices in future weather forecasting operations.
THE OPEN PLAN
Earlier explorations of geospace have revealed that there are two plasma source regions — the solar wind and the ionosphere — and two storage reservoirs — the extended geomagnetic tail and the inner plasma sheet/particle trapping regions. These four basic geospace regions represent four major pieces of the geospace puzzle. They are interconnected by a network of transport processes that determine the behavior of the system as a whole.
These processes, determining the structure and dynamics of geospace, all involve a variety of plasma interactions in which the plasma components can be viewed both as tracers and as carriers of the energy flow through the system. This fact leads to a natural approach for studying the behavior of geospace as a whole — namely, the simultaneous observation of the major sources of plasma that feed the system, and the major regions in the system in which plasma and energy are often temporarily stored. Simultaneous observations of these regions allow the flow of plasmas in Earth's neighborhood to act as natural "dyes” to reveal the interconnections between source and storage regions, to identify energy transformation and dissipation processes, and to measure the energy output to the atmosphere.
The key to the OPEN Program is illustrated in the two diagrams on the right. These depict the relationship between the two plasma source regions and the two geospace storage regions. By placing a properly instrumented spacecraft laboratory in each of the four key regions, scientists can simultaneously observe the entry of plasma into the system, the storage and release of energy within the system, and the transfer of plasma and energy between those key regions as they change with time.
This minimum set of observations will provide experimenters, theorists and modelers with the basic information to assemble and analyze the puzzle for the first time.
THE TOOLS OF OPEN
The OPEN Program will use a set of strategically located spacecraft, together with one central data handling facility, that will provide spacecraft measurements and related ground-based data to the OPEN science team, and eventually, to other practical managers in government and industry who will benefit by access to instantaneous measurements of the energy input from geospace to the near-Earth environment.
The spaceflight segment of the Program will require that a minimum of four suitably instrumented spacecraft be placed where they can simultaneously observe each of the major geospace plasma source regions and the storage regions. The spacecraft and their functions are:
- The Interplanetary Physics Laboratory (IPL), which will measure the incoming solar wind, magnetic fields and particles.
- The Polar Plasma Laboratory (PPL), which will measure solar wind entry, ionospheric output, and the deposition of energy into the neutral atmosphere at high latitudes.
- The Equatorial Magnetosphere Laboratory (EML), which will measure solar wind entry at the sunward nose of the magnetosphere, and the transport and storage of plasma in the equatorial ring current and near-Earth plasma sheet.
- The Geomagnetic Tail Laboratory (GTL), which will measure solar wind entry and acceleration, transport, and storage of plasma in the geomagnetic tail.
The OPEN Program will have a flexible central data handling facility to which the scientific investigators will be coupled by remote computer terminals. All researchers, in fact, will be able to obtain a unified set of data from the four spacecraft in a timely fashion. The central facility will process the raw telemetered data, maintain archives of the processed data, and coordinate the analyses of various investigators.
The OPEN Program will be able to obtain much better science than other programs because of the new capacity to change orbits in space at will. The OPEN laboratories will be launched from the Space Shuttle and each of the OPEN spacecraft will include propulsion systems and ample fuel supplies for moves to different orbits.
The Interplanetary Physics Laboratory (IPL) will be launched into a trajectory that will carry it out of the magnetosphere into the upstream solar wind about 1.5 million kilometers sunward of the Earth toward a point at which Earth's gravitational pull is balanced by that of the Sun. On reaching this point, called a "stable libration point," or "L1" the onboard propulsion system will place the IPL into a small circular orbit around L1. In this "halo orbit,” the IPL will accompany the Earth around the Sun and will serve as an early warning station for measuring the solar wind plasma flowing past it before impacting the magnetosphere about an hour later.
Initially, the Polar Plasma Laboratory (PPL) will be placed in a very eccentric orbit over Earth's poles so that it will reach a maximum height of about 90,000 kilometers. In this orbit, plasma and field instruments can measure the entry of solar wind plasma into magnetospheric regions such as the dayside polar cusps; and optical and X—ray devices can obtain continuous global images of the auroral emissions stimulated by particle energy dumped into the upper atmosphere. The PPL’s onboard propulsion system will drop the orbit later to a maximum altitude of about 25,000 kilometers so that direct measurements of the transfer of plasma between the ionospheric source region and the equatorial storage regions can be made.
At first, the Equatorial Magnetosphere Laboratory (EML) will be placed in an elliptical orbit in the equatorial plane at altitudes ranging from about 6000 to 70,000 kilometers. In the course of one year in this orbit, the EML will provide complete measurements of the near—Earth equatorial storage region in both the daytime and nighttime hemispheres, and will permit detailed investigations to be made of solar wind entry at low latitudes of the magnetosphere. The EML will also use its own propulsion system to move into a deep magnetotail orbit later in the OPEN mission in order to permit more sophisticated studies of the geomagnetic tail storage region to be conducted jointly with the fourth spacecraft.
