Plasma convection in the Earth's polar region under the condition of IMF Bz=0 was investigated by using Akebono electric field (EFD) data. The patterns of convection were different between cases with and without significant y-component of IMF. When IMF had large By, the convection patterns were similar to those of negative Bz cases. When By vanished and thus IMF was parallel to the solar wind velocity, the velocity of convecting plasma was small and no significant pattern appeared.
It suggests that IMF By not only modify convection patterns in certain IMF Bz but play an significant role in solar wind- magnetosphere inter- action.
Acknowledgment: IMP-8 data were supplied by Dr. R.Lepping (NASA/GSFC).

Introduction

A Japanese Interplanetary Spacecraft Sakigake found peculiar interplanetary objects whose magnetic fields deviate largely from the Archimedian field in the Parker wind. In each of the objects, magnetic field was highly variable but restricted in a certain plane at least locally in the vicinity of the spacecraft, thus they were called ``planar magnetic structures" (Nakagawa et al., 1989). The planar field with variation of the direction of the field suggests that the spacecraft encountered a part of a large scale structure of sheared magnetic field. They had higher plasma density and slower bulk speed than those of average solar wind.

The ``planar magnetic structures" were enigmatic because they had no relationship with prominent phenomena on the solar surface such as flares, prominences nor dark filament disappearances (Nakagawa, 1993). Sometimes they were observed recurrently at an interval of 27 days. Since the discovery of active region expansions by Yohkoh (Uchida et al., 1992), the question has arisen whether the ``planar magnetic structures" might be interplanetary extensions of the active region expansions. A systematic search of possible relationship was made by using data obtained by Sakigake and Yohkoh spacecraft during the period from January 6 to November 11, 1993, when there were not too many active regions on the visible solar hemisphere.

Detection of Interplanetary Planar Magnetic Structures

A ``planar magnetic structure" is detected in interplanetary magnetic field data by using two criteria for the field component normal to the plane
B_n < 0.1 B
and for the standard deviation of the magnetic field vector
sigma_B > 0.7 B,
set to pick up planar and variable magnetic field vectors. Here B is the magnitude of the magnetic field throughout each event.

According to the criteria, five typical ``planar magnetic structures" were found on May 18, June 3, June 11, July 1, and July 15.

Solar Sources of Planar Magnetic Structures

They were traced back onto the Sun on the assumption that the solar wind speed increased linearly within the first 3 solar radii, from 10 km/s up to the real value measured at 1 AU. The solar wind plasma was assumed to be launched radially outward from the Sun, although it is not always the case for real solar wind. As the result, sources of 4 events coincided with NOAA low-latitude active regions which appeared within 5 degrees from the equator. The 80% coincidence is extremely high with respect to accidental coincidence, 7%, of Sakigake windows of solar wind observation with active regions.

In the Yohkoh Soft X-ray images at or near the calculated time of the launch of each object, we found expansion activities at the active regions near the central part of the Sun's disk from which ``planar magnetic structures" were supposed to be launched. Very faint loop-like structures were often observed, and the loops, especially their outer portions, seemed to expand repeatedly into interplanetary space. On the source of the interplanetary object which was not traced back to any NOAA active region, we found expansion activity of faint loops. No flares were observed in any of the sources. These observations support that the active region expansions can extend into interplanetary space to be observed as ``planar magnetic structures" at 1AU.

Bibliography

Nakagawa, T. (1993) Solar source of interplanetary planar magnetic structures, Solar Physics, 147, pp.169--197.
Nakagawa,T., Nishida,A., and Saito,T.(1989) Planar magnetic structures in the solar wind, J. Geophys. Res., 94, pp.11761--11775.
Uchida,Y., et al.(1992) Continual Expansion of the active-region corona observed by the Yohkoh soft X-ray telescope, Publ.Astron. Soc. Japan, 44, 5, pp.L155--L160.

( published in Magnetodynamic Phenomena in the Solar Atmosphere - Prototypes of Steller Magnetic Activity , edited by Y.Uchida, T.Kosugi, and H.S.Hudson, Kluwer Academic Publishers, Dordrecht, p503-504, 1996. )

Non-Parker type interplanetary objects, planar magnetic structures, have been examined in relationship to 'active region expansions' discovered in the Yohkoh soft X-ray observations. During the quiet period of the Sun from January 6 to November 11, 1993, solar sources of 5 typical planar magnetic structures, which were detected by Sakigake, were searched for in the Yohkoh soft X-ray telescope data. Loop structures were seen to expand outward above the corresponding active regions in the probable sources.

