Fast shock and Alfvén Waves Acceleration of Protons and Heliums：Formation of H+ / He++ Differential Streams in the Solar Wind

• Main Authors: Cheng-Jia Weng, 翁承嘉
• Other Authors: Lou-Chuang Lee
• Format: Others
• Language: zh-TW
• Published: 2006
• Online Access: http://ndltd.ncl.edu.tw/handle/99302435137919510465

碩士 === 國立成功大學 === 物理學系碩博士班 === 94 === 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 Vaa 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 Vaap = 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 DVap = 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 DVap 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 .