The Geomagnetic Tail Laboratory (GTL) will use a novel technique that involves occasional gravitational kicks from the Moon to keep its orbit in the long comet—like magnetospheric tail. By properly controlling the timing and approach distance of lunar swingbys, NASA will be able to vary the range of distances down the tail from near the Moon's orbital path (about 380,000 kilometers from Earth) to points as far as 1.5 million kilo- meters downstream. Thus, the GTL simultaneously will round out the exploration of geospace by providing the first complete survey of the magnetotail and will provide a key observing laboratory in the quantitative study of the whole geospace system.
TECHNOLOGY FOR OPEN
Two important factors combine to permit the OPEN Program to make a major step toward full understanding of the geospace environment.
First, the findings of earlier exploratory studies enable planners to define a sound, realistic strategy for undertaking such a global program as this one.
Second, the technology necessary for obtaining the needed measurements has just recently become available.
Advances in space plasma instrumentation over the past few years have led to the capability of measuring the properties of plasmas, different species of energetic charged particles, and electric and magnetic fields with the resolution and the range that are required for defining the complex processes involved in the behavior of geospace. Because many of the measurement techniques have been successively refined on earlier flight missions, their successful use on OPEN spacecraft can be anticipated with confidence.
Plans for global, auroral imaging from PPL are representative of the new capabilities to be used in the OPEN Program. An array of detectors that are sensitive to different wavelengths, including visible and ultraviolet light and X-rays, will provide frequent images of auroral formations over the entire polar cap of the Earth. Like the image formed on a television screen by the electron gun in a television set, the auroral emissions provide a direct image of the “footprint” of energy being deposited into the upper atmosphere by charged particles formerly stored in geospace. From its vantage point in a high polar orbit, the PPL will give scientists a nearly continuous view of the aurorae on a global scale, so that the location, area, and magnitude of particle energy input to the upper atmosphere can be measured and related to other plasma processes throughout geospace.
The OPEN Program will be a central element in the comprehensive study of solar—terrestrial relations being mounted in the decade of the 1980's.
Together with other programs now in preparation, it will be possible for the first time to look at the complete solar—terrestrial energy chain: first, the ejection and transport of energy and matter from the Sun through interplanetary space; then the coupling and transport of energy through Earth's magnetosphere; and finally, the deposition and dissipation of energy in Earth's upper atmosphere.
The chief complementary programs begin in the mid—1980’s. Two International Solar Polar Mission (ISPM) spacecraft will be obtaining the first stereoscopic view of the Sun from latitudes high above the ecliptic plane, and will be providing new insights into the three-dimensional structure of the interplanetary medium. Simultaneous measurements by 0PEN’s Interplanetary Physics Laboratory (IPL) will provide an in—ecliptic benchmark for out—of—ecliptic data from the ISPM. The latter can then be interpreted properly and related to the more familiar perspective of the Sun and the solar wind.
During the same time frame, the Upper Atmosphere Research Satellite (UARS) Program is planned in order to conduct a global study of the chemistry and dynamics of Earth's mesosphere and stratosphere (the altitude range extending from about 15 to 85 kilometers). Simultaneous measurements by the four OPEN spacecraft and by UARS will result in the first coordinated investigation of the coupling and transport of solar wind plasma and energy through the magnetosphere and of the consequences of that energy transfer on the upper atmosphere.
The OPEN spacecraft measurements of global behavior of geospace will also complement a number of investigations to be conducted from the Shuttle—Spacelab orbiting laboratory. Many active experiments, such as controlled chemical releases, electromagnetic wave injections, and particle beam accelerators, will benefit greatly from the diagnostic capability of the OPEN Program and the real—time monitoring of magnetospheric activity so that the Spacelab experiments can be conducted under known conditions.
The OPEN Program is the next logical thrust in geospace research.
Such an undertaking will be directly responsive to recent recommendations of the Space Science Board of the National Academy of Sciences.
A quantitative assessment of geospace inputs to the near-Earth environment is feasible and necessary.
The OPEN Program can develop the physical basis required for understanding and monitoring geospace effects on satellite communications, space radiation environment, terrestrial power systems, exploration geophysics systems, and, conceivably, some aspects of weather and climate.
The OPEN Program will be a key element in the overall solar-terrestrial program in the 1980's.
Together, the International Solar Polar Mission, the Upper Atmosphere Research Satellite, and the OPEN Program will permit a coherent study of the flow of matter and energy from the Sun through the heliosphere, the magnetosphere, and the upper atmosphere. OPEN will also complement simultaneous investigations of space plasmas near Earth with the Shuttle Spacelab and of space plasmas near Jupiter with the Galileo orbiter.
Knowledge about the fundamental properties of geospace will have direct, significant applications to other studies in astrophysics and laboratory plasma physics.
Plasma interactions are important throughout the solar system and the universe. Geospace uniquely provides an accessible example of a plasma environment in which significant processes can be studied directly.
The OPEN Program is defined scientifically and technically.
The necessary technological capacity is available, and the program is realistically planned for the decade of the 1980's.
- Space Plasma Physics: The Study of Solar System Plasmas, Space Science Board, A.G.W.Cameron, Chairman (S.A.Colgate, Study Chairman), National Academy of Sciences, Washington, D.C., 1978.
- Solar System Space Physics in the 1980's: A Research Strategy, Space Science Board, A.G.W.Cameron, Chairman (C.F.Kennel, Study Chairman), National Academy of Sciences, Washington, D.C., 1980.