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The Forefront of Space Science

Particle acceleration in space and Jupiterís magnetosphere
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The evidence for the transport of hot electrons into the inner area of the magnetosphere was obtained

As a result of applying the spectral diagnostic for the extreme ultraviolet data of the Io plasma torus that "Hisaki" obtained, the fact was discovered for the first time in the world that hot electrons that can be considered to come from outer magnetosphere, even reducing its density gradually, enter into the inner area of the magnetosphere. (Yoshioka et al. 2014, Science)

Magnetic field lines and the plasma near the Io plasma torus rotate along with the high-speed rotation of Jupiter, and there is strong centrifugal force (force in the direction away from Jupiter) here. On the other hand, plasma in the torus is very dense but the density drop gradually as the distance from Jupiter increases. This means that there is heavier gas as goes into inside even though outward centrifugal force acts the plasma that constitutes the torus. This is similar to the situation where heavy fluid is riding on top of the lighter fluid under the influence of gravity. In such a state, a complex convection phenomenon (instability) is caused under the situation where light things move to the above, and heavy things move to the lower. Based on this concept, it can be expected that instability that centrifugal force is driven (instead of gravity) causes the plasma flow between Io plasma torus and the outside area of it. When we took a high-speed transport driven by such convection into account, hot electron distribution derived from the data of "Hisaki" made sense very well. In other words, this result suggested that large convective (instability) enough to break the strong magnetic field barrier is induced because there is high-density Io plasma torus. Hot electrons enter into the inner area of the magnetosphere in this way would cause a phenomenon of electromagnetic wave motion to accelerate electron more efficiently. And consequently, the strongest Jupiter radiation belts in the solar system would be formed and maintained.

Future Jupiter exploration

Currently, two large Jupiter exploration missions, JUNO and JUICE are in progress in the world.

JUNO is a NASA spacecraft that is underway and aims to be put to Jupiter orbit the summer of 2016. At the time of closest approach, it will approach Jupiter surface up to 4300km (2/3 or less of Jupiter radius). On the other hand, the apojove distance is 25 times that of Jupiter radius, and its orbit is a polar orbit of the ellipse. Therefore, it is expected that detailed information on the Jupiter surface and a state of high latitude regions are revealed. In addition, it would surely bring an important knowledge for understanding interaction between the magnetosphere and atmosphere and the causality between external factors such as the solar wind and activities in the magnetosphere.

JUICE is the Jupiter system exploration program led by the European (ESA) initiative that is expected to be launched in 2022. The main purpose is to explore the world of ice satellites like Europa, Ganymede and Callisto while being conscious of the possibility of life living in underground sea. Exploration from the plasma-physical point of view is also very interesting because these satellites are in Jupiterís magnetosphere, and also Ganymede have an own magnetosphere, which means having its own magnetic field in Jupiterís magnetosphere.

In the beginning of such era that Jupiter exploration is likely to be enhanced, it could be meaningful that "Hisaki" has contributed to develop a better understanding of Jupiterís magnetosphere physics.

(Kazuo Yoshioka)

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