研究者詳細 - 藤木　謙一

論文 - 藤木　謙一

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Streamer belt and chains as the main sources of quasi-stationary slow solar wind 査読有り

Eselevich, M., V. Eselevich, and K. Fujiki

Solar Physics 240 巻 ( 135-151 ) 2007年

記述言語：英語掲載種別：研究論文（学術雑誌）

It has been known that periods of the low geomagnetic activity in near-earth environment coincide with arrival time of the “slow" solar wind (SW) at Earth's orbit. In this connection, sources of the “slow" SW on the Sun are significant issue in the space weather problem. In this work synoptic maps of white-light coronal brightness from SOHO/LASCO C2 and distributions of solar wind velocity obtained from interplanetary scintillation were studied. Regions with velocity V ≈ 300-450 km s-1 and increased density N > 10 cm-3, typical of the “slow" solar wind originating from the belt and chains of streamers, are shown to exist at Earth's orbit, between the fast solar wind flows (with a maximum velocity Vmax ≈ 450-800 km s-1). The belt and chains of streamers are the main sources of the “slow" solar wind. As the sources of “slow" solar wind, the contribution from the chains of streamers may be comparable to that from the streamer belt.

Tracking heliospheric disturbances by interplanetary scintillation 査読有り

Tokumaru, M., M. Kojima, K. Fujiki, and M. Yamashita

Nonlin. Processes Geophys. 13 巻頁： 329-338 2006年

Reconstructed global feature of interplanetary disturbance for the full-halo coronal mass ejection event on 1999 September 20 査読有り

Tokumaru, M., M. Yamashita, M. Kojima, K. Fujiki, and T. Nakagawa

Adv. Space Res. 38 巻頁： 547-551 2006年

Interplanetary consequences caused by the extremely intense solar activity during October-November 2003 査読有り

Tokumaru, M., M. Kojima, K. Fujiki, M. Yamashita

J. Geophys. Res. in press 巻頁： doi:10.1029/2004JA010656 2005年

*Relation between solar wind velocity and properties of its source region 招待有り査読有り

Fujiki, K., M. Hirano, M. Kojima, M. Tokumaru, D. Baba, M., Yamashita, K. Hakamada

Advances in Space Research 2005 巻頁： doi:10.1016/i.asr.2005.05.057 2005年

担当区分：筆頭著者記述言語：英語

Different kinds of coronal holes are sources of different kind of solar winds. A successful solar wind acceleration model should be able to explain all those solar winds. For the modeling it is important to find a universal relation between the solar wind physical parameters, such as velocity, and coronal physical parameters such as magnetic field energy. To clarify the physical parameters which control the solar wind velocity, we have studied the relation between solar wind velocity and properties of its source region such as photospheric/coronal magnetic field and the size of each coronal hole during the solar minimum. The solar wind velocity structures were derived by using interplanetary scintillation tomography obtained at Solar-Terrestrial Environment Laboratory, Japan. Potential magnetic fields were calculated to identify the source region of the solar wind. HeI 1083 nm absorption line maps obtained at Kitt Peak National Solar Observatory were used to identify coronal holes. As a result, we found a relation during solar minimum between the solar wind velocity and the coronal magnetic condition which is applicable to different kind of solar winds from different kind of coronal holes.

