2 Solar Phenomena and Solar wind 2-1 Study of Energy Build-up in Solar Flares KUROKAWA Hiroki To guarantee human beings to work safely and effectively in the space, the forecast for strong solar flares is indispensable. And the study of the build-up and release mechanism of the magnetic field energy, which is the source of flares, is essentially needed. This paper demonstrates which type of sunspot groups or which type of changes in the magnetic field configuration produce strong flares by showing the results obtained during the international coordinated observations in June, 2000. Our important finding is the fact that strong flares occurred right after the emergence of a strongly-twisted magnetic flux rope. We stress, therefore, the importance of further quantitative analyses of the evolution of the twisted magnetic flux rope in more details, and the importance of continuous solar observations from the space to develop the space weather research. Keywords Solar flare, Sunspot group, Emerging twisted flux ropes, Space weather 1 Introduction the surface; to progress further we need to clarify the specific details of the relationship On the surface of the sun, many explosive between the development of the three-dimen- phenomena－such as flares, prominence erup- sional field configuration as twisted flux ropes tions, and coronal mass ejections (CMEs)－ emerge, and the formation of Hαfilaments, unexpectedly occur, releasing strong electro- flares, and CMEs. This will deepen our magnetic waves of various wavelengths (from understanding of the process behind the ener- gamma rays and X-rays to radio waves), as gy buildup and the trigger mechanism of well as high-energy particles and plasma explosive phenomena on the solar disk, thus clouds. The high-energy particles and power- enabling us to forecast the occurrence of such ful radiation pose a threat to space-based phenomena. Furthermore, in relation to human activity, which will become increasing- research into space weather forecasting, we ly common in the future as satellites and space expect to investigate the processes by which stations come into more widespread practical magnetic fields emerge repeatedly in the solar use. Therefore, it is urgent that we elucidate convection zone, and also how twisting of the the outbreak mechanisms of these phenomena magnetic field occurs. It is also necessary to to enable accurate forecasting. investigate the causes of solar activity and to Recent high-resolution observations have elucidate the origin of periodic changes in the revealed that the main cause of the explosive solar magnetic field and solar irradiance. phenomena on the sun is the emergence of the There have been many recent observations solar magnetic field to the surface from the of explosive phenomena from celestial objects internal convection zone and the sudden outside of the solar system at levels several release of energy stored in the distorted mag- orders of magnitude higher than the energy netic field. accompanying solar flares; these energetic However, this research has only scratched objects include flare stars, cataclysmic vari- KUROKAWA Hiroki 5 ables, X-ray transients, and gamma-ray bursts. will be required, in addition to observations It is important that we now elucidate the using X-rays, Extreme-Ultraviolet rays mechanisms behind the build-up and release (EUVs), and radio waves. of this energy, so that we may better under- From this perspective, this paper will pres- stand the structure and evolution of the uni- ent part of our research on the morphologies verse. The sun is the only celestial object that of emerging twisted flux ropes, focusing on permits analysis of the details and dynamics of recent results from coordinated international the physical structures which produce the vari- observations of the active region NOAA9026 ous high-energy explosive phenomena that (June 2000). We will conclude by suggesting occur on its surface. Thus the sun serves as a how research on solar-flare forecasting should laboratory in which we can investigate the be conducted in the future, and the types of basic processes of high-energy phenomena in observation equipment that will be required. space plasmas. Clarifying the causes of solar flares and 2 Hida－La Palma－TRACE coor- applying this knowledge to space weather dinated international observa- forecasting and to a broader understanding of tions the universe, requires an investigation of vari- ous physical processes, including those that We performed observations from Hida drive energy build-up, energy release, high- Observatory from May through June 2000, in temperature plasma heating, the acceleration coordination with observations carried out by of non-thermal particles, and the propagation the SOUP observation team of the Lockheed of magnetic plasma and shocks. These mech- Martin Solar and Astrophysics Laboratory anisms are all interrelated and connected (U.S.) using the 50-cm Swedish Vacuum Solar through the solar magnetic field. Obviously, Telescope on the Island of La Palma in West research on solar flares has