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2 and 2-1 Study of 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 energy, which is the source of flares, is essentially needed. This paper demonstrates which type of 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 research. Keywords , Sunspot group, Emerging twisted flux ropes,

1 Introduction the surface; to progress further we need to clarify the specific details of the relationship On the surface of the , 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 waves), as gy buildup and the trigger mechanism of well as high-energy particles and explosive phenomena on the solar disk, thus clouds. The high-energy particles and power- enabling us to forecast the occurrence of such ful 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 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 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 . 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 and the sudden outside of the 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 , 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- 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 Laboratory anisms are all interrelated and connected (U.S.) using the 50-cm Swedish Vacuum Solar through the solar magnetic field. Obviously, 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 also carried out radioastronomy and X-ray observations, such observations of the same region during this as those recorded by the recent period. satellite, have begun to reveal the of Fortuitouly, during these coordinated energy release due to 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 , 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 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 (white ) 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 . 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 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. The flares that occurred in the active NOAA9026 region over the solar disk. We region NOAA9026 are marked with can see from this figure that it passed close to squares.

KUROKAWA Hiroki 7 According to the high-resolution Hα June 6 to 7. Also, by as early as June 8, the images observed continuously by the Dome- core portion of theδ-type had less at Hida Observatory (Fig. already completely decayed. 3), the sunspot group was too close to the east- As is clear from the GOES satellite data ern edge on June 1 to investigate its structure shown in Fig.1, this region produced intense in detail; however, from June 2 to 4 we can consecutive flares (two X-class and two M- clearly observe two pairs of newly emerging class) from 13 UT to 15 UT on June 6. This dipole magnetic field regions and the develop- is characteristic of the flare activity in this ment of the corresponding sunspot pairs. The region. M-class flare activity seen in Fig.2 was These flares occurred at night in Japan; induced by the appearance of these newly however, at La Palma, where it was day time, emerging magnetic field regions. Although it the flares could be observed perfectly. Fig.4 is certainly important to study in detail the shows the Hαimages of the flares. All of variation of the magnetic field configuration these were homologous flares occurring along during this period, here we shall focus on and the strongly sheared magnetic neutral line describe the development of the sunspot withinδ-type sunspots. region before and after June 6, when more What happened before and after this intense, X-class flares occurred consecutively. intense flare activity? What kind of changes Fig.3 shows the development and the occurred in the sunspot group or in the mag- decay of the active region between June 4 and netic field configuration during this period? June 8, including June 6. These images were What happened to the emerging flux ropes to taken at Hida Observatory. Here we can see cause these changes? We will approach these that a strong magnetic shear structure devel- problems in the following sections. oped on the magnetic neutral line of theδ- type sunspot group from June 4 to 6, and that the magnetic neutral line rapidly rotated from

Hα center

Hα-5.0A°

(Images taken at the Domeless Solar Telescope at Hida Observatory.)

Fig.3 Development and decay process of active region NOAA9026 These observations were made by the Domeless Solar Telescope at Hida Observatory. Note that the Hα filament channel (the magnetic neutral line) rapidly rotated from June 6 to 7.

8 Journal of the Communications Research Laboratory Vol.49 No.3 2002 tioned at both edges of the sunspot group showed a rotational motion, suggesting interconnection. These observations strongly suggest that the sunspots were connected by a collection of flux ropes. Fig.5 shows the sudden decay of theδ- type sunspots, which took place immediately after 10 UT on June 6. Note that the decay of the core portion of theδ-type sunspots was Provided by Richard Shine of the Lockheed Martin Solar and Astrophysics Laboratory initiated between 10 UT and 11 UT. Fig.4 Intense flares occurring consecutively along the magnetically neutral line of We constructed a model for the emergence NOAA9026 of the flux ropes that can simultaneously Photographs taken by the Swedish 50-cm Vacuum Solar Telescope on the Island of explain the slow growth of theδ-type sunspot La Palma. group during the earlier stage and the sudden decay in the later stage, in addition to the and rotational motion of the 4Proper motion of the sunspots individual sunspots described above. We will immediately before the occur- discuss this model in Chapter 6. However, rence of the X-class flares first we would like to call attention to the par- ticularly intriguing proper motion of the TRACE performed continuous observa- sunspots. tions of region NOAA9026 from June 3 through 12, providing valuable data on the development of this sunspot group. We com- piled the observed 5000A images into a movie in order to study the proper motions of the group. The analysis of this movie and of the Hα images from Hida Observatory has revealed the following with respect to the growth, decay, and motion of the sunspot group. (1) Theδ-type sunspots formed slowly in the Fig.5 Sudden decay of theδ-type sunspots center of region 9026 from June 3 to 6. (TRACE5000Å images) (2) The magnetic shear on the neutral line of Note that theδ-type sunspots suddenly began to decay between 10 UT and 11 UT. theδ-type sunspots gradually became stronger from June 4 to 6. (3) Theδ-type sunspots, which had developed Before the sudden decay of theδ-type slowly, began to rotate and then suddenly sunspots that induced the intense X-class decayed after 10 UT on June 6. flares, high-speed proper motions were (4) After the intense flare activity from 13 UT observed at their eastern edge; we believe this to 16 UT on June 6, the decay of theδ- triggered the decay of the sunspots. Fig.6 type sunspots proceeded further, and after shows the temporal variation of the position of another X-class flare on June 7, most of sunspot N5. "L" indicates the variation along the core portion of these sunspots had dis- the direction of heliographic longitude and appeared by June 9. "B" indicates the variation along the direction (5) The decay of the sunspot group from June of heliographic latitude. In this figure, we can 6 to 8 occurred over the entire sunspot see that N5's high-speed motion began six group. Furthermore, the sunspots posi- hours before the occurrence of the X-class