Resolving the enigmatic solar wind disappearance event of 11 May 1999 査読有り

Janardhan, P., K. Fujiki, M. Kojima, M. Tokumaru, and K. Hakamada

Journal of Geophysical Research 110 巻頁： 10.1029/2005JA010535 2005年

記述言語：英語掲載種別：研究論文（学術雑誌）

On 11 and 12 May 1999, the Earth was engulfed by an unusually low-density (<1 cm^-3) and low-velocity (<350 km s^-1) solar wind for a period of over 1 day. Extensive studies of this unusual event that occurred during Carrington rotation 1949 (CR1949), using both ground-based and space-based in situ observations, have not as yet been able to identify the cause or the solar source of this event. Using solar wind velocity measurements from the four-station IPS observatory of the Solar-Terrestrial Environment Laboratory (STEL), Toyokawa, Japan, we investigate the structure of the solarwind in May 1999 during CR1949. IPS observations from STEL were used to make tomographic velocity maps to identify and delineate the extent and morphology of the stable solar wind flows during CR1949 in the vicinity of the Earth. Combined with in situ measurements of the interplanetary magnetic field (IMF), potential field computations of the solar magnetic fields in the period, and HeI 10830A° observations of coronal hole boundaries during CR1949, we have identified the source region of the unusual flows and have shown that the flow responsible for the "disappearance event" was a stable unipolar flow originating in the vicinity of a large midlatitude active region AR8525, located at 18N and between heliographic longitudes 280 and 300. Earlier workers have speculated that such events may be caused by the large-scale restructuring of the solar magnetic field at the maximum of each solar cycle. However, by identifying the solar source and nature of this event, we believe that at least in this particular case, the association with global, large-scale solar phenomena like the periodic 11-year solar polar field reversal is most likely to be coincidental.

Correlation between expansion rate of the coronal magnetic field and solar wind speed in a solar activity cycle 査読有り

Hakamada, K., M. Kojima, T. Ohmi, M. Tokumaru, K. Fujiki

Solar Physics 227 巻頁： 387-399 2005年

Origin of the slow solar wind 査読有り

Ohmi, T., M. Kojima, M. Tokumaru, K. Fujiki, and K. Hakamada

Adv. Space Res. 33 巻頁： 689-695 2004年

記述言語：英語掲載種別：研究論文（学術雑誌）

The origin of the slow solar wind is an interesting subject on its own, and is also important in relation to the origin of the solar wind including the fast solar wind, especially if the slow wind originates directly from a coronal hole. We have been studying the solar wind whose speed is comparable to, or slower than, that measured in the heliospheric plasma sheet. We derived solar wind velocity distribution maps on the source surface using an interplanetary scintillation tomography analysis, and low-speed regions in them were investigated in relation to a coronal hole using potential field magnetic field lines.
We found the slow solar wind which originated from an equatorial coronal hole located in the vicinity of active regions and also from a polar coronal hole which was about to disappear at the solar activity maximum. The properties of the slow solar wind from an equatorial coronal hole were investigated using spacecraft in situ measurements. Magnetic polarity in this wind was uniform, as expected from a potential field analysis. The helium relative abundance was as large as the fast wind, and variances of density, velocity, and helium abundance were as small as the fast wind from a large coronal hole. In spite of the small equatorial coronal hole origin, the density and ion freeze-in temperature were as large as observed for the slow wind in the heliospheric plasma sheet.
This study clearly showed one of the origins of the slow solar wind and solved the age-old puzzle as to why a potential neutral line does not tend to pass through the locus of a low-speed stream. We think this work provides crucial information to reveal what determines the acceleration efficiency of the solar wind: physical properties in the coronal base or/and flux expansion.

Fast solar wind after the rapid acceleration 査読有り

Kojima, M., A. R. Breen, K. Fujiki, K. Hayashi, T. Ohmi, and M. Tokumaru

J. Geophys. Res. 109 巻頁： (10.1029/2003JA010247) 2004年

記述言語：英語掲載種別：研究論文（学術雑誌）

Since Grall et al. (1996) reported the acceleration profile of the polar solar wind near the sun region, the acceleration of the polar wind has been thought to be rapid. However, their velocities measured with interplanetary scintillation (IPS) method have a large velocity spread with some measurements being over 1000 km/s. Coles and Harmon (2001) have shown that density fluctuations produced by oblique Alfve´n waves can cause a large overestimation of flow velocities in the IPS measurement especially within 20 Rs. They attributed the wide spread of radial velocities obtained in IPS measurements near the Sun to this biasing effect. Therefore, the acceleration profile of the fast solar wind, how it is accelerated and how it approaches asymptotically the final velocity, has not yet elucidated, and it is crucially important for the study of solar wind acceleration.
We have studied the radial dependence of the velocity of high-latitude fast solar wind in the heliocentric distance range of 0.13–0.9 AU. The analysis was made for the IPS observations during 3 years, from 1995 to 1997, when solar activity was minimum. For this study a new IPS tomographic analysis method was developed to obtain velocity distribution maps on two reference spheres at 0.13 and 0.3 AU form IPS observations. From this analysis, average velocities of 770–780 km/s were obtained at distances of 0.13–0.3 AU, which were 19 ± 17 km/s lower than those at 0.3–0.9 AU. The results from this work, taken together with measurements of SOHO/LASCO, EISCAT and MERLIN [Breen et al., 2002], Helios [Schwenn et al., 1978], and Ulysses [McComas et al., 2000], indicate that the fast wind is accelerated almost to its final flow velocity within 20 Rs and a small but not negligible acceleration exists beyond 30 Rs which tends to become smaller at farther heliocentric distances.
This work revealed that the so-called rapid acceleration does not mean the completion of the acceleration within 10 s

