The Sun is a very interesting astrophysical plasma laboratory close to us. The solar surface (photosphere) is approx. 6,000 deg. C. High-temperature plasma over 1 million deg. C exists about 2,000km above the sun’s surface. This is the atmosphere of the Sun called “corona.” Solar flares (explosions on the solar surface) release energy suddenly in the corona. The flares and coronal mass ejections have a close association with the occurrence of magnetic storms on Earth. Today, for mankind to utilize outer space, it is important to gain an understanding of the space environment between the Sun and the Earth, so-called space weather, as more activities are conducted in space by astronauts, such as the construction of the International Space Station.
Observational research on the Sun has been performed for a long time, starting with sketches of sunspots by Galileo Galilei about 400 years ago. Although we think we know everything about the Sun, actually it is a star that is not yet fully known, even about fundamental matters such as the physics of flaring activities in the solar corona, the mechanism for the existence of the solar corona, or the origin of its 11-year periodic cycle of solar activity.
A soft X-ray telescope onboard the solar observation satellite “YOHKOH” has been continuously observing the solar corona (Fig. 1) for over 10 years since its launch in 1991. The movies of the X-ray Sun greatly impressed the public, and aroused various scientific interests among researchers with impacts. When observed in soft X-ray, the Sun’s surface is seen as a black disk. The image also shows that the corona is composed of structures with significant contrast, unlike our impressions of the solar surface. The temperature of the solar surface is approx. 6,000 deg. C, whereas the corona is composed of high-temperature plasma of 1 to 3 million deg. C, allowing us to view the faint corona over the bright solar surface clearly and easily by using X-ray light.
Why does the plasma with temperature several hundred times higher than that of the surface exists above the solar surface? This is the mysterious phenomenon known as “coronal heating problem” that has denied our efforts at comprehension for a long time. Though it is an unsolved puzzle, it is certain that coronal heating is closely related to the existence of magnetic fields, because it is more intense in regions with stronger magnetic fields.
Coronal heating and micro/nano-flares
What physical process is working for heating the corona and how? Two kinds of mechanisms have been proposed. One is the “wave-heating” mechanism, in which electro-magnetic waves propagating along the magnetic field lines dissipate in the corona. The other is the “micro-flare heating” mechanism, in which a great number of extremely small-scale flares dissipate magnetic energy in the corona. Neither of these mechanisms has yet fully explained the actually observed properties of the corona. Thus, right now we have not understood the origin of the corona at all.
Of the two mechanisms, observational understanding of the micro-flare heating has been advanced by the continuous soft X-ray imaging observations by YOHKOH, etc. In observations before YOHKOH, the energy scale of flares possible to detect was 10^29 to 10^32 erg. With YOHKOH’s high-imaging, high-temporal resolution and low scattering observations, we become able to observe easily that small explosions of two or more orders of magnitude occur frequently. Furthermore, we may detect some signatures of extremely small explosions with levels of 10^24 to 10^25 erg. Such small explosions are called micro- or nano-flares, since their energy scale is smaller by six to nine orders of magnitude than the largest flares. According to our observations, the amount of energy released by the micro-flares is insufficient to heat the corona. Therefore, our focus of observation is shifting to the nano-flare energy range.