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The Forefront of Space Science

The Forefront of Space Science:New Aspects of Magnetosphere DynamicsNew Aspects of Magnetosphere Dynamics
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Outer Space known as the Magnetosphere

When talking about outer space, the first image that comes to mind is probably “a vacuum.” In fact, the air gets thinner as soon as we move up from the ground. At about 100km above ground, gas in a different state from the air on the ground emerges. This is ionized gas (plasma), and the atoms and molecules composing it are separated into ions and electrons. Ionized gas is the part of the earth’s atmosphere ionized by sunlight. As we ascend further, the density of the gas lessens. At space over 300km above ground, ultra thin ionized gas, originally blown from the sun, gradually prevails.

Now, let’s look at the aspect starting from the sun. The solar atmosphere is in an ionized state due to the extremely high temperature of 1 million deg C. The high-temperature atmosphere is not held by solar gravity, and is blown continuously away from the sun (the solar wind). The earth floats in the solar system’s space filled with solar wind. Because the earth has its own magnetic field, the solar wind (plasma flow) collides with the magnetic field to cause that region of space known as the magnetosphere. Outer space, spreading above 300km from the ground, is the plasma world formed in this way. The density of gas there is just 10 gas particles (ion and electron) per 1cm3. This density is 18-digits lower than air on the ground, so the first impression of a vacuum is not far wrong. Plasma, however, is present there, though thin, and thus dynamic phenomena develop there - such as the aurora’s spectacular dancing glow in polar nights. It is a fascinating world where, owing to its thinness, dynamics are governed by a physical process known as the “collisionless process,” which would be unimaginable on the ground.
Most of the universe is filled with thin plasma. In this sense, it can be said that the magnetosphere is typical of outer space. Because the magnetosphere is present near to and surrounding earth, it is the only region of outer space where we can make in situ observation by scientific satellites. Since the mechanism ruling outer-space plasma is different from that explained by common sense on the ground, detailed verification of the space-plasma theory by in situ observation is essential. Also, it can be said that the magnetosphere offers us research opportunities with universal values. Assuming that mankind continues to move into space, the magnetosphere is also a space where mankind will live and act. For such activity we will need advance forecasts of the dynamic phenomena of the magnetosphere plasma (space weather forecasts), just like the weather forecasts on the ground, since such phenomena can be dangerous. This shows the significance of advancing our understanding of the magnetosphere and space plasma physics, and verifying by scientific satellite observation.

From magneto-hydrodynamics (MHD) to inter-scale connection

We are basically interested in large-scale, dynamic phenomena in outer space. It is common knowledge that, if the spatial scale of a phenomenon is large, magneto-hydrodynamics (MHD) can regenerate the phenomenon fully and correctly. MHD adds the following two items to regular motion hydrodynamics equation: (a) force imposed on the plasma by the magnetic field; and (b) formula defining development in time of the magnetic field, in that the magnetic line of force moves and transforms along with the plasma gas (“Frozen-in Concept”). This approximation system is very helpful, enabling us to easily image the change of magnetic line of force by the Frozen-in Concept. In terms of understandability, the system is very complete. I suppose we would be more relaxed if we could say, " that's all." The purpose of this article is to say " however, in fact..."

According to MHD, we can obtain the following figure (Fig. 1) for the magnetosphere. The magnetosphere’s shape in daytime side shows the dipole-shape magnetic line of force a little squashed, since the solar wind collides with the earth’s dipole magnetic field. Meanwhile, in the nighttime side, it is stretched due to interaction with the solar wind. It is also said that the stretched magnetic line of force faces the opposite direction magnetic line of force across the current layer. The magneto-tail current layer often becomes thin and explosive phenomena occur inside. This is the primary source of magnetosphere activity.

Figure 1. Magnetosphere
Figure 1. Magnetosphere

I use vague expressions such as" interaction "and" explosive phenomena "above. I will not go into detail here, but I would like to point out that “magnetic reconnection” plays an important role in the phenomena. Magnetic reconnection is a phenomenon where the magnetic lines of force, in opposite directions and facing each other, cross and reconnect. This is one of the most important phenomena in space plasma in that it changes the topology of the magnetic lines of force and releases the energy stored in the magnetic field state by converting it to plasma thermal and kinetic energy (Fig. 1). Here we face a problem, however, in that the dynamic phenomena in space plasma represented by magnetic reconnection cannot be fully regenerated by MHD alone, even when on a large scale as a whole.

To try to understand magnetic reconnection within the scope of the approximation system of MHD, we have to introduce a non-ideal electrical resistance. Since electrical resistance is a common concept on the ground, you may think that this poses no problems. In space plasma, however, there is no collision of particles with each other (collisionless plasma), which causes electrical resistance and is a natural phenomenon on the ground. In other words, an effect similar to electrical resistance is generated by a different process from the ground and causes magnetic reconnection. In the process, minute-scale dynamics that are neglected in MHD, such as ion scale or even electron scale, are substantially involved. That is, if we try to understand magnetic reconnection exactly, we should not assume electrical resistance by extrapolating from common sense on the ground, but we should elucidate the mechanism generating the same effect as electrical resistance, in the interaction among the dynamics at different scales, from MHD scale to ion/electron scales. We would think that this change in viewpoint is quite effective in understanding every large scale and dynamic space plasma phenomenon. We call this perspective the "cross -scale coupling. " To formulate the perspective is very challenging because we have to link modules of several-digit differences in space-time scale. We believe that formulation will be possible by the approach, as discussed
below, of systematic integration of verification by next-generation observation instruments, and the findings from ever-advancing large-scale plasma particle simulations.

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