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The Forefront of Space Science

GEOTAIL Satellite - Detailed Story of Astrophysical Gamma-ray Observation
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Main figure
Positions on the galactic system coordinate of known Soft Gamma-ray Repeaters (SGR, four red circles) and Anomalous X-ray Pulsars (AXP, six yellow squares, believed to have similar characteristics to SGR). SGR1806-20, where a huge flare occurred recently, is located closer to the center of the galactic system. The last huge flare occurred in SGR1900+14 and before that in SGR0526-66.
© NASA Marshall Space Flight Center

“GEOTAIL” is the name of the magnetosphere exploration satellite launched in July 1992 and now actively engaged in data acquisition. GEOTAIL is a coined word from “geo” meaning earth and “tail” from magnetotail. In science history books, we can find many cases of unexpected data triggering and facilitating new development of research. On a personal level, however, such cases are rare and uncommon. Indeed, just a few months ago I never would have dreamed that I would observe astrophysical gamma rays with GEOTAIL, and as a result, be pushed to the forefront of astronomy.

As its name suggests, the GEOTAIL project is intended to study the earth’s magnetosphere, in particular its tail, and was planned and developed jointly by the former Institute of Space and Astronautical Science (ISAS) and NASA. The important points in magnetosphere research are high-energy particles, the electromagnetic field and its fluctuations (waves and oscillations) as well as the plasma density, velocity and temperature in the magnetosphere. These observations have been almost perfectly performed by GEOTAIL.

Meanwhile, YOHKOH’s outstanding achievements in the solar corona and flare field (research neighboring magnetosphere research) are still fresh in the memory. Though plasma density and magnetic field strength differ by several orders of magnitude between the earth’s magnetosphere and the solar flare, it has been proved that the common physical process called “magnetic reconnection” governs both. Magnetosphere and solar-research groups in Japan, using data from GEOTAIL and YOHKOH satellites, played a decisive role in the advancement of the research. YOHKOH made remote-sensing observations with X-rays through gamma rays to study magnetic reconnection because, unlike GEOTAIL, it did not conduct in-situ observations.

GEOTAIL captured the solar flare’s gamma rays

Figure 1
Figure 1 Upper : LEP ion-counter data are expressed in artificial color along with energy level and time (E-t figure). The vertical stripe in the center indicates the arrival of solar gamma rays at GEOTAIL.
Lower : Temporal profile of gamma rays (50 to 90keV) from the Sun observed with YOHKOH. Its peak position coincides with the vertical stripe in the upper figure.

The story begins five years ago, early in 2000. While I was watching data from the LEP, GEOTAIL’s plasma-observation instrument, consisting of ion and electron counters, I noticed that strange vertical stripes sometimes appeared on the data. The upper diagram in Fig. 1 shows the ion data from the LEP for 30 minutes from 11:40 to 12:10 (UT) on November 6, 1997. The vertical axis indicates the energy level of the ions while the horizontal axis shows time. The number of ions counted in corresponding time and energy levels is expressed in the artificial color. At that time, since GEOTAIL was in the plasma sheet of the magnetotail, the surrounding high-temperature ions were continuously rushed into the counter. The magnetotail is the earth’s tail, similar to that of comets, formed by the phenomenon whereby the earth’s magnetic field is blown by solar wind. In the center of the magnetotail, high-temperature plasmas reaching tens of millions deg. C (several keV) are confined and form a sheet-like structure called the plasma sheet.

In the figure, the plasma sheet is seen as a colored belt extending horizontally between 0.2 to 10keV. The color changes over time suggest that the amount of ions varies. In the center of the diagram at about 11:53 to 11:55, we can see a vertical stripe colored in blue, yellow and red. If this is caused by ions, it means that there is an ion-burst event with a wide energy range extending from the minimum 0.02keV, the lower measurable limit, to the maximum 40keV. No such event, however, has been recorded before.

While I was examining the credibility of the LEP data, I found data on gamma-ray photons (HXT/H) observed by YOHKOH at the same time (lower graph in Fig. 1). YOHKOH observed the increase in number of photons at the same time as the vertical stripe appeared in the upper figure. The data from YOHKOH was due to an ultra-large solar flare at a class of X 9.4. By investigations made by Yasuhiro Takei, a graduate student at that time, we found many cases where vertical stripes similar to the above were seen in LEP observations due to large solar flares (>X3). Further, with detailed comparison of data from the LEP and YOHKOH, it was proved that the LEP data’s vertical stripe represents the intensity of gamma-ray photons over 50keV (note that the energy scale on the vertical axis in the upper diagram in Fig. 1 is not applicable to the gamma ray).

LEP sensitivity is low for gamma rays, under about 1/1,000 compared to instruments used exclusively for gamma-ray measurement. Such instruments, however, spend much time without seeing the Sun because they are often in the earth’s shadow. On the other hand, since GEOTAIL flies 10 to 30 earth radii away from the earth’s center, the period flying in the earth’s shadow is negligible, allowing it to watch the Sun almost continuously. Thus, GEOTAIL was able to provide solar-flare researchers with data on solar gamma rays, even when data from other instruments were lacking.

Then, early in 2005, when we were about to end solar gamma-ray observations with LEP because the number of flares was decreasing along with a decline in solar activity, it happened.

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