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Aurora Show Every Day:
Observations of Fine Auroral Structures by the Reimei Satellite

The Reimei satellite, meaning "dawn" in Japanese, was launched on 24 August 2005 primarily to investigate fine auroral structures. Almost two years have passed since Reimei started scientific observations. Reimei is still sending beautiful auroral data every day, and most of them are immediately available from DARTS.

Aurora is an atmospheric fluorescent phenomenon taking place at an altitude of 100-300 km above the Earth's polar regions. Aurora's energy source is energetic electrons precipitating from the Earth's magnetosphere. Namely, auroral phenomena may be considered to be a "show" projected on the atmospheric screen, in which actors are physical conditions of surrounding terrestrial space.

Most significant characteristics of Reimei is that it can simultaneously (1) obtain auroral images in high time and spatial resolutions (so precise that was only possible from ground observations) and (2) observe precipitating electrons which cause auroral emission and (3) the ion outflow accompanying auroral phenomena. This is made possible for the first time thanks to the sensitive instruments, low-altitude orbit optimized for auroral observations, highly accurate attitude control, and so on.

Previous satellites could not do these, since they are more concerned about global auroral distributions and activities. On the other hand, Reimei is a small satellite that was devoted for auroral image and particle observations. In order to simultaneously observe high-quality auroral images and detect auroral particles, Reimei satisfies the following requirements:

1. Having a sun-synchronous orbit at 00:50-12:50 local time, with which polar regions can be repeatedly observed where auroral phenomena are particularly active
2. Having an altitude of about 640 km which is suitable to observe auroral emission and particles from space
3. Being able to detect auroral particles coming from various directions at high time resolution, while operating the high-performance CCD camera

Below we show four aurora examples observed by Reimei. The Multi-spectral Auroral Imaging Camera (MAC) is able to take auroral images with three wave-lengths (red 670.0 nm, green 557.5nm, and blue 427.8 nm) every 120 msec. In the southern hemisphere, however, two-band observation (red and green) is nominal due to power constraint. The following aurora movies (mpeg format) are available from DARTS, so please take a look (It may be better to save the movies before playing them, because the size of the files is large.):

Movie 1 (2005-12-26): Stream-like fine and multiple auroral arcs embedded in a largely structured auroral band
Movie 2 (2006-04-24): Blinking (pulsating) auroras together with clouds and then multiple fine auroral arcs parallel each other
Movie 3 (2006-09-17): Three successive appearances of bright and drastically varying auroral forms
Movie 4 (2007-08-07): Complicatedly streaming and curling auroral forms

These movies will be replayed about five times faster than the real. While the stream in these movies is primarily due to satellite orbital motion, the aurora itself is changing its shape and location. In the movies, what does not change the shape at all are probably clouds or Earth's terrain illuminated by moonlight. Several permanent bright spots are CCD noises, caused by secular radiation damage.

Corresponding to these auroral images, we show aural particle data (electrons in these examples) taken by the Electron/Ion Energy Spectrum Analyzer (ESA/ISA). In each movie above, you may have noticed a small moving square, which is projection of the Reimei position along the geomagnetic field line to a typical auroral emission altitude (110 km). Namely, Reimei is observing the auroral electrons which are precipitating into this point, causing auroral emission. Figure 1 shows the energy-time (E-t) spectrogram (where abscissa is time, ordinate is electron energy, and the color indicates the flux) of the downward electrons corresponding to Movie 1, together with auroral images.

Figure 1

From Figure 1, you can notice that auroral electron variations (both energy and flux) are associated with the variation of fine structures of the auroral emission. It has been known that the auroral global structure is corresponding to the region where high-energy (about 10 keV) electrons are responsible for large amounts of the energy flux. On the other hand, Reimei is revealing auroral fine structures which reflect small modulations of the electron kinetic energies and the energy fluxes (compare the fluctuations of electron fluxes with the auroral structures, as guided by light red lines).

Figure 2

In Figure 2, which corresponds to Movie 2, we show the E-t spectrograms of the electrons in not only downward direction but also upward and scattered directions. The amount of the upward electrons (mirrored by the Earth's magnetic field) is tiny, and most of the downward electrons are lost due to collision with the Earth's atmosphere. In this example too, you can see that the auroral fine structures are correlated with auroral electron energies and energy fluxes.

Figure 3    Figure 4

In Figures 3 and 4, we show only auroral images corresponding to Movie 3 and 4. If you compare these "static" images with "dynamic" movies, you may have completely different impression. We wish you notice auroras, meaning "dawn" in Latin, are so dynamic and beautiful natural phenomena.

Masafumi Hirahara (The University of Tokyo)

November 2007