摘 要
在1AU處的高速太陽風有以下的重要特性：(1)高速太陽風的速度將近800 km/s；(2)質子(H+)在垂直磁場方向的溫度較平行磁場方向的溫度高；(3)氦離子(He++)的數目約佔高速太陽風組成的5 %；(4)次要離子(含He++ 離子)與質子有著將近相同的熱速度；(5) He++ 和H+ 離子的流速差Vap= 0.1 ~ 0.9 VA0，其中VA0為Alfvén速度，Vap = | Va-Vp |，而Va及Vp分別為He++ 離子及H+離子與磁場平行的流速。傳統的加速機制是以Alfvén波對He++ 離子做共振加速。我們的模擬研究發現，這個Alfvén波的加速機制並不能有效地產生高速太陽風中的速度差Vap = 0.1 ~ 0.9VA0。本研究認為流速差的產生是在太陽附近(r≦5R⊙)，先由快震波(fast shock)對H+ 與He++ 的不同加熱及加速所產生。利用快震波的加速機制[Lee and Wu, 2000]及混合粒子碼的數值模擬，結果所得在太陽表面附近之H+ 與He++ 離子的流速差Vap約為0.13VA0~200 km/s，而當此H+ 與He++ 離子行進至地球附近時，Alfvén波之速度逐漸減為VA0 =40~ 60
km/s，此過程中之H+ 與He++ 離子的流速差在太陽風裡不減少，則在地球軌道附近將變為Vap= (3~5)VA0。我們再利用Alfvén波對粒子的迴旋共振理論，設定共振作用還沒開始前，H+ 與He++ 離子之間具有大於一個Alfvén速度的流速差Vap，且分別假設兩種不同的初始條件，一是模擬開始時就有Alfvén波的存在，另一則研究作用還沒開始前並沒有Alfvén波的存在，由於粒子與質子的平均速度差Vap>1.0VA0，且因電漿體的不穩定而引發出Alfvén波來迴旋共振。模擬結果顯示作用後的流速差
Vap將下降至小於一個Alfvén速度，此結果可符合太空船Ulysses的觀測Vap = 0.1 ~ 0.9VA0。

At 1AU, the high-speed solar wind has the following characteristics: (a) the velocity of high-speed solar wind is nearly 800km/s；(b) the temperature of protons in the perpendicular direction to magnetic field is larger than in the parallel direction；(c) the number of the helium ions He++ is approximately 5 of the high-speed solar wind；(d) the secondary ions (including the He++ ions) have a thermal velocity similar to the protons；(e) the differential streaming velocity between the He++ and H+ ions are Vap = 0.1 ~ 0.9 VA0 . Here VA0 is the Alfvén speed, Vap = | Va-Vp |, and Va and Vp are respectively the parallel velocities of the He++ and H+ ions. The traditional acceleration mechanism is the cyclotron resonant acceleration of helium ions by the Alfvén waves. Our simulations show that the Alfvén wave acceleration mechanism cannot effectively produce the differential streaming velocity Vap = 0.1 ~ 0.9 VA0 in the high-speed solar wind. This study proposes that the differential streams are produced nearby the sun ( r≦5R⊙). Firstly fast shocks produce the differently heating and acceleration of H+ and He++ ions. By using fast shock acceleration mechanism [Lee and Wu, 2000] and hybrid code simulations, the differential streaming velocity are approximately Vap = 0.13VA0 ~ 200 km/s near the solar surface. Secondly, the H+ and He++ ions move torward the Earth, and at this time the Alfvén speed in the solar wind is gradually reduced to VA0 = 40 ~ 60 km/s. If the differential streaming velocity maintains the same value, the differential streaming velocity would become Vap = (3~5)VA0 near the Earth's orbit. However, the ion-cyclotron resonant interactions between Alfvén waves and particles during the propagation of solar wind protons and helium ions may reduce the differential streaming velocity. The Alfvén waves may in the solar wind or produced by instabilities when Vap VA0. We simulate cases where the initial differential streaming velocity between H+ and He++ ions are larger than the Alfvén speed. We consider two kinds of initial wave conditions: at t = 0, the Alfvén waves are present for case(A) and no Alfvén wave for case(B). Because the Vp is larger than the Alfvén speed, and plasma instability can lead to the production of the Alfvén waves. After the interaction, the differential streaming velocity between the H+ and He++ ions drop to a value smaller thanthe Alfvén velocity, and the results can conform to the spacecraft Ulysses's observations of Vap =
0.1 ~ 0.9 VA0 .

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