KUROKAWA Hiroki 9 Fig.6 High-speed motion of the sunspots, which induced the decay of the δ-type sunspots "L" indicates the motion in the direction of heliographic longitude, and "B" indicates the motion in the direction of heliographic latitude. The numbers inside the frame at the bottom indicate the classification levels of the X-ray flares, as observed by the GOES satellite. We can see that N5 and N1 began their high-speed motion about six and two hours before the occurrence of the X-class flares, respectively. flares. It is worth noting that sunspot N1, to N1 suddenly began high-speed motions about the west of theδ-type sunspots, also initiated six and three hours before the occurrence of a high-speed motion at about 10 UT, when the the flares, respectively. δ-type sunspots began to decay. Fig.7 shows the speed of the proper 5Variation in the magnetic field motion of the sunspots shown in Fig.6. Here immediately before the occur- again, it is clearly shown that sunspots N5 and rence of the X-class flares

We can study the photospheric magnetic field of this region using continuous observa- tions of the radial magnetic field by SOHO/MDI and vector magnetograms from Huairou Solar Observatory, Beijing Astronom- ical Observatory. First we made a movie rep- resentation of the MDI observation from June 3 through 10. As with the TRACE movie, we could clearly see the decay of the entire mag- netic field configuration as the magnetic neu- tral line in theδ-type sunspot region rapidly rotated after June 6. By June 10, a large part Fig.7 Variation in speed of the proper motion of the sunspots before the of the magnetic field in theδ-type sunspot occurrence of the flares region had disappeared. This suggests that the

10 Journal of the Communications Research Laboratory Vol.49 No.3 2002 rapid rotational motion of the magnetic neutral magnetically neutral line and the development line in theδ-type sunspot region plays a key of magnetic shear, in addition to the decay of role in the occurrence of the X-class flares theδ-type sunspots described in the previous (Fig.8). section. This is a very significant observation in light of the emerging features of the flux ropes. Fig.9 shows magnetograms from Huairou Solar Observatory, Beijing Astronom- ical Observatory, which show the incursion of the southern polarity region into the northern polarity region. Next, Fig.10 shows the results we obtained regarding the time variation of the radial mag- netic flux using MDI data. The divisions of the region used in our measurements are shown in the MDI on the right. For example, "p1" indicates the total northern- polarity magnetic flux within the area, and "f1" indicates the total southern-polarity mag- netic flux within the area. From this figure, Fig.8 Rotation of the magnetic neutral line we can see that the magnetic flux in these of theδ-type sunspots three areas increased or remained constant before the intense flare activity on June 6, but In addition, we discovered another charac- that it suddenly began to decrease immediate- teristic variation: a sudden incursion of the ly before the flare activity. southern polarity region into the northern The observed variation of the magnetic polarity region of theδ-type sunspots immedi- field can be summarized as follows. ately before the X-class flare activity on June (1) The overall magnetic flux in theδ-type 6. This synchronized with the rotation of the sunspot region increased from June 3 to 6,

Red contour lines:south-pole magnetic field Yellow contour lines:north-pole magnetic field Blue arrows:horizontal component of the magnetic field (Provided by Huairou Solar Observatory, Beijing Astronomical Observatory)

Fig.9 Development of magnetic shear structure before the occurrence of the X-class flares There was an incursion into the northern-polarity magnetic field by the southern-polarity region with developed switch-back (returning) magnetic shear.

