Explosive aurora - what is a substorm?
The mysterious aurora shining beautifully and colorfully in the night sky is a phenomenon occurring in the upper atmosphere over the north and south polar regions. The aurora often brightens suddenly around midnight and expands explosively while it moves violently. This intensive auroral activity is called the auroral breakup (auroral expansion). When the event occurs, a huge current from several hundred thousand to several million amperes runs in the upper atmosphere, causing geomagnetic disturbance. In the region called the magnetosphere where the Earth’s magnetism governs (Fig. 1), there are also violent changes of electromagnetism and plasma (i.e., charged gas).
The energy causing these events comes from the Sun. The energy is transported by a flow of plasma called the solar wind to space around the Earth. Part of that energy intrudes into the magnetosphere, where it is stored temporarily in the area where the magnetosphere is elongated like a tail on the Earth’s nightside (i.e., opposite side of the Sun). This is called the magnetotail. When the energy has accumulated to some degree, some kind of phenomenon occurs inside the magnetotail and the accumulated energy is released explosively. As a result, an intense change occurs in the magnetosphere and part of the energy approaches the Earth to cause the auroral breakup. A series of energy-release processes is called a substorm (auroral storm) and occurs several times a day on average. The auroral breakup is an important signature of the occurrence of a substorm. We have yet to find a clear answer for what triggers such energy release of substorm. This remains a big question in magnetospheric research.
The substorm is typical of the violent events that occur in space near the Earth. Now I would like to comment about meaning of the substorm research. In the modern society, the utilization of near-Earth space is indispensable for humankind. However, the occurrence of violent events there caused by solar activity poses various troubles and damage: such as the destruction of satellites, radiation exposure to astronauts staying in space, communication troubles, damage to electric-power transmission systems and electric-power outages. These problems have a large impact on our activity and economy. To minimize such impact, it is important for us to conduct basic research on space weather (changing environmental conditions in near-Earth space) and apply to space-weather forecast to predict such changes.
The substorm is also considered a typical explosive event arising in space. Intense-change events in electromagnetism and plasma such as substorms are observed commonly in the universe. They also emerge in the magnetosphere of planets, e.g., Mercury, Jupiter, and Saturn, in solar flares, and in other celestial bodies, although their environment is different from the Earth. Accordingly, the elucidation of substorms leads to an understanding of similar explosive events and common space-plasma phenomena in the universe. Therefore, substorm research is vital in terms of space weather and space-plasma physics.
In the next paragraph, I will discuss a process believed to be a substorm trigger. After that, I will introduce our latest research results.