2.空間等離子體

1、1,2.空間等離子體和單粒子的運動、Kinetic Theory 和 MHD 簡介,空間物理學(xué)（I）,Outline:,等離子體的定義Debye長度和等離子體的特性空間等離子體單個帶電粒子的運動回旋波Guiding center motion,磁矩守恒量磁鏡效應(yīng)Trapped particles輻射帶電離層極光的產(chǎn)生磁凍結(jié)效應(yīng)Kinetic theory簡介MHD簡介,2,3,等離子體,等離子體的定義:

2、 由自由電子和自由離子組成的集合體（氣體），其正負電荷數(shù)目相同。在平均作用的意義上，等離子體對外呈現(xiàn)電中性。,等離子體的直接特性： 自由離子和自由電子占主導(dǎo)地位（即帶電粒子所受到的勢能遠小于粒子熱運動的動能），隨機的熱運動克服庫侖勢。,4,http://en.wikipedia.org/wiki/Plasma_%28physics%29,5,See http://en.wikipedia.org/wiki/Plas

3、ma_%28physics%29,Plasma lamp,分離的正離子和電子是否會在電場力的庫侖相互作用下結(jié)合成電中性原子或者分子？頻繁的帶電粒子碰撞對等離子體的影響是什么？,6,什么樣的條件使得等離子體穩(wěn)定存在？,1.保持電中性2.自由粒子的存在3.發(fā)生碰撞的幾率低,7,德拜屏蔽(Debye Shielding),等離子體存在一個特征長度，在此特征長度以外，帶電粒子間庫侖勢場因為屏蔽效應(yīng)迅速減弱。這個特征長度定義為Debye le

4、ngth。,8,Debye Length由溫度和帶電粒子的密度共同決定,9,Debye and Coulomb potential,庫侖勢,德拜勢,10,德拜屏蔽(Debye Shielding),統(tǒng)計物理指出自由粒子在相空間*的分布大致如下：,*相空間指（vx, vy, vz, rx, ry, rz）組成的坐標空間,11,解Poisson方程,12,德拜長度的快速估算,Debye Length in cmTe in kn0 in

5、cm-3,13,研究對象尺寸要求,保持等離子體電中性,14,典型的德拜長度,http://en.wikipedia.org/wiki/Debye_length#Debye_length_in_silicon,15,Debye length vs spacecraft,Spacecraft potential光電子效應(yīng)產(chǎn)生的電荷,16,Debye sphere,德拜球的定義如下：,ND討論了自由帶電粒子的中的“自由問題”,17,,,18

6、,如何聯(lián)系自由帶電粒子和ND？,庫侖勢能遠小于粒子熱運動的動能,考慮極限條件下：,于是：,,19,Plasma frequency,,,+,+,+,+,+,-,-,-,-,-,,,,,Possible electron plasma oscillation,20,單個電子的運動方程,整理后：,Harmonic oscillator equation,21,諧振子方程的特征頻率,等離子體對碰撞的限制,22,Relationship bet

7、ween Plasma Frequency, Debye length, and thermal speed,23,散射問題（碰撞）,和電中性粒子的碰撞和帶電粒子的碰撞,24,碰撞頻率,,Hot Plasma is collisionless,25,研究對象是否是等離子體的判據(jù)總結(jié),Debye shieldingFree charged particleLess collision with neutral,26,等離子體中的量

8、子效應(yīng)？,Heisenberg’s uncertainty principle,,,,LHS:,,The boundary of classical and quantum regime,27,空間中的等離子體,太陽風(fēng)磁層輻射帶電離層,空間等離子體的典型參數(shù),太陽風(fēng)：ne=5 cm-3, Te = 105k, B=5nT磁層--輻射帶： ne=1 cm-3, Te = 107k, B=100-1000nT磁層—磁尾：

9、ne=10-2 cm-3, Te = 105k, B=30nT電離層： ne=105 cm-3, Te = 103k, B=104nT,28,29,Quiz 3,何為“等離子體”？為什么太陽風(fēng)等離子體能夠保持電離狀態(tài)而不會發(fā)生離子和電子的復(fù)合？1AU處太陽風(fēng)質(zhì)子熱速度為35km/s，試推算太陽風(fēng)質(zhì)子溫度。,30,研究等離子體的方法,單粒子運動流體力學(xué)的方法（Magnetohydrodynamics）Kinetic the

10、ory,31,單個帶電粒子的運動,庫侖力和洛侖茲力,32,回旋運動,受洛侖茲力影響的帶電離子的運動方程,,假設(shè)在均勻磁場中,33,Cyclotron Frequency,有磁場存在的等離子體,34,帶電粒子回旋運動的軌跡,Cyclotron Frequency (gyrofrequency),Note:,gyroradius,35,Guiding center and pitch angle(俯仰角),36,Drift motion –

11、 Electric Drift,37,其它幾種漂移運動,Polarization DriftMagnetic Gradient DriftGeneral Force DriftCurvature Drift,38,Guiding Center motion,39,磁矩守恒,Magnetic moment,40,磁鏡效應(yīng),41,磁鏡效應(yīng)推導(dǎo),42,Loss cone,43,Van Allen輻射帶,Trapped charged

12、particles,44,The outer belt,The outer belt consists mainly of high energy (0.1–10 MeV) electrons The large outer radiation belt extends from an altitude of about three to ten Earth radii (RE) or 13,000 to 19,000 kilomet

13、res above the Earth's surface. Its greatest intensity is usually around 4–5 RE.,45,Inner belt,The inner Van Allen Belt extends from an altitude of 100–10,000 km (0.01 to 1.5 Earth radii) Ions energy exceeding 1

14、00 MeV and electrons in the range of hundreds of keV, trapped by the strong (relative to the outer belts) magnetic fields in the region It is believed that protons of energies exceeding 50 MeV in the lower belts at lowe

15、r altitudes are the result of the beta decay of neutrons created by cosmic ray collisions with nuclei of the upper atmosphere. The source of lower energy protons is believed to be proton diffusion due to changes in the m

16、agnetic field during geomagnetic storms,46,地面實驗室模擬的輻射帶,47,木星的輻射帶,48,Aurora,http://en.wikipedia.org/wiki/Aurora_(astronomy),49,Kinetic theory,Distribution function,50,Kinetic theory,Note:,51,等離子體中的Vlasov方程,52,類似Maxwellian

17、分布曲線,53,衛(wèi)星位于地球bow shock以外,衛(wèi)星位于磁尾,,,54,離子束流的速度、壓強和溫度,,T,The 1st order moment,The 2nd order moment,55,典型Flux的范圍,Non-Maxwellian distribution function,Kappa distribution---power law distribution,56,57,MHD簡介,流體力學(xué)的理論在于尋求宏觀參量的演

18、化方程；（宏觀參量指：n(r,t), v(r,t), p(r,t), T(r,t) etc.）；當(dāng)單粒子理論中的粒子所帶電荷、質(zhì)量無限變化時（如：e’=e/M, m’=m/M），等離子體的德拜長度、回旋半徑和回旋頻率等保持不變（可作為流體處理）。,58,連續(xù)性方程,59,磁凍結(jié)（frozen-in condition）,Faraday’s law,60,magnetopause,61,磁壓,等離子體beta常數(shù),62,磁聲速（Alfv

19、en velocity）,Note: sound speed,63,MHD使用范圍,64,地球舷激波周圍離子加速的情況,65,Dynamic of the Earth’s Bow Shock,The dynamic of the Earth’s bowshock creates variations of the localΘBn, which can lead to particle release in turn.,66,