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The Forefront of Space Science

Particle acceleration in space and Jupiterís magnetosphere
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Dilute gas filling the space is affected by the electromagnetic field because it is composed of plasma consisting of ions and electrons. On the contrary, the movement of the plasma will vary the electromagnetic field. Furthermore, in the dilute state, there are no frequent collisions between particles. Therefore, part of the small number of particles is able to earn a very high energy. In fact, the abundance of high-energy particles (cosmic rays) that fill the space is one of the basic amount that defines the space environment, and it is believed that they are accelerated in the shock wave that is caused with the supernova explosion that star causes at the end of life. In addition, the corona atmosphere of the sun also varies the space environment around the planet by particles accelerated in the explosion phenomenon flying. Thus, understanding the mechanism of particle acceleration in space is one of the important issues in space physics.

Our solar system is filled with plasma ejected from the sun (the solar wind). On the other hand, Earth and Jupiter have a magnetic field like a giant magnet. These magnetic field lines (blue line in Figure.1) work as obstacles to the solar wind because they capture the plasma and constrain its movement. The collision between the solar wind and the planetary magnetic field caused by this phenomenon creates a region of "magnetosphere" around the planet. The magnetosphere is filled with magnetic field lines of a planet that has been deformed by the solar wind plasma and plasma that has originated in the solar wind and planetary atmosphere. Jupiter's magnetosphere that is the stage of this research has three major characteristics; very strong magnetic field; rotation of the magnetic field lines with high-speed rotation of the 10-hour cycle; gas ejected from the satellite Io volcanoes and floating around the peripheral Jupiter as high-density plasma.

The magnetic field line nearby Jupiter has the same shape close to a dipole as the bar magnet and is not deformed to a large extent. This area is called the inner magnetosphere (Figure.1 A). On the other hand, the magnetic field line at the remote area from Jupiter is stretched by the centrifugal force of rotation. Since the magnetic field line can be flexibly deformed in this area, they collide and connect with each other (magnetic reconnection). As a result, the magnetic field line that stores energy like a stretched rubber gives the kinetic energy to the plasma particles, and the hot electron of 10keV (the same as an electron which is accelerated by 10 000 volts) is made (Figure.1 B).

The electromagnetic wave is generated when the plasma enters the area of the strong magnetic field, and it accelerates electrons more. With the nature of the plasma that the ratio of magnetic field strength and particle energy remains constant, a big acceleration can be expected if plasma can enter the inner area of the Jupiterís magnetosphere that has the very high magnetic field strength. The area in the vicinity of Jupiter should store high-energy particles because dipole magnetic field of the inside of the magnetosphere is a barrier for the plasma. The region where high-energy particles are caught in this way is called the radiation belt (Van Allen belts in the Earth) (Figure.1 C). In terms of Jupiter, electron energy of the radiation zone is known to reach even 50MeV. Jupiter works as a maximum and strongest particle accelerator in the solar system.

The strong magnetic field works as a barrier and captures the plasma as described above. In other words, it means that plasma intrusion is not easy. That is to say, a barrier to understand the acceleration mechanism of radiation belt particles was the question whether or not electronic entry into the inner area of the magnetosphere happens steady (Figure.1 D). Although there was no example that captures the observational evidence so far, we were able to grab the evidence for the first time in the world by using the data of the planet spectroscopic observation satellite, "Hisaki".

Figure 1
Figure 1. Schematic diagram of Jupiter's magnetosphere [Click for larger image]
There is a high-density plasma region originating from the volcanic ejecta from satellite Io situated in the inner area of the Jupiter magnetosphere (Io plasma torus), and ultra-high energy particles gather in further inside (radiation belts). To generate and maintain these energy structures, the hot electrons that received the magnetic field energy through magnetic reconnection at the outer (magnetotail) have to be transported to the vicinity of Jupiter. However, the invasion to the inner area of the magnetosphere that is protected by the strong magnetic field is not self-evident.

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