KUROKAWA Hiroki 11 and the magnetic shear on the neutral line (2) Theδ-type sunspots continued to grow, also became stronger. accompanied by development of a sheared (2) The incursion of the southern polarity into magnetic structure (June 4 to the first half the northern polarity began immediately of June 6). before the intense flare activity on June 6; (3) Triggered by the rapid proper motion that at the same time, rapid rotation of the neu- began in the sunspot penumbra at the east- tral line and a sudden decrease in the mag- ern edge of theδ-type sunspots, the netic flux began. By June 10, the magnet- sunspot group suddenly became unstable ic field of theδ-type sunspots had almost and began to decay. This was accompa- disappeared. nied by rotation of the neutral line, incur- sion of the southern polarity into the north- ern polarity region, and the shear motion of the sunspots along the central line. We will now consider models for the emerging flux ropes, which can explain this striking development of region NOAA9026. The correct model must be able to explain the rotational motion of theδ-type sunspots, the shear motion inside these sunspots, the varia- tion in the magnetic field, and the incursion of the southern polarity into the northern polarity, in addition to the magnitude of the proper motion and the direction of the rotational motion of the surrounding sunspots related to the rotational motion of theδ-type sunspots. In the following sections, we will consider two twisted flux rope models, distinguished by their respective approaches to the two types of helicities (twists): "writhe helicity" and "twist helicity." The former type of helicity is pro- duced through twisting of the central part of Fig.10 Time variation of the magnetic flux in the NOAA9026 region the flux rope; and the latter is produced Note that the sudden decrease in magnetic through twisting of both ends of the flux rope. flux began immediately before the occur- The difference between the two models in rence of the X-class flares. question is based on the type of helicity seen as present (as the initial condition) in the con- 6 Models for the emergence and vection zone. energy release of the twisted flux ropes 6.1 Model of the emerging twisted flux rope (A) (writhe-helicity type) From the analysis in the previous sections, As shown in Figs.11 and 12, a positive we can provide an outline of the development writhe helicity is applied to the central part of of the NOAA9026 region, which induced the flux rope. As a result, a negative twist intense consecutive flare activity (including helicity occurs over the entire flux rope due to three X-class flares), as follows. the law of conservation of helicity. The twist- (1) A new dipole emerging flux region (EFR) ed region at the center corresponds to theδ- appeared in the existing sunspot region, and type sunspot, and we can assume that energy theδ-type sunspot group grew (June 2 to 4). is released as the twisted flux rope loosens.

12 Journal of the Communications Research Laboratory Vol.49 No.3 2002 time, it can also explain the proper motion of adjacent sunspots such as N 1, N 2, N 4, and S 4, which are connected to theδ-type sunspot. The flux rope continued to ascend slowly, and although the magnetic shear of theδ-type

Photospheric sunspot became stronger from June 4 to 6, it disk was stable during this period with no exten- Time sive energy release. We note, however, that when the flux rope ascended to a certain level, it suddenly became unstable and, as shown in Emerging twisted flux rope induced large-scale flares in this region. Fig.13, the entangled flux rope rapidly Fig.11 Model of the emerging twisted flux untwisted, thus initiating the observed proper rope (A) motion and rotation of the sunspots, the decay The positions of the photospheric disk are given by the three planes corresponding to of the magnetic field configuration, and the times T 1, T 2, and T 3. In this figure, the intense flare activity. This model explains the emerging flux rope is seen as fixed, rotational motion of the sunspots well. Fur- whereas the photospheric disk lowers as time passes; here the charge in the forma- thermore, it does not contradict the recent tion of the positions where the flux rope observation that negative twist helicity is breaks through the surface of the disk (at each time) corresponds to the observed dominant in active regions in the northern motion of the sunspots. hemisphere (Pevtsov et al. 1995; Longcope et al. 1998). However, this model has drawbacks Below is a photograph of a model we con- in its explanation of the following points: (a) structed to represent the twisted flux rope. the channel with the switch-back structure, as shown in Fig.9, continued to grow longer (with a narrow width) even after the X-class flare activity on June 6, and the development of the strongly sheared structure continued until the occurrence of the X-class flare on June 7; (b) it is uncertain whether it is possible to twist the central part of the flux rope locally by the Coriolis force and to create a strong writhe helicity.

Fig.12 Photograph of the model of the twist- ed flux rope, based on the diagram in Fig. 11 The twist loosens as it rotates Figs.11 and 12 show the three positions of the photospheric disk at times T 1, T 2, and T 3. Although the flux rope is in fact emerging Rotational motion Rotational motion from the surface, these figures treat the devel- of the sunspots of the sunspots opment over time in terms of the relative low- ering of the photospheric disk. Thus the loca- tions of the sunspots at each time T corre- Fig.13 Twist-release model for the emerging spond to the sites where the surface of the flux rope photospheric disk is broken by the flux rope. This model can explain the growth of a 6.2 Model of the emerging twisted flux strongly shearedδ-type sunspot by the emer- rope (B) (twist helicity type) gence of the twisted flux rope. At the same In this model, we first assume that a nega-