Solar cycle dependence of high-latitude solar wind 査読有り

Fujiki, K., M. Kojima, M. Tokumaru, T. Ohmi, A. Yokobe, and K. Hayashi

Solar Wind Ten, AIP Conference Proceedings 679 巻頁： 141-143 2003年

Radial dependence of propagation speed of solar wind disturbance 査読有り

Yamashita, M., M. Tokumaru, and M. Kojima

Solar Wind Ten, AIP Conference Proceedings 679 巻頁： 754-757 2003年

Global structure of interplanetary coronal mass ejections retrieved from the model fitting analysis of radio scintillation observations 査読有り

Tokumaru, M., M. Kojima, K. Fujiki, and M. Yamashita

Solar Wind Ten, AIP Conference Proceedings 679 巻頁： 729-732 2003年

Toroidal-shape interplanetary disturbances associated with the halo coronal mass ejection event on July 14, 2000 査読有り

Tokumaru, M., M. Kojima, K. Fujiki, M. Yamashita, and A. Yokobe

J. Geophys. Res. 108 巻頁： 1220 2003年

Evidences for low-speed streams from small coronal hole 査読有り

Ohmi, T., M. Kojima, K. Hayashi, A. Yokobe, M. Tokumaru, K. Fujiki, and K. Hakamada

Solar Wind Ten, AIP Conference Proceeding 679 巻頁： 137-140 2003年

Polar low-speed solar wind reappeared at the solar activity maximum of cycle 23 査読有り

Ohmi, T., M. Kojima, M. Tokumaru, K. Fujiki, K. Hayashi, and K. Hakamada

Geophy. Res. Lett 30 巻頁： 1409-1412 2003年

Time-dependent tomography of heliospheric features using interplanetary scintillation (IPS) remote-sensing observations 査読有り

Jackson, B.V., P.P. Hick, A. Buffington, M. Kojima, M. Tokumaru, K. Fujiki, T. Ohmi, and M. Yamashita

Solar Wind Ten, AIP Conference Proceedings 679 巻頁： 75-78 2003年

MHD tomography using interplanetary scintillation measurement 査読有り

Hayashi, K., M. Kojima, M. Tokumaru, and K. Fujiki

Journal of Geophysical Research 108 巻頁： (10.1029/2002JA009567) 2003年

Basic results of MHD tomography analysis method for IPS observation 査読有り

Hayashi, K., K. Fujiki, M. Kojima, and M. Tokumaru

Solar Wind Ten, AIP Conference Proceedings 679 巻頁： 144-147 2003年

*How did the solar wind structure change around the solar maximum? - From interplanetary scintillation observation 査読有り

Fujiki, K., M. Kojima, M. Tokumaru, T. Ohmi, A. Yokobe, K. Hayashi, D. J. McComas, and H. A. Elliott

Annales Geophysicae 21 巻頁： 1257-1261 2003年

Solar wind velocity structure around the solar maximum observed by interplanetary scintillation 査読有り

Fujiki, K., M. Kojima, M. Tokumaru, T. Ohmi, A. Yokobe, K. Hayashi, D. J. McComas, and H. A. Elliott

Solar Wind Ten, AIP Conference Proceedings 679 巻頁： 226-229 2003年

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