Golden age of solar observation
Since the 1990s, solar observation by satellites has been widely and actively conducted. Japan began solar observation with the HINOTORI satellite in 1981, followed by the YOHKOH satellite launched in 1991, leading the golden age of solar observation. Since then, solar observations by satellites have become active and popular in the world. SOHO and TRACE are typical solar-observation satellites and still in service. Japans HINODE satellite started operation in 2006, once again spearheading solar observation in the world.
In 1999, when I started solar research, the three satellites YOHKOH, SOHO and TRACE were in full-service while the HINODE (called SOLAR-B at that time) project was just initiated. Thus, I was fortunate to enter the solar research world at a good time.
Why do we observe the Sun? It is because the Sun is the ideal object to study the physics of a variety of plasma phenomena. In the outer atmosphere of the Sun, a high-temperature plasma coronaEof more than one million kelvin exists constantly. When an explosive event called a flare occurs, it produces extremely high-temperature plasma and high-energy particles of more than ten million kelvin. Such plasma phenomena are common in the universe, namely other stars, planetsEmagnetosphere, interplanetary space, galaxies, and galaxy clusters. The phenomena are not peculiar to the Sun only. The Sun is the only star that allows us to observe the phenomena in detail with spatial resolution. With solar observation, we are able to identify the physical quantity and spatial distribution of high-temperature plasma. Further, we can diagnose magnetic fields, which are essential to understand the plasma phenomena.
As solar observation by satellites becomes popular, our understanding of the solar plasma phenomena has steadily deepened. In this article, I would like to focus on observational study of coronal heating.
The mystery of coronal heating
Although the temperature of the solar surface (i.e., photosphere) is about 6,000 kelvin, corona of several million kelvin exists just several thousand km above the photosphere. It is thought that magnetic lines transport energy between gases with different temperatures. Lately it has become possible to observe such interesting aspects in great detail, which provides great motivation for solar-plasma research. However, we have yet to reach a conclusion on the production of the high-temperature corona, i.e., coronal heating mechanism.
There are two theories for the heating mechanism of the solar corona: nanoflare heating theoryEand wave heating theory.EThe nanoflare heating theory posits that extremely small energy explosions (accordingly called nanoEflares) arise numerously to heat the corona. The wave heating theory posits that convection in the photosphere excites waves that propagate above to heat. Both theories have strong and weak points respectively. Thus, no conclusion has been reached yet. Specifically, regarding nanoflare heating, it becomes evident that, even if we add together all the energy generated by the flares, the amount cannot cover the energy necessary to heat the corona. Meanwhile, regarding wave heating, no one has observed waves propagating from the photosphere to the upper corona. In addition, we do not yet understand how the conveyed waves disperse in the upper. Since nanoflares and waves, which are currently thought to be key to understand the coronal heating, are both extremely small energy phenomena, it is hard to distinguish them in observations. Perhaps this is one reason why coronal heating has remained a mystery for such a long time.
To elucidate observationally less-visible phenomena is a challenging task for researchers. We have attempted observational research on coronal heating from two approaches: (1) If we cannot resolve the tiny phenomena, then we should try to analyze superposition of the phenomena to scrutinize a minute event. (2) With new observational instruments, we should try to take a close look at minute events that had been invisible to date.