developed in con- Africa's Islas Canarias. Observations at Hida junction with recent advances in observation Observatory were concluded approximately at equipment, with a growing ability to observe a 08-09 UT, when observations began at La wide range of wavelengths. Research using Palma; these coordinated observations thus optical observations of chromospheric flares enabled continuous Hαimage observations and the sunspot magnetic field, beginning with unprecedented spatial resolution. In from the late 1940s to the 1960s, in addition to addition, the TRACE satellite also carried out radioastronomy and X-ray observations, such observations of the same region during this as those recorded by the recent YOHKOH period. satellite, have begun to reveal the physics of Fortuitouly, during these coordinated energy release due to magnetic reconnection observations, active region NOAA9026 in the corona. Furthermore, the movies appeared from the eastern edge of the sun, and obtained by the LASCO instrument on the as it passed near the center of the solar disk, SOHO satellite provide vivid and dramatic an intense flare occurred. Of the many active images of the corona plasma (CME) flowing regions appearing on the solar disk, the num- out dynamically into interplanetary space. ber of regions that produce X-class flares, This energy originates in the magnetic flux which are defined as those seriously affecting ropes that are twisted in the convection zone the space environment, is considerably limit- as they emerge. To investigate the behavior of ed. If we look at the current cycle, 1,758 these flux ropes－emerging from underneath active regions designated with NOAA num- the photosphere to regions above the chromos- bers (NOAA regions) have appeared during phere and corona, and flowing out into inter- the five years from January 1997 through planetary space－simultaneous high-resolu- December 2001. However, only 25 of them tion and high-accuracy optical observations (1.4 percent) produced X-class flares at least 6 Journal of the Communications Research Laboratory Vol.49 No.3 2002 once. Also, during the 11 years of the previ- the center of the solar disk from June 6 to 7, ous cycle, from 1986 through 1996, of the when it developed most significantly, produc- 3,089 NOAA regions during this period only ing intense flare activity. 69 (2.2 percent) produced X-class flares (Ishii 2002). Furthermore, among these occur- June 2 June 6 June 9 rences few are suitable for analysis. This is because in order to study the variations of the photospheric and chromospheric magnetic field configurations before and after the occur- rence of a flare in detail, and in order to ana- lyze the relevant energy build-up and energy release trigger mechanisms, the region needs to produce a flare near the center of the solar disk. In addition, such a region must appear Big Bear Solar Observatory (white light) during coordinated observations, and the Fig.1 Heliographic passage of the active region NOAA9026 weather must be suitable for proper operation of the high-resolution ground telescopes. If Fig.2 shows the variation of the soft X-ray we consider the above requirements in terms flux recorded by the GOES satellite from June of probabilities, we can conclude that the X- 1 to 11 as this region passed across the solar class flare occurrence in NOAA9026 certainly disk. It can be seen that several intense flares represented an extremely rare occasion. occurred. These coordinated international observa- The flares that occurred in region 9026 are tions were organized by Allan Title, Tom Berg- marked with squares. er, and Richard Shine of the U.S., and, in addi- As can also be inferred from the photo- tion to the author, by Takako Ishii, Keiji graphs in Fig.1, since there were no other sig- Yoshimura, Hiromichi Kozu, Taro Morimoto, nificant active regions besides NOAA9026 Ayumi Asai, Reizaburo Kitai, and Satoru Ueno during this period, most of the flares in Fig.2 at Hida Observatory. The following results originated within this region. were obtained from coordinated research con- From June 2 to 4, this region produced ducted by the author, Wang Tong Jiang, and five M-class flares, but no X-class flares. Sig- Takako Ishii (Kurokawa et al. 2002). nificantly, after subsiding for two days, the The data used for this analysis are: Hα region produced three consecutive X-class images obtained by the domeless solar tele- flares from June 6 to 7. scope at Hida Observatory; Hαimages taken by the Lockheed team; 5000A continuum images and 1600A images from the TRACE Large flares satellite; radial magnetograms from SOHO/MDI; and vector magnetograms Midium-sized flares obtained at Huairou Solar Observatory, Bei- jing Astronomical Observatory. Small flares 3 Development and sudden The horizontal axis indicates the time, and the decay of active region vertical axis indicates the x-ray energy flux of NOAA9026 the flares 171Å image from the TRACE satellite Fig.2 Flares that occurred in the active region NOAA9026 Fig.1 shows the passage of the x-ray flux recorded by the GOES satellite.