An Overview of the Space Radiation Environment

An Overview of the Space Radiation Environment

J. E. Mazur

Space systems operate in an environment whose effects and descriptions are unusual compared with the weather in Earth's atmosphere. Engineering new systems to survive and perform in space is still a challenge after more than 40 years of spaceflight.

Space systems operate in conditions that are much different from terrestrial weather. The space environment, just as any environment on Earth, contains phenomena that are potentially hazardous to humans and technological systems; however, many of these hazards involve plasmas and higher-energy electrons and ions that are relatively uncommon within Earth's atmosphere (see sidebar, Space Environment Impacts). The description of the space environment requires new terminology for both the hazards and the places they occur. These hazards exist in broad spatial regions that change with time. Typical satellite orbits cross many of these regions and spend a variable amount of time in each.

The space environment is populated with electrons and ionized atoms (ions). The unit of kinetic energy for these particles is the electron volt. At high energies (millions of electron volts), these particles have sufficient energy to ionize atoms in materials through which they propagate. At lower energies (below thousands of electron volts) their effects range from charge accumulation on surfaces to material degradation.

The interaction of space particles with spacecraft materials and electronics is complex to describe and difficult to simulate with ground-based test facilities. It is also not possible to fully specify the space radiation environment for a given mission because of unknowns in mapping it and unknowns in the processes that generate it. The space environment also changes with time, often in unpredictable and undiscovered ways, making it a challenge to completely assess the hazards in any orbit.

Interplanetary Space

The sun and most planets in the solar system generate magnetic fields. The space outside the local effects of planetary magnetic fields contains its own population of particles. Several satellites near Earth continuously monitor the intensity of the particles and electromagnetic fields in interplanetary space. These and other space probes have shown that the radiation environment in the solar system is highly variable, but the consistent locations of intense radiation are the planetary magnetospheres.

Earth's magnetosphere is a teardrop-shaped cavity formed by the interaction of the solar wind (gold) with Earth's magnetic field. The solar wind becomes subsonic at the bow shock (blue). The magnetosphere contains the Van Allen radiation belts (orange) and other particle populations. (NASA)

The space between the planets is not a vacuum, but at about 10 particles per cubic centimeter, the particle density is many orders of magnitude below typical densities of materials found on Earth. However, what counts for radiation effects is not only the particle density, but also how the energy is distributed among the particles. By combining measurements from a large number of space particle instruments as well as ground-based detectors, researchers have shown a tremendous range in both particle intensity and energy, with fewer and fewer particles at higher and higher energies.

Solar Wind

Most of the particles in interplanetary space are in the form of a hot, ionized gas called the solar wind; it flows radially from the sun with a speed at Earth that varies from about 300 to 1000 kilometers per second, representing a mass loss of about 10¹⁴ kilograms per day. The mechanism that heats the upper solar atmosphere to roughly 1 million degrees is intimately linked to the creation of the solar wind. The heating mechanism is unknown, but may originate in constantly reorganizing magnetic fields. X-ray images of the solar atmosphere at low altitudes show regions of varying intensity. The brightest and hottest regions, with temperatures at several million degrees, lie above sunspots. The darker areas are coronal holes—large, cooler volumes of the atmosphere filled with magnetic field lines that extend into interplanetary space. In the coronal holes, the solar wind travels about twice as fast as it does from regions on the sun with magnetic fields that loop back to the surface. Coronal holes can last many solar rotations and will be the dominant feature in the solar atmosphere from 2003 to 2005, when the sun approaches its activity minimum.

Explosive ejections of large volumes of the solar atmosphere, known as coronal mass ejections, draw out complex loops of magnetic field into interplanetary space. The magnetic field's direction and strength determine how energy from the solar wind gets transferred into the planetary magnetospheres.

Solar Energetic Particles

Many highly variable sources produce interplanetary particles with energies typically between 10 thousand and 100 million electron volts. These energetic particles originate in acceleration processes in the solar atmosphere, sometimes close to the sun and sometimes beyond Earth's orbit. The transient nature of these particle populations is directly linked to the sun's activity.

An increase in solar energetic particles is only one manifestation of a complex sequence of events that begins with a large energy release at the sun. While these energy releases are generally called "proton events," and it is true that protons are the most abundant ion produced, these events also energize ions as heavy as iron. Both the protons and the heavy ions are hazardous to spacecraft: The more abundant protons are primarily responsible for anomalies resulting from the total radiation dose, while heavy ions contribute most to anomalies known as single-event effects.

