Abstract:After being released in the ionosphere, alkali-metal atoms will be rapidly photoionized by solar UV, producing positive ions and electrons, and forming artificial plasma cloud. Based on a three-dimensional two-species fluid model, considering both the loss of barium atoms due to photoionization and oxidation and the influence of horizontal wind field in the release region, the spatial-temporal evolution of the artificial plasma cloud is discussed. By taking into account the electromagnetic field force, pressure gradient, particle collisions and ion inertia, the ionospheric disturbance effects caused by barium and cesium are compared with each other. The simulation results show that the alkali metal rapidly expands after being released in the ionosphere, and the generated plasma cloud gradually forms an ellipsoidal structure from the inside to the outside under the constraint of magnetic field with considering no wind. Meanwhile, the expanded plasma cloud pushes away the background oxygen ions, forming an oxygen ion density hole in the release center and two symmetrical density bumps on both sides. In the absence of neutral wind, the plasma cloud is dominated by the movement along magnetic field, while considering the background neutral wind, the plasma cloud and background disturbance area will move along the direction of wind, so that the density gradient of plasma cloud becomes steepening on the upwind side. Although the movement of ion cloud across the magnetic field is constrained, the neutrals can pass through the magnetic field freely, so the ion cloud and neutral cloud will separate from each other slowly. Also, the presence of horizontal wind field will make a greater disturbance to the background oxygen ion. By comparing the simulation results of barium and cesium we can see that, qualitatively, the expansion characteristics of Cs+ and Ba+ as well as their effects on the background O+ are similar. Due to the small diffusion coefficient of cesium, the barium cloud expands more rapidly and the coverage area of Ba+ cloud is wider. Because of the large photoionization rate of cesium, the ionization yield of cesium is higher than that of barium when the same mass is released. In addition, the snowplow effect of Cs+ is stronger than that of Ba+, and the oxygen ion density holes and bumps caused by Cs+ are also larger. Keywords:artificial plasma clouds/ neutral diffusion model/ three-dimensional two-species model/ snowplow effect
为探讨金属原子在电离层释放后产生的扰动特性和释放区域主要粒子的时空演化规律, 分别模拟了10 kg钡和铯在300 km高度处的释放, 释放云团的初始特征半径均取2 km, 计算域为X = Y = Z = [–25 25] km. 所有的剖面图都是经过释放中心的平面, 图中的等值图代表粒子数密度分布, 带电粒子的速度用矢量场表示(图中黑色箭头). 33.2.1.无中性风场时 -->
3.2.1.无中性风场时
图2和图3描述了300 km高度释放10 kg Ba原子, 释放后5, 30 和200 s钡离子及背景氧离子的数密度分布. 不考虑背景风速时, 钡离子云的早期密度分布为球对称的, 由于磁场的存在, 钡离子云在垂直磁场方向上的膨胀受到束缚(图2), 而在沿磁场方向, 由于钡离子云在密度梯度作用下的运动不受限制, 离子云团逐渐沿着磁场方向被拉伸, 逐渐变成椭球状结构(图3). 由于碰撞作用, 钡离子和氧离子的动量相互耦合, 钡离子平行于磁场的动能传递给氧离子, 促使氧离子沿着磁场向两侧运动, 不断将中心处的氧离子往外传送, 形成氧离子密度“空穴”; O+到达两侧后, 又受到背景热压梯度的作用, 阻止了其向外的膨胀, 最终造成氧离子在两侧的堆积, 产生两个密度尖峰, 即所谓的“扫雪机效应”[14], 氧离子空穴处的形态分布与钡离子云团形态是一致的. 5 s时, 空穴区的氧离子密度比背景降低了27.6%, 密度凸起处比背景高14.3%, 钡离子的数密度峰值达到1.332 × 107 cm–3. 图 2 无中性风场时, 300 km高度释放10 kg钡后钡离子和氧离子的离子数密度分布(x-y平面) (a) Ba+, t = 5 s; (b) Ba+, t = 30 s; (c) Ba+, t = 200 s; (d) O+, t = 5 s; (e) O+, t = 30 s; (f) O+, t = 200 s Figure2. Density distribution of Ba+ and O+ (in x-y plane) after 10 kg barium released at 300 km while no neutral wind is considered: (a) Ba+, t = 5 s; (b) Ba+, t = 30 s; (c) Ba+, t = 200 s; (d) O+, t = 5 s; (e) O+, t = 30 s; (f) O+, t = 200 s.
图 3 无中性风场时, 300 km高度释放10 kg钡后钡离子和氧离子的粒子数密度分布(x-z平面) (a) O+, t = 5 s; (b) O+, t = 30 s; (c) O+, t = 200 s; (d) Ba+, t = 5 s; (e) Ba+, t = 30 s; (f) Ba+, t = 200 s Figure3. Density distribution of Ba+ and O+ (in x-z plane) after 10 kg barium released at 300 km while no neutral wind is considered: (a) O+, t = 5 s; (b) O+, t = 30 s; (c) O+, t = 200 s; (d) Ba+, t = 5 s; (e) Ba+, t = 30 s; (f) Ba+, t = 200 s.