( published in Solar Wind Eight, edited by D.Winterhalter, J.T.Gosling, S.R.Habbal, W.S.Kurth, M.Neugebauer, AIP Press, New York, p465-468, 1996. )

通常の太陽風磁場と異なり，Parkerのモデルで説明することのできない平面状磁場構造の発生域を調べると，近年「ようこう」軟Ｘ線画像で発見された太陽表面の「活動域膨張」に一致することがわかってきた。従来，フレアなどの場合を除いては，磁場が強いためプラズマを放出しないと考えられてきた活動域であるが，惑星間空間への質量流出に寄与していることが示唆される。
一方，すべての活動域のプラズマが異常な太陽風として検出されるわけではない。93年１月から１１月の間に 1 AU 付近で「さきがけ」によって観測された太陽風のうち発生源が活動域に相当するものが１６例あったが，平面状磁場構造（Bn<0.1B,σB>0.7B）が５例，その他の異常磁場（スパイラル方向からずれた磁場が85%以上の時間にわたって観測された）が４例で，残りの７例（44%）については，磁場・プラズマ密度とも平均的な太陽風と違いがなく，活動域のプラズマが引き出されたとは考えにくい。
　本研究では，プラズマと磁場が惑星間空間へ引き出される活動域と惑星間空間で検出されない活動域の違いを周辺の磁場構造との関連から調べる。

Tomoko Nakagawa
Release of Active Region Plasma into Interplanetary Space

The interplanetary objects, 'planar magnetic structures', whose magnetic field cannot be explained by Parker's model field, have been identified as the active region expansions found by Uchida et al.[1992] in the Yohkoh soft X-ray images, from which loop-like structures of active-region corona expand outward repeatedly at speeds of a few to a few tens of km/s even in the absence of any sizable flares. The active region expansions are expected to play an important role in mass loss from the Sun. On the other hand, not all the active regions were observed as planar structures in interplanetary space. In 16 Sakigake observations of solar wind plasma whose solar sources coincided with active regions, 9 had non-spiral magnetic field including planar structures, high ion density or both of them, but the other 7 (44%) exhibited no significant difference from average solar wind with spiral field. This study attempts to find difference between active regions which expand into inter- planetary space and which do not.

　平面状磁場構造は、磁場ベクトルがある平面に平行を保ったまま数 時間にもわたって激しく変化する現象で、太陽から 0.8-1 AU の惑星 間空間で「さきがけ」によって初めて発見された。 85-93年の「さき がけ」、83-86年のISEE3の観測を見る限り、この現象はバルク速度の 勾配による太陽風の圧縮域とは無関係で、惑星間空間で形成されたと いうより、太陽表面の磁場構造が 1AUまで引き出されたものと考えら れる。一方、Neugebauer等は78-82年のISEE3の観測から、平面状磁場 構造の多くはコロナルマスイジェクションとその前面の衝撃波の間の 圧縮・ドレーピングによって形成されたと考えた(JGR98,1993)が、 これでは速度勾配と無関係であった観測を説明することはできない。 近年Crooker等はUlyssesの観測例から、速度勾配が無い場合でも圧力 勾配が周辺磁場を圧縮して平面状構造を作る可能性を示唆した(JGR, in press,1995)。
　本研究は、このCrookerの考えが「さきがけ」の平面状構造を説明 できるかどうかを調べた。93年に観測された、速度勾配のない５つの 平面状構造のうち、２つはガス圧と磁気圧の和が変化していたが、他 ３例には有意な変化を見ることはできなかった。右上図は圧力勾配の ある例、右下図は圧力勾配の無い例である。（電子の圧力は測定され ないため、イオンの圧力およびそれを２倍したものを示してある。） これらより、圧力勾配は平面状構造の本質的な生成機構とは考え難い。

Is Pressure Gradient a Cause of Planar Magnetic Structures in Interplanetary Space?
Tomoko Nakagawa

Pressure gradient in the solar wind is examined as a candidate cause of 'planar magnetic structures' whose magnetic field is highly variable but parallel to a plane. In five of Sakigake planar magnetic structures with no bulk-speed gradient, two werein the regions of pressure gradient but the others were not.

