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The Forefront of Space Science

Geospace Exploration Satellite - ERG Project
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What is Geospace?

Geospace is the region of outer space surrounding the earth, where dynamic interactions occur between the magnetic fields and the upper atmosphere of the earth with the plasma flow (solar wind) from the sun. Emerging from the earth’s surface into outer space, the environment changes from a neutral atmosphere to plasma (i.e., ionized gas). The region also has many names including thermosphere, ionosphere, and magnetosphere. Fig. 1 shows a conceptual illustration of Geospace. The well-known Aurora, for example, is the most attractive event arising in Geospace. Light emission from the Aurora is caused by electrons and ions from outer space entering the earth’s upper atmosphere.

Geospace is also the region where many satellites related to our lives operate. The meteorological satellite HIMAWARI and many communications satellites fly at an altitude of 36,000 km in geosynchronous orbit. The International Space Station (ISS) flies closer to the earth in the upper area of the magnetosphere at several hundred km altitude. Thus, Geospace is both a region utilized by mankind and also a familiar universe for our exploration.

Plasmas and particles with a wide range of energies are present in Geospace. These include: the “plasmasphereEwith “coldEplasmas of less than 1 electron volt (eV) (10,000 deg. C in temperature); the “plasma sheet/ring currentEwith an energy of several to several tens KeV that flows as a strong current in Geospace and causes the Aurora to shine; and the “radiation beltEwith the highest energy in Geospace of over MeV. Thus, Geospace is formed by plasmas and particles with various energy bands extending in a six-digit range.

Geospace and space weather forecast

Geospace is never quiet. When the violent solar wind caused by disturbances such as explosive events on the sun arrives at Geospace, a large fluctuation phenomenon called a Geospace storm occurs. During this event, a strong current runs across Geospace and intensive Aurora activities arise over the north and south poles.

Particles in the radiation belt exhibit a very mysterious behavior in a Geospace storm. When the storm occurs, electrons present in the radiation belt suddenly vanish. When the storm ends, they emerge again. Even more strangely, compared to before the storm, their number is often increased.

Why do high-energy electrons increase in a Geospace storm? Various possible mechanisms have been proposed but there is no still clear answer (see ISAS News, May 2006). One theory is “external supply.EIt presumes that the electrons are supplied to the radiation belt from the so-called plasma sheet shown in Fig. 1. Another interesting new idea is “internal acceleration.EIt presumes that plasma waves and electrons exchange their energies in the radiation belt to increase high-energy electrons in the belt. In this acceleration, all the plasmas and particles of the differing-energy hierarchy, such as ring current/plasma sheet and plasmasphere, are dynamically connected through the plasma waves and contribute to the electron increase in the radiation belt (“cross-energy couplingsE.

Fluctuation phenomena in Geospace such as Geospace storms are dangerous and can threaten our space activities. In the high-latitude regions, large-scale power outages occur unpredictably because electricity-supply facilities can be damaged by the strong electric current released in a Geospace storm. Furthermore, disturbance of the magnetosphere may affect signals from communications/broadcast and GPS satellites. High-energy particles from the radiation belt may not only cause problems and malfunctions in satellites but also have an impact on astronautsEhealth.

The phenomena in outer space closely related to our daily lives are called space weather. To understand the Geospace environment and forecast its changes is indispensable for our utilization of space. In particular, understanding and forecasting particle fluctuation in the radiation belt is considered to be the most important issue in space-weather research.

Figure 1
Figure 1. Conceptual illustration of the sun-earth system
The earth’s magnetosphere is formed by plasma flow (solar wind) that pours from the sun at supersonic velocity. The region to be explored by the ERG satellite is called the radiation belt. (Courtesy of K. Seki, Nagoya University)

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