To See the Invisible

A half-ton satellite carrying some of the most sophisticated imaging instruments ever flown in near-Earth space will soon help researchers predict "space storms," which can disrupt power grids and communications worldwide.

by James L. Burch, Ph.D.

The Imager for Magnetopause-to-Aurora Global Exploration (IMAGE) mission is the first in NASA's MIDEX (Mid-size Explorer) program. It is also the first satellite dedicated to imaging the Earth's magnetosphere, the region of space controlled by the Earth's magnetic field, which contains extremely tenuous plasmas of both solar and terrestrial origin. These invisible populations of ions and electrons have traditionally been studied by making localized measurements with charged particle detectors, magnetometers, and electric field instruments. Instead of taking such in situ measurements, IMAGE will use a variety of imaging techniques to "see the invisible" and to produce the first comprehensive global images of plasma in the inner magnetosphere. With these images, space scientists will be able to observe, in a way never before possible, the large-scale dynamics of the magnetosphere and the interactions among its plasma populations. The Instrumentation and Space Research Division at Southwest Research Institute (SwRI) manages the IMAGE program and leads the IMAGE science investigation.

Dr. James L. Burch, IMAGE principal investigator, is vice president of the SwRI Instrumentation and Space Research Division. In addition to directing a variety of division activities, he has served as principal or co-investigator on more than 10 space science missions, including Cassini, Rosetta, ATLAS-1, and Dynamics Explorer 1.

Solar interactions

The sun interacts with the Earth to create a dynamic magnetosphere, which envelops the atmosphere and shields humankind from interplanetary space. As the technological age has advanced, the importance of the magnetosphere as a medium that must be monitored and predicted has rapidly become apparent. For not only does the magnetosphere protect the Earth's upper atmosphere from exposure to the supersonic flow of charged particles from the sun, known as the solar wind, but it is also the site of disturbances -- geomagnetic storms and magnetospheric substorms - that can adversely affect human activities. For example, such disturbances generate strong electrical currents, overhead in the ionosphere and below the Earth's surface as well, that can disrupt communications and power systems. Electric fields in these disturbances accelerate magnetospheric ions and electrons to extremely high energies, and these high-energy particles can damage satellites and other electronic systems in space. These particles also pose potentially serious health risks to astronauts because of high radiation doses.

The sun contributes to the formation of the Earth's magnetosphere in two important ways.

First, the solar wind applies mechanical pressure to the geomagnetic field, compressing it on the dayside and stretching it into a tail many hundreds of Earth radii long on the nightside. While only a small fraction of the solar wind plasma enters the magnetosphere, a larger fraction of the solar wind electric field is admitted, driving the large-scale flow of the plasma within the magnetosphere.

Second, solar X-rays and ultraviolet radiation strip electrons off atoms in the upper atmosphere, creating the partially ionized region of the atmosphere known as the ionosphere. Like all plasmas, the ionosphere is electrically conducting and thus strongly influenced by electromagnetic forces. Accelerated by electric fields and guided by the Earth's magnetic field, ions and electrons flow continuously upward from the high-latitude ionosphere into the magnetosphere. This outflow -- of which IMAGE is expected to provide the first-ever images -- is an important source of magnetospheric plasma and an important loss of oxygen in the atmosphere.

The Earth's aurora is produced by magnetospheric electrons and protons spiraling downward along magnetic field lines and colliding with oxygen and nitrogen atoms in the upper atmosphere above the polar regions. Especially dramatic auroral displays occur when the near-Earth space environment is in a highly disturbed state as a result of the increased transfer of energy from the solar wind into the magnetosphere. While auroral emissions excited by precipitating electrons have frequently been imaged, both from the ground and from space, the proton aurora -- illustrated by this simulated image -- will be made visible for the first time from space with the IMAGE far ultraviolet imager.

