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The Forefront of Space Science

New Aurora Shape Captured by REIMEI
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Aurora B is a dynamically dancing aurora. A part of the aurora is dominated by magenta color, which was caused by prevalence of red and blue emissions by nitrogen molecule and nitrogen molecule ion, respectively. In the past, it has been thought that magenta aurorae were caused by high-energy incident electrons. REIMEI, however, revealed that it is not always the case. We can explain this rationally as follows: when the region of incident particles moves in horizontal direction, a magenta aurora appears at the leading edge first, followed by a green aurora because it takes longer time to shine the green aurora than the magenta aurora.

Aurorae A and B are produced by downward-accelerated electrons. Hereinafter, we call them “acceleration-type aurora.EThe shape of the acceleration-type aurora directly reflects the acceleration area. The aurora’s thickness in the horizontal direction is used to verify the generation mechanism of the acceleration-type aurora. According to past research, the acceleration-type aurora is sometimes very thin and, is as thin as about 70m. Borovsky reexamined 21 acceleration mechanisms proposed so far and he concluded that none of them could explain an aurora which is thinner than 100m.

Aurora C continuously flashes with a period of several seconds and is known as a “pulsatingEaurora. A prominent hump is not present in the energy distribution of incident-electron flux. The means that this aurora was not generated by accelerated electrons, but the electrons were strongly scattered in the near-earth space before entering the earth’s atmosphere. Hereinafter, we call this type the “scattering-type aurora.EIt is known that the scattering-type aurora is generally vague in shape or, if it has clear shape, it turns on and off repeatedly and periodically like a pulsating aurora. In recent years, however, scattering-type aurorae that do not turn on and off have been reported. Spatial scale of such aurorae is reported to be 5 to 30 km. However, REIMEI observed minuter scattering-type aurora than previously reported.

Minute, strangely shaped, scattering-type aurora

At left in Fig. 2, a mosaic image of a minute scattering-type aurora discovered by REIMEI is displayed. It has a strange shape like a combination of geometric patterns. Since it reminded me of the Nazca Lines in Peru, South America, when I first saw it, I thought it might reflect some sort of construction on the ground. By scrutinizing the observation data of its incident electrons, I was confident that this was surely aurora itself. The right side of Fig. 2 shows the brightness of the aurora and the energy flux of the incident electrons obtained along the magnetic foot of REIMEI. This shows that the brightness of the aurora increases sharply at six points from A through F as the satellite passed through the emitting areas, which have shapes as shown at left. Simultaneously, the energy flux of incident electrons increases. The fact that both light emissions and the incident electron flux increased almost in the same timing is evidence that this strange pattern is the aurora. Because of the lack of a hump in the energy distribution of the incident electrons, and the lack of flashing, we concluded that it was a scattering-type aurora. More interestingly, the thinnest part of the aurora is about 0.6 km, thinner than any scattering-type aurorae reported in the past.

Figure 1
Figure 2. Strangely shaped minute aurora discovered by REIMEI
Left: Auroral images captured by REIMEI, which is composed of overlapping square-shaped fields of view. The white line is a trace of REIMEI’s magnetic foot. Upper right: Brightness of the aurora along the satellite’s magnetic foot. Lower right: Energy flux of incident electrons measured by REIMEI, which moved from upper right (northeast) to lower left (southwest) to cross the complex-shape aurora from point A to F.

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