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The Forefront of Space Science

Science of Space Weather
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The aurora is a luminous atmospheric phenomenon where electrons rushing to the earthís polar regions from outer space at extremely high-speed collide intensively with the earthís atmosphere. Aurora light shines mostly at an altitude of 100 km above ground. For the aurora to shine, electrons must be accelerated. Observations by satellites revealed that the voltage to accelerate electrons exists over aurora and electric power generation is made on a global scale. During a large aurora event, satellites flying above it start to malfunction caused by direct hits from aurora particles.

At the same time, plasma, which cannot become an aurora, receives vibration energy from electromagnetic waves and magnetic fields. It turns into very high-energy particles (radiation belt particles) and does significant damage to satellite devices and surfaces. Aurora electrons also emit strong X-rays, which cause a dangerous situation for astronauts too.

The changing space environment, or space weather, describes the changes in the space environment that cause damage to satellites, astronauts, communications, etc. Around the world, scientists are making efforts on ďspace weather forecastingĀEto predict the occurrence of solar or terrestrial storms.

Figure 2
Figure 2. Radiation particle environment in space

Scientific understanding of a space storm

One world organization is trying to forecast space weather. It is the International Space Environment Service (ISES) based in Boulder, Colorado, U.S.A. Over 10 years ago, I was transferred from the former ISAS to become manager of Space Environment Research Section at the former Communications Research Laboratory (CRL) of the Ministry of Posts and Telecommunications. In the new office, I started to establish space-weather forecasting in Japan. At that time, the CRL commenced full-scale research on solar activity in addition to research on the ionosphere, which had been addressed by its former organization, the Radio Research Laboratory. My brief was to add space-weather forecasting in the earthís magnetosphere to the conventional research.

Around the earth, a radiation belt known as the Van Allen belt exists (refer to the article in the Forefront of Space Science in ISAS news : ĀEa href="../../2006/miyoshi/index.shtml">Quest to Solve the Mystery of the Highest Energy Particles in Geospace,ĀEdated July 18, 2006). The farthest region of the radiation belt is the altitude of geostationary orbit. Solar flares emit coronal gas as well as light, radio waves and solar cosmic rays. The gas aggregation is very large and heavy. Its speed sometimes reaches up to 2,000 km/sec and it carries a huge amount of energy. In about two days, the coronal gas bound for the earth collides with the earthís magnetosphere to cause terrestrial storms known as magnetic storms. It was discovered that the Van Allen belt is greatly vitalized in the magnetic stormís recovery process. Solar wind changes, especially in the equatorial plane of the solar windís magnetic fields, play an important role in this vitalization. It becomes clear that, if the magnetic field direction is kept downward (i.e. southward), radiation particles (especially electrons) of the Van Allen belt increase.

Figure 3
Figure 3. Aurora image shot from space by the magnetosphere observation satellite AKEBONO

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