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The Forefront of Space Science

Particle acceleration in space and Jupiterís magnetosphere
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Io plasma torus and spectrum diagnosis

In the satellite Io located in the inner area of the Jupiter magnetosphere, the most a volcanic activity in the solar system is induced under the influence of tidal action from Jupiter, and volcanic gas reach into space and is ionized. As a result, Io plays the role of plasma sources that provide1000 kg / s. By the way, the orbital speed of Io to Jupiter is 17km per second. On the other hand, the rotational speed of the Jupiterís magnetic field at Ioís orbit is 71km per second. This means that volcanic gas once ionized is caught and accelerated by the magnetic field line that passes Io at relatively 54km per second. Thus plasma caught by the magnetic field surrounds Jupiter in a donut shape along the orbit of Io. This is called the Io plasma torus.

Ion emits light of specific wavelength determined by the electron configuration of the nucleus (emission line). However, in order to emit the lines, it is necessary to increase the energy level of the orbital electron by the collision with the solar irradiation and particles. This emission line is an important indicator to explore the inner area of the Jupiter magnetosphere.

Line intensity is dependent on ion density. In the case of the emission by electron collisional excitation, in addition to ion density, it includes information on temperature distribution and density of the electrons that is the side to make excited. The important point here is that the ions of certain types emit multiple emission lines at the same time. Temperature distribution and density of the impinging electron determine conditions such as whether or not each bright line is easy to light. Therefore, temperature distribution and density of the impinging electron that itself does not emit light can be derived from simultaneous observations of a number of emission lines. This technique is known as spectral diagnostics and has an advantage in that the temperature and density of electrons and ions can be derived through the optical observation that excels in understanding of spatial structure.

Besides, many of the emission line of sulfur ions, which is the main component of Io plasma torus, are located in the area called the extreme ultraviolet (wavelength 50-150 nanometers). Therefore, high-resolution spectroscopic observations in this wavelength band were what we have been waiting for because it was required for the derivation of electron temperature and density distribution of the inner part of the Jupitar magnetosphere.

Features of "Hisaki" and Observation of "Hisaki"

In fact, high precision extreme-ultraviolet spectroscopic observation are not easy. First problem to be overcome is that the efficiency of the optics is low. Since extreme-ultraviolet light has a high energy as compared with the visible light and penetrates deep into the mirror, the reflectance is low. Furthermore, the optical system is not suitable for large-sized in order to obtain a sufficient quantity of light because a satellite must be out to space in order to observe extreme-ultraviolet light that is absorbed by the earth's atmosphere. Therefore, the data suitable for spectrum diagnosis was not obtained so far. Then, the development team of "Hisaki" devised solutions as follows.

  • They improved the primary mirror reflectance by depositing high-purity silicon carbide to the surface of the mirror.
  • They dug a trench at a rate of 1800 per 1mm on the same high reflectivity mirror as the primary mirror and used it as a high-efficiency diffraction grating (spectral element).
  • They created the system to always preserve the detector in a vacuum to prevent sensitivity degradation and continued evacuating until just before the launch. (Thanks for the help of the rocket team, the launch site team, and many other people)
  • They realized the directional stability of high precision by feeding back the position information of the planet image that the observer caught into the attitude control system.
As a result of these devices, extreme-ultraviolet spectrometer of "Hisaki" realized several times higher sensitivity compared to previous ones and a high wavelength resolution suitable for spectrum diagnosis (Figure.2). In addition to these innovations, it was one of the factors to be able to obtain excellent data that "Hisaki" was a space telescope for planetary observation and was able to continue observing the same subject for a long time.

Figure 2
Figure 2. Observation of Io plasma torus by "Hisaki" [Click for larger image]
The extreme-ultraviolet spectrum of high wavelength resolution was obtained by placing visual field (slit) of "Hisaki" in parallel with Io plasma torus. Frames to separate the slits represent spatial resolution (spatial pixels). Each band of color corresponds to the emission lines that are emitted by various ions (primarily sulfur). Kyokutan is a mascot of "Hisaki". (Upper left of Io of image:©NASA/JHU APL/SwRI)

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