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The Forefront of Space Science

New Aurora Shape Captured by REIMEI
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The shape of a scattering-type aurora is determined by where electrons are scattered in the near-earth space. Some scattered particles descend toward the earth, resulting in the aurora. Plasma waves are the most probable candidate for a mechanism to cause electron scattering. The growth rate of the plasma wave depends strongly on the density of cold plasma in the near-earth space. Therefore, we can presume that the shape of a scattering-type aurora reflects the spatial distribution of cold plasma in the near-earth space. If so, there are two questions regarding the shape.

The first one is complexity in shape. A possible candidate to produce the complex shape is interchange instability. This type of instability is caused by a pressure imbalance between the plasma and the magnetic field in the near-earth space. According to past theoretical research, instability tends to occur when the plasma pressure drops sharply with increasing distance from the earth. For example, if hot plasma is injected locally in the near-earth space, which is caused by large-scale disturbances called substorms, the conditions for interchange instability can be set up (Fig.3).

Figure 4
Figure 3. Simulation of cold plasma distribution around the earth
By the injection of hot plasma, the interchange instability grows, resulting in disturbance of cold plasma. In the box of white lines, a complex pattern on a scale of several 1,000 km is shown, but it is possible that, in fact, a minute pattern on a scale of 10 km is produced.


The next issue is minuteness. Since the charged particles exhibit helical motion around a magnetic line, they cannot form a shape smaller than their radius of helical motion. It had been thought that high-energy ions contribute to the plasma’s pressure and determine the plasma pressure distribution in near-earth space. The radius of helical motion of ions with typical energy is about 100 km on the magnetic equatorial plane in the near-earth space. Meanwhile, if we project the aurora’s 0.6 km thickness onto the equatorial plane, its radius is only 9 km. We cannot explain the minuteness of plasma pressure distribution by the conventional theory that ions contribute to the plasma’s pressure. However, if electrons, whose radius of helical motion is smaller than that of ions, contribute to the plasma pressure too, the observed minute structure can be formed. When REIMEI observed the minute, strangely shaped aurora, the geosynchronous satellite LANL-97A had just observed a plasma injection event where only hot electrons increased while hot ions remained almost unchanged. I think that this provides indirect evidence to support the above hypothesis.

Conclusion

It was believed that plasmas are relatively smoothly distributed in the near-earth space. Our finding with REIMEI may prompt reconsideration of the plasma environment in the near-earth space. Then, a new question was raised: “How minute is plasma distribution?EThe SPRINT-B/ERG satellite, now planned as the second in the SPRINT series of small scientific satellite series, will conduct comprehensive in-situ observation of particles, magnetic fields, electric fields and waves in the region where the above instability is expected to occur. In this region, it is also thought that electrons are accelerated to nearly the speed of light, and the radiation belt is created. The SPRINT-B/ERG satellite will capture the real nature of the highly diverse universe near the earth.

Yusuke Ebihara

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