The many spacecraft that have explored the magnetosphere since the beginning of the space age have identified its average configuration and its "cellular" organization into plasmas, energetic particles, and even voids. At the boundaries between such cell-like regions, strong shear flows of plasma and changes in plasma pressure create current systems that link the outer magnetosphere with the upper atmosphere. This coupling waxes and wanes with the occurrence of geomagnetic storms and the more impulsive and shorter-lived magnetospheric substorms. The most dramatic and familiar manifestations of these disturbances are the aurora borealis and aurora australis -- the Northern and Southern Lights. These polar lights are like images on a television screen upon which is projected the global distribution of magnetospheric charged particles and currents.

In situ measurements over the past 35 years have yielded a wealth of statistical information about the magnetosphere and its constituent plasmas. They also have provided many examples of the dynamic changes of magnetospheric plasma parameters at specific times and places in response to changes in the solar wind input and to internal disturbances related to substorms. Statistical global averages and individual events, however, aren't enough to understand the dynamics and interconnection of this highly structured system. Fundamental questions remain unanswered concerning plasma entry into the magnetosphere, global plasma circulation and energization, and the global response of the magnetospheric system to internal and external forcing. These processes occur in minutes to hours, yet currently available statistical averages are on time scales of months to years. Understanding the physical processes that affect the magnetospheric plasma requires the nearly instantaneous measurement of its structure, which can only be obtained by magnetospheric imaging.

The imaging perspective

To obtain useful images of the Earth's magnetosphere, a trade-off has to be made between obtaining a sufficiently wide field of view and achieving good resolution and signal-to-noise ratio. The most favorable observing location is one that is high above the North Pole but still within the magnetosphere.

The IMAGE satellite will be launched into an elliptical polar orbit. Its altitude at apogee, the farthest point from the Earth, will be 7 Earth radii (44,646 kilometers), while at perigee, the point closest to the Earth, the spacecraft will pass over the Southern Hemisphere at an altitude of 1,000 kilometers. The launch window, scheduled for the 1999-2000 winter season, is set by the need to maximize the amount of sunlight received by the spacecraft's solar array to allow all the instruments to operate continuously throughout the two-year mission.

Imaging the parts

To image a region as large and complex as the magnetosphere, which is invisible to traditional astronomical techniques, it is necessary to image its component parts. The components that can be imaged are the total plasma density; the density of low-energy helium ions; and the density, velocity, and mass of energetic atoms. Ultraviolet emissions produced in the aurora by electrons impacting the atmosphere can also be imaged, as can those ultraviolet emissions made by energetic protons that pick up electrons from the upper atmosphere as they precipitate into it.

The total plasma density can be remotely sensed by a radar that transmits radio waves with selected frequencies from regions of relatively low plasma density. The waves will propagate until they reach a region of higher density with a natural resonant frequency equal to the wave frequency, at which point it will reflect. This is the same type of reflection that is responsible for the well-known ionospheric skipping of radio waves, which allows them to propagate beyond the horizon. By receiving the reflected waves and measuring the time delay, the directions and distances to regions with particular plasma densities can be mapped out. This technique is referred to as radio plasma imaging (RPI) and will be used on IMAGE to map large portions of the magnetosphere, from its inner boundary in the upper atmosphere to the magnetopause, its outer boundary with the solar wind. This technique is used routinely to sound the Earth's ionosphere from the ground, and has been used from low-orbiting spacecraft to sound the upper part of the ionosphere from above. It has not been used to sound the magnetosphere from Earth orbit.

Another technique that IMAGE will employ is extreme ultraviolet (EUV) imaging. This technique makes use of the fact that helium ions on the sun emit ultraviolet light at a wavelength of 30.4 nanometers. This light is absorbed and then rapidly re-emitted by helium ions in the magnetosphere. The greatest concentration of helium ions is in the plasmasphere, which is the high-altitude extension of the ionosphere and consists of "cold," or low-energy, plasma. Most of the ions in the plasmasphere are hydrogen, but hydrogen ions produce no photon emissions. On the other hand, there is a nearly constant fraction (about 15 percent) of helium in the plasmasphere that can be used as a tracer for the region. By imaging the helium ions from above the plasmasphere, IMAGE will be able to track the erosion of the outer regions of the plasmasphere during magnetic storms and observe its refilling with ionospheric plasma during quiet times.

