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The Forefront of Space Science

Space Gas Vortices Carrying Source Particles of Aurora
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Visualization of space gas

Most of scientific instruments onboard satellites for our discipline measure directly plasmas, magnetic fields, etc. “in-situ”. This forms a striking contrast to astronomical satellites that take 2D images of celestial bodies, outer space gas, etc., using remote-sensing techniques such as telescopes. From those 2D images, one can easily see what type of object is where and what the object looks like. For instance, the HINODE satellite launched in September 2006 shows very clearly and precisely the structure and movement of solar magnetic fields and corona gas. On the other hand, in-situ observation has a big advantage that very detailed measurement is possible, but it is often very difficult to extract scientifically meaningful information from the obtained data. As mentioned above, in order for us to be able to unambiguously identify 2D or 3D structures such as vortices, we had to launch a fleet of satellites like CLUSTER that perform formation-flight observation.

In this situation, we are now trying to develop a new data-analysis technique for obtaining 2D information about gas structures in space from in-situ observations. Fig. 2 shows a 2D image of the magnetospheric boundary layer produced from data recorded by the GEOTAIL satellite, an ISAS and NASA joint mission. The figure represents the plasma structure (flow field and density) around the satellite path, reconstructed using the observed values of density, velocity, temperature of plasmas, and of magnetic field as the initial conditions. At first glance, we can find that a row of vortices, each with length of about 20,000km and width of several thousands km, existed and GEOTAIL passed through the vortices or their surrounding region.

The current version of the reconstruction method of 2D images is based on the ideal magnetohydrodynamic theory, that holds when collisionless plasmas behave as fluids. In addition, the present reconstruction is limited to 2D structures in a equilibrium state. Since these conditions are not always satisfied, a rigorous discussion usually requires the help of formation-flight observations. From flow-line maps as shown in Fig. 2, however, we can trace the routes of plasma movement. We hope therefore that, after further improvements of this technique, we will be able to answer questions such as, via which path does the solar wind penetrate into the magnetosphere?

Figure 2
Figure 2. Vortex structure reproduced from observations by the GEOTAIL satellite. Black lines in the figure are flow lines and white arrows show the actually measured plasma velocity vectors (X axis is the satellite path).

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