Diagram of Earth's Van Allen radiation belts.

Galactic Cosmic Rays

Galactic cosmic rays are the highest-energy particles in the solar system—even Earth's magnetic field is usually not sufficient to deflect them. They originate somewhere outside the solar system (possibly in supernova shocks) and probably represent the accumulated output of many particle sources and acceleration processes. Always present at Earth, they consist of about 87 percent protons, 12 percent helium nuclei, and 1 percent heavier ions.

During several years around solar maximum, the sun is more likely to eject disturbances into interplanetary space. As these disturbances propagate, they carry tangled magnetic fields that scatter the lowest-energy galactic particles. Hence, the galactic particle intensity at Earth varies inversely with the solar cycle (it also varies with radial distance from the sun and latitude above the ecliptic plane, although these effects are small compared to the solar cycle variations). Because of the solar cycle, one might even consider a long-duration mission to Mars at solar maximum rather than at solar minimum because the galactic radiation—which is impossible to shield against—is at lower levels during solar maximum.

Earth's Magnetosphere

Earth's magnetic field establishes a volume of space within which the magnetic field dominates charged particle motion. Close to Earth, the magnetic field is roughly a magnetic dipole that is tilted 11.5 degrees from the rotational axis and offset from the center of the planet. For most purposes, the dipole approximation is poor, and there are more sophisticated models that account for the steady changes of the central field as well as the dynamic outer boundaries.

The magnetosphere is complex and dynamic because of its interaction with the variable solar wind and transient phenomena from the sun. On the sunward side, the magnetosphere extends about 10 Earth radii (roughly 60,000 kilometers). On the opposite side, the magnetotail extends beyond 200 Earth radii. The sunward dimension can change by more than a factor of two depending on the interplanetary magnetic field and solar wind upstream from Earth.

The magnetosphere contains a mixture of plasmas with incredibly diverse sources. Some populations of charged particles are trapped within the magnetosphere while others vary on many time scales. The magnetosphere has its own weather, with complex processes of particle transport and acceleration during geomagnetic storms that contribute to surface charging and internal charging of spacecraft.

A charged particle in a constant magnetic field experiences a force perpendicular to its motion. The resulting trajectories of ions and electrons in the magnetosphere are a complex superposition of motions as each particle travels in a spiral around a magnetic field line, bounces back and forth between the North and South Poles, and drifts around the planet, with electrons drifting east and protons drifting west.

Influenced by Earth's magnetic field, charged particles engage in a complex dance of motions as each one spirals around a magnetic field line, bounces back and forth between the hemispheres, and drifts around the planet—electrons to the east, and protons to the west.

Stable trapping of particles occurs, given the right combination of particle charge, energy, and magnetic field strength. As these particles are trapped on time scales ranging from days to years, they execute their gyration, bounce, and drift motions around Earth, resulting in spatial zones of trapped radiation known as the Van Allen belts. The inner zone is the proton belt (peak intensity at about 3000 kilometers from Earth's surface) and the outer zone the electron belt (peak intensity from about 12,000 to 22,000 kilometers from the surface).

There are trapped electrons and protons throughout the magnetosphere, but the division into two zones is reasonable because the radiation dose from trapped particles is usually highest in these regions. Also, the particles that contribute most to the radiation dose in the inner zone are protons and those contributing most in the outer zone are electrons. Occasionally, new radiation belts form between the inner and outer zones when interplanetary shock waves from coronal mass ejections hit the magnetosphere.

Different processes produce and sustain the proton and electron belts. Galactic cosmic rays collide with atoms in Earth's atmosphere and produce showers of secondary products. Some of these products are neutrons that subsequently decay into energetic protons; thus, cosmic rays are the most important source of energetic particles in the inner zone. The telltale clue for the decay source is the dominance of protons over other types of ions. Another clue is the relative stability of the inner zone, which results from a combination of long particle lifetimes in this part of the magnetic field and the slowly varying cosmic ray input.

The Van Allen radiation belts and typical satellite orbits. Key: GEO—geosynchronous orbit; HEO—highly elliptical orbit; MEO—medium Earth orbit; LEO—low Earth orbit. (Illustration by B. Jones, P. Fuqua, J. Barrie, The Aerospace Corporation. View larger image)

The offset of Earth's magnetic dipole from the geometric center of the planet causes a weaker field region over the South Atlantic Ocean and an opposing region of stronger field over northern Asia. As the trapped inner-zone particles execute their bounce motion along field lines, they can reach lower altitudes at a region known as the South Atlantic Anomaly. All spacecraft in low Earth orbit penetrate the inner zone in the South Atlantic Anomaly even if their altitude is below the belt at other positions in the orbit.