33.2.2.考虑中性风场的情况 -->
3.2.2.考虑中性风场的情况
图4给出了x方向存在1 km/s的中性风时, 300 km高度释放10 kg钡的模拟结果(x-z平面), 释放中心位于(–15, 0, 0) km处, 由于带电粒子的密度分布在空间上是柱对称的, 这里不再给出x-y平面的图像. 与无风场的结果相比, 中性风场的存在使得释放的中性原子有了一个水平方向的运动, 生成的等离子体云团不再是对称的椭球状结构, 而是一个逆风侧的密度梯度“较陡”, 顺风侧的密度梯度较为“平缓”的不对称结构. 比较图3和图4可以发现, 中性风场存在时, 释放相同质量钡原子在早期对背景氧离子的密度扰动更大, 由于中性云的移动, 扰动区域也有所增加. 5 s时, 空穴区的氧离子密度比背景降低30.7%, 密度凸起处比背景高17.7%, 由于分布区域变广, 钡离子的数密度峰值略微降低, 为1.223 × 107 cm–3. 图 4 存在x方向大小为1 km/s的中性风时, 300 km高度释放10 kg钡后钡离子和氧离子的粒子数密度分布(x-z平面) (a) O+, t = 5 s; (b) O+, t = 30 s; (c) O+, t = 200 s; (d) Ba+, t = 5 s; (e) Ba+, t = 30 s; (f) Ba+, t = 200 s Figure4. Density distribution of Ba+ and O+ (in x-z plane) after 10 kg barium released at 300 km with a neutral wind of 1 km/s in the x direction: (a) O+, t = 5 s;(b) O+, t = 30 s; (c) O+, t = 200 s; (d) Ba+, t = 5 s; (e) Ba+, t = 30 s; (f) Ba+, t = 200 s.
图5中右侧的绿色球表示中性钡原子, 蓝色部分代表钡离子, 虽然钡离子的运动在垂直磁场方向上受到磁场的束缚, 但中性钡原子不受磁场力的约束, 可以自由地穿过磁场线, 因此钡离子云团与中性云团会慢慢分离, 如图5所示, 30 s时离子云和中性云的分离已经较为明显. 中性钡原子在背景中性风的作用下向x轴正方向移动, 由于移动过程中持续的光电离作用, 钡中性云运动后方形成了拉长的离子结构(蓝色部分), 较早产生的钡离子已经沿磁场方向被拉伸(蓝色球左侧沿z方向拉伸的区域). 图 5 钡中性云团(绿色)和离子云团(蓝色)在释放后30 s时的三维分布示意图 Figure5. Three-dimensional density distribution of barium neutral cloud (green sphere) and ion cloud (blue sphere) at 30 s after release
33.2.3.铯释放结果 -->
3.2.3.铯释放结果
10 kg Cs原子释放后5, 30 和200 s铯离子及背景氧离子密度分布的演化过程如图6所示, 除释放物质外, 其他释放参数与 3.2.2节中一致. 虽然铯由于电离势较低容易发生热电离, 但在300 km高度及有日照的条件下, 其自身的热电离与光电离相比是微不足道的, 因此这里没有对铯的热电离过程进行探讨. 定性地说, Cs+和Ba+的膨胀特性以及对背景O+的扰动效应是类似的, 虽然铯比钡原子的质量小, 但铯原子的极化率较钡原子要大, 因而铯的扩散系数较小, 钡云的膨胀更为迅速, 钡离子云团的覆盖区域更广; 同时, 由于铯的光电离率较大, 释放相同质量下铯的离子产率更高, 5 s时铯离子云的峰值数密度达到2.248 × 107 cm–3, 是相同条件下产生的钡离子云密度的近两倍; 此外, Cs+-O+的碰撞频率较Ba+-O+更大, Cs+的扫雪机效应比Ba+扫雪机更强, 氧离子密度空穴和凸起更大, 30 s时钡和铯释放产生的背景氧离子的最大扰动分别为74.2%和75.1%. 图 6 存在x方向大小为1 km/s的中性风时, 300 km高度释放10 kg铯的粒子数密度分布(x-z平面) (a) O+, t = 5 s; (b) O+, t = 30 s; (c) O+, t = 200 s; (d) Cs+, t = 5 s; (e) Cs+, t = 30 s; (f) Cs+, t = 200 s Figure6. Density distribution of Cs+ and O+ (in x-z plane) after 10 kg cesium released at 300 km with a neutral wind of 1 km/s in the x direction: (a) O+, t = 5 s; (b) O+, t = 30 s; (c) O+, t = 200 s; (d) Cs+, t = 5 s; (e) Cs+, t = 30 s; (f) Cs+, t = 200 s.
图7给出了背景中性风场存在下10 kg铯和钡在300 km高度释放后, 生成的等离子体云团密度的最大值以及背景氧离子密度凸起最高点的值随时间的变化情况对比. 释放初期, 伴随着持续的光电离, 钡离子和铯离子数密度持续增加, 之后由于扩散作用逐渐下降, 铯离子的数密度最大值比钡离子大, 但随时间的变化趋势是一致的; 随着等离子体云团密度的增加, 增强的密度梯度使得越来越多的氧离子被推开, 两侧密度凸起处的值不断增大, 随后, 密度梯度开始下降, 氧离子扰动也逐渐回复. 图 7 生成的等离子体云团的密度最大值(a)和背景氧离子的最大扰动值(b)随时间的变化 Figure7. The maximum density of artificial plasma cloud (a) and the maximum disturbance of background oxygen ion (a) versus time.