Interplanetary planar magnetic structures, which are distinctive features of interplanetary magnetic field that is highly variable but parallel to a plane, are re-examined to test a model of flattened, intertwined flux tubes of high-beta plasma (Crooker et al., JGR 101, 2467, 1996).
The idea of flux tubes or ropes seems to be in good accordance with the correlation found between interplanetary planar magnetic structures and expansions of photospheric loop structures in active regions observed by Yohkoh, because magnetic loops are likely to be twisted by the motion of their footpoints.
The model flux tubes with various degrees of helicity account for the highly variable magnetic field observed in planar magnetic structures. Flattening the tubes by compression effect or by kinematic distention is essential for explaining the magnetic field vectors that were all parallel to a plane in each of the structure, although compression was not always detected.
High-beta plasmas in planar magnetic structures are likely to cause kinematic distention as suggested in the model, but in high-beta plasma, the structure of the model field cannot be force-free as is often used to model low-beta magnetic cloud. The magnitude of the core field of the rope structure should be weaker than that of force-free field, which may cause kink instability.

　平均的なspiral方向から外れた惑星間空間磁場(IMF)は、太陽と太陽風をつなぐ手がかりを与えてくれる。　平面状磁場構造 (planar magnetic structure) は、太陽から0.8-1.0AUの惑星間空間で「さきがけ」によって発見された非spiral磁場構造の一つであり、IMFが数時間にわたりある平面に平行となっていながら、不規則に激しく変化する現象である。この平面状磁場構造の３次元構造を考えることは、その発生域と考えられる太陽表面の活動域の磁場構造と比較する上でも重要である。
本研究では、平面状磁場構造をflux ropeと呼ばれる形状の磁場で説明できるかどうか検討する。flux ropeは、中心の高い圧力を外縁のらせん状磁場のピンチ効果で抑える構造であり、ガス圧が磁気圧に比べて小さい場合は、ローレンツ力の生じない force-free fieldという安定な構造となることが知られている。
　flux ropeモデルは、平面状磁場構造と太陽面の活動域膨張との関係や平面状磁場構造中の高βプラズマを説明する上でも好都合である。活動域のループ状の磁場はその足元がねじられていると考えられるし、らせん状の磁場は内部にプラズマを保持する性質があるからである。しかし、flux ropeは、探査機がその中心軸を通過するような特殊な場合を除いては平面に平行な磁場として観測されないこと、単一の flux ropeでは平面に平行な方向のほとんどを覆う広範な磁場方向変化を説明できないこと、高βプラズマ中では安定な構造とはいえないことから、これまでは平面状磁場構造のモデルとしては不適当と考えられてきた。
　近年、圧力勾配などによって平たくつぶれたflux ropeが多数集まれば平面状の磁場として観測され得るという提案がCrooker他(1996)によってなされた。本研究では、1993年に「さきがけ」によって観測された典型的な平面状磁場構造をこのモデルに当てはめてみた。個々のflux ropeを通過中は磁場の回転方向が変らないことを使うと、回転方向の変化からflux ropeの境界を推定でき、個々のflux ropeのピッチ角やflux ropeの総数が概算できる。この方法によれば、個々のflux ropeを通過中に観測される磁場の回転は大きくなく、ひとつの平面状磁場構造には20本以上のflux ropeが含まれるという結果が得られた。平面状磁場構造中の磁場の回転は、らせん状の形状のためというよりもflux ropeの軸がさまざまな方向を向いているためと考えられる。

Examination of a flux-rope model of planar magnetic structures in the solar wind
Tomoko Nakagawa

Interplanetary planar magnetic structures, which are distinctive features of interplanetary magnetic field that is highly variable but parallel to a plane, are re-examined to test a model of flattened, intertwined flux tubes of high- beta plasma (Crooker et al., JGR 101, 2467, 1996). In order to reproduce a planar magnetic structure, a large number of flux ropes with small helicity are required. Variation of field direction is mainly due to variation of orientation of rope axes, rather than to field rotations in each of ropes.