Mission Investigates Radiation Arriving at Earth

NASA's Solar, Anomalous, and Magnetospheric Particle Explorer (SAMPEX) satellite, an international collaboration with Germany, is the first of a series of scientific research missions called "Small Explorers." In 1989, NASA chose three initial missions to research energetic radiations observed near Earth and radio frequency emissions given off by stars in early development. In addition to the pursuit of scientific research, the Small Explorer program emphasizes small spacecraft and rapid development of each mission - in 3 years or less [Baker et al., 1991].

The SAMPEX mission yields new information on the high velocity radiations arriving at Earth from the Sun and interstellar space. SAMPEX is named after the primary sources of the radiations: "Solar" ions and electrons, which are swept up by huge explosions in the solar atmosphere, "Anomalous" cosmic rays, which are believed to be interstellar gas atoms accelerated at the edge of the solar system, and "Magnetospheric" electrons, which plunge into the upper atmosphere and may influence its chemistry.

To study radiation, SAMPEX investigates ions - atoms stripped of one or more negative electrons - of such chemical elements as oxygen and iron and also the ions of electrons. An electron is the basic unit of negative electric charge.

These radiations come from different places in the solar system and the Galaxy, and they carry information about the site of their origination. Researchers want to know more about these radiation types since they can provide information about the Sun and its interaction with the Earth, about the local interstellar medium, as well as about the violent explosions in the galaxy called supernovae.

SAMPEX also measures galactic cosmic rays of very high velocity ions that are believed to come from supernovae or pulsars. The mission makes use of the Earth's magnetic field and new technology instrument designs of unprecedented resolution and sensitivity to study features of these radiations that were inaccessible to earlier measurements. SAMPEX has been described in detail by Baker et al. [1993].

Some remarkable discoveries have already been made by SAMPEX. For example, the source and fate of electrons with energies greater than one million electron volts (MeV) confined in the outer regions of the Earth's magnetosphere has long been a puzzle.

These electrons may be lost from the magnetosphere by moving along magnetic field lines and then penetrating into the Earth's atmosphere at mid- to high-latitudes. The interaction of these precipitating electrons with the atmosphere near 50- to 70-km altitude initiates ion-chemical processes that lead to the formation of oxides of nitrogen, compounds important to the balance of the Earth's global ozone.

Previous studies have shown that the variation of the high speed plasma flows - the so-called solar wind - emanating from the solar corona modulates the number of these energetic electrons within the magnetosphere. Over the course of the 11-year solar activity cycle, time-averaged electron radiation levels observed at geostationary orbit have varied by up to 800% from solar activity maximum to solar activity minimum conditions with peak electron levels occurring during solar minimum conditions. Such radiation variations have been used to estimate the magnitude of the number of electrons penetrating the atmosphere.

Results derived from an atmospheric model have suggested that large electron variations in the outer magnetosphere may lead to a change of global levels of middle-atmospheric odd nitrogen compounds and, through complex catalytic cycles, in global ozone [Callis et al., 1991].

The highly inclined (82°) polar orbit, the ability to acquire measurements at all longitudes, and the at least 3-year mission lifetime provide the SAMPEX mission with an opportunity to measure the magnetospheric electron radiation and electrons penetrating into the atmosphere (see Figure 1). Such measurements are establishing the extent to which this phenomenon provides a strong and continuing coupling between variations in the solar wind, modulations of the near-Earth space environment, and middle atmospheric processes.

Fig. 1. This image is derived from data from the Proton/Electron Telescope aboard the SAMPEX satellite. It shows, in the irregular gray-shaded collar in the Northern Hemisphere, where very energetic electrons penetrate the atmosphere and may form nitrogen compounds that influence global ozone levels.

In the realm of high-energy ion measurements, SAMPEX has also located a third radiation belt surrounding the Earth, one that traps material from the nearby interstellar medium. The newly discovered belt, which dips closest to Earth over the South Atlantic, is embedded in the lower Van Allen belt (see Figure 2).

Fig. 2. As indicated by the key on this illustration, a newly discovered radiation belt around the Earth is shown as a darkly cross-hatched crescent. The belt, where cosmic rays collect, is embedded within the inner of the two Van Allen radiation belts. The Van Allen belts are represented by the lighter-shaded crescents. The belt is most intense above a 5,000-mile (8,050-km) strip of Atlantic Ocean between the southern tips of South America and Africa. The radiation is intense along this strip because the Earth's magnetic field is not centered perfectly, and this is where it allows the trapped particles to get closest to the Earth's surface. This off-centeredness can be seen in the illustration: Note that the belts on the left appear slightly farther away than those on the right.

SAMPEX pinpointed the new belt, the existence of which was first predicted 15 years ago [Blake and Friesen, 1977], and it is now measuring its composition and monitoring its intensity variation. In 1991, a team of Russian and U.S. scientists detected this cloud of nuclei trapped in the Earth's magnetic field, but they were unable to fix its location.

The new belt consists of trapped heavy ions, including the nuclei of atoms of nitrogen, oxygen, and neon, which are part of the anomalous cosmic ray component [Cummings et al., 1993]. These gases help make up the thin cloud that fills the void between stars in the Milky Way. This material, called the interstellar medium, represents the debris from the birth and death of stars, as well as particles that have been drifting around since the creation of the universe about 15 b.y.a.

Interstellar gas, which is electrically neutral, can penetrate the heliosphere - the bubble of solar wind that envelopes the solar system. Some of these neutral interstellar atoms are ionized, which means that they lose a single electron, by solar UV radiation. They are then accelerated to cosmic ray energies at the solar wind termination shock. If one of these singly charged cosmic rays brushes the Earth's atmosphere and loses its remaining electrons, it may become trapped in the Earth's magnetic field. Once inside the new belt, these atoms may bounce back and forth between the Earth's magnetic poles for weeks before leaking out into space or into the atmosphere. As a result, the amount of matter inside the new belt increases and decreases. It doubled, for example, from August to November 1992.

The SAMPEX instrumentation is a collaborative effort between the University of Maryland, the Max-Planck-Institut for Extraterrestrial Physics (Garching), California Institute of Technology, Aerospace Corporation, Goddard Space Flight Center, and the Langley Research Center; the spacecraft was developed at Goddard. SAMPEX demonstrates that small, relatively inexpensive satellites can return large scientific dividends. It is expected to return data for much of the current solar cycle before re-entering the atmosphere in 1997 or 1998.

References

Cummings, J. R., A. C. Cummings, R. A. Mewaldt, R. S. Selesnick, E. C. Stone, and T. T. von Rosenvinge, New evidence for geomagnetically trapped anomalous cosmic rays, Geophys. Res. Lett., in press, 1993.

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