This extreme ultraviolet (EUV) helium ion image simulates one that IMAGE will take once in orbit. It shows a portion of the Earth's plasmasphere observed from above the North Pole. The Earth is at center, and the sun is lower right. The "bite" in the upper left is on the Earth's night side, where the EUV input from the sun is blocked.

Direct measurements of the ions from spacecraft have led to intriguing, but unproven, conclusions that long plasma tails or detached plasma islands are produced during magnetic storms and driven out through the dayside boundary of the magnetosphere by high-altitude winds. The helium ion images from IMAGE will allow scientists to test these theories immediately. Although the helium ion imaging has not yet been performed in the magnetosphere, researchers know this technique will work because the same emissions that IMAGE will use to study the erosion and recovery of the plasmasphere have been a serious source of "noise" for ultraviolet astronomy observations from low-altitude spacecraft. In this case, one person's noise is another's signal.

Different populations of energetic ions have been identified in the magnetosphere. Because the ions are trapped by the Earth's magnetic field, they can be imaged on a global scale from remote locations only if they are converted to neutral atoms and released from their magnetic confinement to travel along line-of-sight paths to a remote imager. This conversion occurs by means of a process known as charge exchange. In this process, a few percent of the energetic ions in a particular population pick up electrons from the neutral hydrogen atoms that occupy the outer reaches of the Earth's atmosphere. Charge exchange preserves the parent ion's direction, speed, and mass so that suitable energetic neutral atom (ENA) cameras can obtain detailed information about the magnetospheric ion populations.

The IMAGE polar orbit maximizes observation time over the North Pole during the spacecraft's two-year mission. The orbital period is 13.5 hours.

A tricky part of neutral atom imaging is the translation of the neutral atom images into images of the parent ion populations. The generation of ion images from neutral atom images requires knowledge of the hydrogen atom density of the upper atmosphere, which will be provided by one of IMAGE's far-ultraviolet (FUV) instruments, and the application of complex image processing techniques. The regions being imaged are "optically thin," or transparent. For such a transparent medium, the signal obtained is the integrated signal along the line of sight through the imaged region.

ENA cameras have been used in interplanetary space to detect interstellar neutral atoms; one onboard the Cassini spacecraft is on its way to Saturn to image that planet's magnetosphere, and others have flown on low-altitude Earth orbiters. However, IMAGE will be the first spacecraft to carry ENA cameras to high altitudes in the Earth's magnetosphere, providing the needed perspective for global magnetospheric imaging. Here again, researchers know the technique will work because two different ion detectors on spacecraft in high earth orbit have inadvertently obtained images of neutral atoms by detecting the neutral atom background noise when the spacecraft were in locations with ion fluxes too low to be observed.

Finally, IMAGE will carry two instruments to detect the ultraviolet auroral emissions produced by electrons and protons as they precipitate into the Earth's upper atmosphere. The auroral images will reveal the state of the magnetosphere, thus providing a context for the interpretation of the ENA, EUV, and RPI imaging of various plasma regions and boundaries as they change in response to varying magnetospheric conditions.

The payload and observatory

The IMAGE payload is made up of seven camera systems, the radio plasma imager, and a central computer that provides communications between the payload and the spacecraft system.

In April, the IMAGE payload was transported from SwRI to the Lockheed Martin Missiles and Space facility in Sunnyvale, California, for integration into the IMAGE spacecraft. It was then tested under vibration conditions experienced during launch and under temperature extremes encountered in the high vacuum of space.

Launch and mission operations

In January 2000, IMAGE will be transported to the Western Range launch facility, located at Vandenberg Air Force Base, California. There, the spacecraft will be mated with the third-stage solid-rocket booster. This combination will then be mounted atop a two-stage, liquid-fueled Delta II launch vehicle, one of the most reliable available. Launch is scheduled for February 2000.