KUROKAWA Hiroki 13 tive twist helicity is given to the flux rope in variation of the magnetic field configuration) the convection zone. This agrees with obser- summarized in Chapters 4 and 5 above. vations of the northern hemisphere described above. This can be experimentally demon- 7 Summary strated by twisting both ends of a plastic tube (Fig.14). As it ascends, the strongly twisted We have discussed the development of a flux rope transforms its twist helicity into δ-type sunspot region which induces intense writhe helicity at the center, forming a knot at solar flare activity through the example of this location. δ-type sunspots are thus active region NOAA9026, which produced formed. Linton et al. (1999) and Fisher et al. consecutive X-class flares. This region devel- (2000) have discussed this theoretical behav- oped on the eastern side of the solar disk, and, ior of the flux rope. as it passed close to the center of the solar Subsequently, when the flux rope passes a disk, it produced exceptionally intense flare certain boundary within the photosphere as it activity, providing us with rare and valuable ascends, a sudden transformation from twist data for the study of the relationship between helicity to writhe helicity due to a strong kink the development of the magnetic field config- instability occurs. The model suggests that uration and X-class flare activity. These this phenomenon would manifest itself in the observations have revealed that the sudden rapid rotation and decay of theδ-type decay of the sunspots and the decrease in the sunspots, the incursion of the south pole into magnetic flux began immediately before the the north pole, and the formation of the X-class flares and that at the same time a switch-back structure of the magnetic neutral strongly sheared switch-back-type magnetical- line. A model based on this suggestion is ly neutral line formed, rotating in the process. shown in Fig.14. This model can adequately All of these observations could be explained explain all of the observations (the behavior well in terms of emerging twisted flux ropes. and motion of theδ-type sunspots and the More precisely, it seems clear that a flux rope twisted in the convection zone transformed its twist toward its central portion, forming a twisted knot (corresponding to theδ-type sunspots) as it emerged; when it had emerged to a certain degree, it became unstable and induced consecutive X-class and M-class flares, accompanied by rapid rotational motion, shear motion, and decay of the sunspots. Although the relationship between emerg- ing twisted flux ropes and intense flare activi- ty has been clarified in previous studies (Kurokawa 1987, 1989, 1996; Tanaka 1991; Fig.14 Model of the emerging twisted flux rope (B) Ishii et al. 1998, 2000), this is the first discus- The twist helicity extending over the sion of a flux rope model that explains in entire flux rope transforms into writhe helicity at its center as the flux rope detail the variation of the magnetic field con- emerges (left column). On the right we figuration and the movement of sunspots see the surface of the photosphere and observed immediately before the occurrence where it is broken by the northern-polarity sunspots N 2, N 3, and N 4 and by the of X-class flares. The success of the endeavor southern-polarity sunspots S 2, S 3, and S owes a great deal to our use of the continuous 4, in addition to the positions of the mag- netic neutral line. The temporal variation data obtained by the TRACE and SOHO satel- seen in this model (downwards) coincides lites. well with the observations.

14 Journal of the Communications Research Laboratory Vol.49 No.3 2002 The main aim of the next-generation from point L5, since this Japanese solar observation satellite, SolarB, is will allow us to begin monitoring and analyz- to help elucidate the mechanism of coronal ing the development of an active region in heating by observing minute magnetic field detail before it directly faces the . In this activity with a super-high spatial resolution of context we again stress the importance of 0.2 arc seconds. However, in order to investi- space missions in making short-term and gate the mechanisms of solar activity in the imminent space weather forecasting possible, context of space weather research, another by advancing research on the prediction of solar satellite (which will complement the use solar flares, prominence eruptions, and CMEs. of SolarB) will be required to monitor the gen- eration and development of active regions Acknowledgments from a broader perspective, using a moderate spatial resolution (1 arc second) as it continu- The author would like to express his sin- ously analyzes the magnetic field activity of cere thanks to the solar group members of the entire sun. As we stated at the beginning Kwasan and Hida Observatories and to the of Chapter 2, and as the analysis of the Lockheed solar group for their cooperation in NOAA9026 region we have presented here obtaining such useful observational data. He shows, it is important to observe phenomena also thanks Drs. T. Ishii and T. Wang for this as close to the center of the visible disk as coanalysis of the data. Thanks are also due to possible for analysis of the magnetic field con- Dr. D. Brooks for his improvements to the figuration in the active regions. Thus it will English version. be significant if we can perform continuous

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KUROKAWA Hiroki, Ph. D. Professor, Graduate School of Science, Kyoto University Director of Kwasan and Hida Observa- tions, Kyoto University

KUROKAWA Hiroki 15