While relative stability is one key property of the inner zone, variability is the outstanding characteristic of the outer radiation belt. The solar wind and interplanetary magnetic field affect this weaker field region of the magnetosphere more than the inner zone, leading to shorter lifetimes of trapped particles and more dynamics. Details of how the magnetosphere accelerates electrons to millions of electron volts in a few seconds have been recently glimpsed; however, the mechanism that accelerates the electrons more routinely in geomagnetic storms has not been established even after 40 years of research. Observations over many years with well understood space environment instruments will be needed before researchers can understand the outer zone's variability and its extreme behavior.

Examples of Current Research

Several factors continually press the need for a better specification and understanding of the space environment. One is the increase in spacecraft lifetimes, leading to questions about longer-term exposures than have been tested in the past. Another is the growing interest in uncommon orbits, where the residence time in different hazard areas is unlike what has been experienced. A third is the use of new materials, which need to be assessed for suitability in space. Aerospace has been conducting research to address the concerns raised by these and other issues (see table, Space Environment Hazards).

Measured radiation dose versus distance from Earth reveals the inner and outer radiation belts. (Air Force Research Laboratory)

Plasma Effects on Surfaces

Space plasmas can change the physical properties of exposed surfaces. For example, optical coatings are used to increase the efficiency of solar arrays; their performance depends in part on their transmittance, which can change after a long exposure to the space plasma environment.

Aerospace is beginning to derive preliminary specifications of the low-energy plasmas around Earth based on data from previous and active science missions. As is often the case, instruments designed in the past were not optimized to answer new questions and suffer from a lack of sensitivity and coverage in the orbits of interest. These new questions pose a challenge as researchers try to quantify and understand a relatively unexplored regime of space particles.

Extreme Value Analysis

The highest intensity of outer-belt electrons in the past 16 years occurred in a geomagnetic storm on March 28, 1991. One question important for space systems design is whether a similar or more intense event will occur during a future mission.

Aerospace has used mathematical tools known as the statistics of extreme events to help answer this question. The analysis indicates that the March 1991 event was equivalent to a 20-year storm, so the likelihood is high that a storm of that intensity and duration could take place in the next few years. In fact, the period from about 2003 to 2005 will have intense outer-belt events because high-speed solar winds usually occur during the upcoming phase of solar activity. The analysis also suggests that a 100-year storm could be about twice as intense. This mathematical approach does not predict when such events will occur, but it has potential to specify the extreme environment, thereby satisfying an important engineering requirement.

Typical variability of outer-zone electron intensity measured from low Earth orbit on the NASA/SAMPEX satellite, shown for the entire year 1997. The intensity is the logarithm of the electron count rate.

Future Needs

Just as every terrestrial flood or hurricane is different, so too are the events in the radiation environments of Earth and interplanetary space. Averaging the rainfall in southern Florida can reveal long-term weather trends, but could never describe the effects of a single hurricane. Similarly, a multiyear average of the intensities in Earth's electron radiation belt reproduces the average environment appropriate for a total-dose estimate, but could never describe a single geomagnetic storm. Longer-duration missions with more capable instrumentation, augmented with more precise theories of space environment phenomena, will help designers specify the environment better and characterize its extreme events more accurately as well.

Space systems must meet their performance requirements regardless of the space weather, so the specifications that affect the engineering on the ground are crucial to their success. This is especially true as mission planners explore the use of different orbits, new materials and technologies, and longer satellite lifetimes. Thus, more support is needed for the development of new space environment specifications and models based on modern and more comprehensive data sets.

Current missions are expanding databases of measurements of trapped radiation, Earth plasmas, solar energetic particles, and galactic cosmic rays. The combination of better data and theories will yield better models, but the models will only be useful to the engineering of space systems if their focus from the start is on their application to actual missions.

Acknowledgements

The author would like to thank P. C. Anderson, J. B. Blake, M. W. Chen, J. F. Fennell, H. C. Koons, M. D. Looper, T. P. O'Brien, J. L. Roeder, and R. S. Selesnick for their contributions to this overview.