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The Forefront of Space Science

Science of Space Weather
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Solar storms

In October 2003, high energy particles (solar cosmic rays) emitted from the Sun caused instantaneous breakdowns and malfunctions on more than several tens of satellites. The accident was caused by the largest solar flare in history. Most satellites were returned to normal status after recovery work after the accident. Nonetheless, some measuring and experimental instruments failed, most notably observation equipment onboard the U.S.’s meteorological satellite.

The effects of high energy particles (solar cosmic rays) emitted from solar flares began to be seen in satellites in the mid 90s. Solar cosmic rays destroyed space devices, which are now extremely integrated and high-performance. Many satellites have been rendered irreparable.

Solar flares are large-scale energy-release phenomena (or explosive phenomena), which arise in the area from chromosphere to coronal region on the solar surface with their centers in grown sunspots. We can quickly detect the occurrence of solar flares because of the light or microwaves emitted. Within a few moments, solar cosmic rays rushes to satellites near the earth.

For safe space development, it is crucial to know when, where and to what degree the Sun reaches a dangerous state based on solar-physics results. Another concern is the impact of solar cosmic rays on astronauts. It is important to protect them from exposure to space radiation.

X-rays coming from the Sun have also a negative effect on the earth’s ionosphere. Specifically, they ionize the atmosphere located around 80km altimeter and, as a result, form a new ionosphere called the D-layer. Although this phenomenon is limited to daytime (dayside) sunlight, the D-layer greatly absorbs radio waves (especially, short wave) passing through it. In some albeit rare cases, radio wave signals from satellites do not reach the ground properly.

Space weather

Generally it is thought that outer space is a vacuum of nothingness. However, this is incorrect. It is, for example, earth’s upper atmosphere. At 500km above ground, a little higher than the orbit of the International Space Station, the atmosphere is 10-8 Pa (pascal). Rising to 1,000 km from the ground, the atmospheric pressure becomes 10-9Pa. At any higher altitude than that, the majority of substances change to plasma or ionized gas.

Plasma and the earth’s magnetic fields coexist in the outer space region around the earth. Various matter forms there and the energy interchange is very intensive. Energy is stored on the night side of the earth. In addition, in the region a little closer to the earth than the geostationary orbit, energy-storing areas completely encompass the earth.

Looking at the Sun, some coronal gases, which are observed clearly at solar eclipse, escape from solar gravity to stream out into the solar system’s interplanetary space. It is known as “solar windEand arrives at the earth in three days on average. The space around the earth is called “magnetosphereEand is surrounded by magnetic fields and plasma. The solar wind interacts with the magnetic fields and plasma in the magnetosphere and continuously inputs energy and momentum there. The magnetosphere constantly receives about the same amount of energy from solar wind as that consumed daily by a large country such as Japan.

Looking at weather phenomena on the earth, low-pressure areas or typhoons appear occasionally. This mechanism transfers energy stored at the equatorial region to the cold polar regions. Similarly, in the magnetosphe, excessively stored energy is released from time to time. One such release is the aurora storm. In addition, there are also occasional storms called “magnetic storms,Ewhere magnetic fields of the earth largely decrease.

Figure 1
Figure 1. Solar corona observed by HINODE satellite

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