In orbit, IMAGE will take about one month to deploy the RPI antennas and activate the instruments. After science operations begin, the instruments will be operated with a 100-percent duty cycle. Except for the RPI, the time resolution of the instruments will be set by the two-minute spin period of the spacecraft. The RPI will have a time resolution of one minute. A minimum of mode changes is anticipated, and spacecraft command uploads will normally be limited to once per week.

Data will be downlinked to the NASA Deep Space Network once per orbit (every 13.5 hours). For space forecasting purposes, the full IMAGE data rate of 44 kilobits per second will be broadcast in real time. Although acquisition of the real-time data is not part of the IMAGE mission, the data can be received by any suitable antenna. Plans are in place for the Communications Research Center in Japan and possibly other groups to receive and process the real-time data.

An important aspect of the IMAGE program is its completely open data set. Within 24 hours of data acquisition by the Science and Mission Operations Center at the NASA Goddard Space Flight Center, data will be available on the Internet. The data will include an orbital plot and images from each instrument on a two-minute time scale, which can be viewed as a movie over any selected time period. In addition to these images, the complete set of science data will be held on-line at the National Space Science Data Center at Goddard. There, a full set of data processing and analysis software will be available for download along with the data. In this way, the space science community will have the same access to the data as the IMAGE science team.

Conclusion

IMAGE is the first spacecraft dedicated to imaging the Earth's magnetosphere. All known magnetospheric imaging techniques are included in the mission, including neutral atom imaging, ultraviolet imaging, and radio plasma imaging.

To date, the regions that IMAGE will study have been investigated largely on the basis of measurements made by spacecraft at single, isolated points in space. As expected, the knowledge derived from such measurements is partial and fragmented. IMAGE will provide the missing global context -- the "big picture" -- that will allow space researchers to study the Earth's magnetosphere as a coherent global system of interacting components.

The IMAGE mission coincides with solar maximum, a period of the most intense solar activity, during which the Earth will be continually buffeted by explosive eruptions of plasma from the sun. The images of the Earth's inner magnetosphere that will be captured by the instruments onboard the IMAGE spacecraft are thus expected to be dramatic ones indeed.

Team IMAGE

A large team of scientists has been working on the development of instrumentation, data analysis software, numerical modeling, image inversion techniques, and education and public outreach over the past several years. The team consists of 24 co-investigators, who generally include the instrument leads and representatives from the major contributing institutions. In addition, a similar number of participating scientists have also been heavily involved in the development of the IMAGE satellite. The instruments (and their lead development institutions) are:

The high energy neutral atom (HENA) imager images neutral atoms at energies from 10 keV to 500 keV. (Johns Hopkins University Applied Physics Laboratory)

The medium energy neutral atom (MENA) imager takes images of neutral atoms at energies from 1 keV to 30 keV. (Southwest Research Institute)

The low energy neutral atom (LENA) imager images neutral atoms at energies from 10 eV to 500 eV. (NASA Goddard Space Flight Center)

The extreme ultraviolet (EUV) imager takes ultraviolet images of 30.4 nm emissions from helium ions. (University of Arizona)

The far ultraviolet (FUV) imager images the aurora produced by the electron bombardment of oxygen (135.6 nm) and nitrogen (140-190 nm) and by energetic protons (121.8 nm). The instrument also images the exosphere (121.6 nm). (University of California at Berkeley)

The radio plasma imager (RPI) performs radio sounding of plasmas with densities from 0.1 cm-3 to 105 cm-3. (University of Massachusetts at Lowell)

The central instrument data processor (CIDP) performs payload command and data handling. It is the interface between the payload and the spacecraft. (Southwest Research Institute)

Comments about this article? Contact Burch at (210) 522-2526 or jburch@swri.org. Additional information is also available on the Internet at pluto.space.swri.edu/IMAGE/.

Published in the Fall 1999 issue of Technology Today®, published by Southwest Research Institute. For more information, contact Maria Martinez. Technology Today®, published by Southwest Research Institute. For more information, contact Maria Martinez.

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