Abstract:The research on space transmission characteristics of terahertz wave is of great significance for the application of terahertz wave in space. In order to study the transmission characteristics of terahertz wave in sand and dust storm weather, according to the lognormal distribution of dust particle sizes, Mie scattering theory and Monte Carlo method are used to analyze the attenuation characteristics of six dry sand modes of sand and dust storm in different regions of China in a frequency band of 1–10 THz, and the relationship of the extinction parameters and attenuation rate to the frequency is given. The results show that with the increase of frequency, the attenuation rate of 1–10 THz terahertz wave first increases and then decreases. Different mode of sand and dust storm leads to different frequency range of strong attenuation of terahertz wave. In order to analyze the influence of sand dust particle moisture content on terahertz wave propagation attenuation, the relationship of three efficiency factors to water content of sand dust particles with different sizes is calculated. The results show that the influence of water content on extinction is different from that of the particle size. Monte Carlo method is used to calculate the attenuation of terahertz wave by sand and dust storm in two kinds of wet sand modes, and the relationship of the attenuation rate and water content to the frequency is given, the results are compared with those from the dry sand mode, showing that the albedo of wet sand mode is obviously lower than that of dry sand mode with the same size distribution. The absorption of wet sand particles increases with water content increasing. The extinction of wet sand and dust storm results from scattering and absorption. With the increase of water content in sand particles, the frequency band with strong attenuation of terahertz wave by wet sand and dust storm moves toward low frequency. When the water content is less than 5%, the attenuation rate of terahertz wave increases significantly with the increase of water content. Sand and dust storms with higher humidity have a greater influence on the transmission attenuation of terahertz wave. Keywords:terahertz wave/ sand and dust storm/ Monte Carlo/ attenuation rate
根据以上分析, 选取能见度Vb = 1 km, 频率范围f 为1—10 THz, 对每一模态的沙尘暴, 首先根据(6)式、(8)式和表1计算出沙尘粒子数密度N0, 再根据Mie理论和(7)式、(12)式和(14)式分别计算出不同频率相应的消光系数μ、反照率ω 及不对称因子g. 积分计算时, 取沙粒半径r的范围为 0.1—150 μm, 半径间隔dr = 0.3 μm. 从计算式可以看出, 3个消光参量与群粒子尺度分布密度函数p密切相关, 对于同一频率的太赫兹波, 由于不同模态的沙尘暴群粒子尺度分布不同, 3个消光参量必然不同. 图4(a)、图4(b)和图4(c)是基于Mie散射理论及沙粒尺度分布函数计算得到的不同模态沙尘暴的3个消光参量随太赫兹波频率的变化情况, 图4(d)是基于蒙特卡罗方法计算得到的衰减率A随太赫兹波频率的变化情况, 计算时取光子数N = 106. 图 4 六种干沙模态沙尘暴对太赫兹波的衰减与频率的关系 (a) ω vs. f ; (b) g vs. f ; (c) μ vs. f ; (d) A vs. f Figure4. Relationship of THz wave attenuation caused by six dry sand and dust storms to frequency: (a) ω vs. f ; (b) g vs. f ; (c) μ vs. f ; (d) A vs. f .
选取频率分别为1, 2, 3, 5和8 THz的太赫兹波, 在19 ℃时, 水在这些频率处的复介电系数分别为4.06 + i2.26, 3.70 + i1.66, 3.53 + i1.69, 2.68 + i1.81和2.25 + i0.94[20], 首先根据(5)式计算不同含水量的湿沙粒子的等效复介电系数, 其余计算过程与第3节干沙模态类同. 在能见度Vb = 1 km的条件下, 计算得到两种湿沙模态沙尘暴对几种太赫兹波的消光参量、衰减率与沙粒含水量的关系如图5和图6所示. 图 5 黄土沙尘暴对太赫兹波的衰减与含水量的关系 (a) ω vs. p; (b) g vs. p; (c) μ vs. p; (d) A vs. p Figure5. Relationship between THz wave attenuation and water content of loess sand and dust storms: (a) ω vs. p; (b) g vs. p; (c) μ vs. p; (d) A vs. p.
图 6 海岸沙尘暴对太赫兹波的衰减与含水量的关系 (a) ω vs. p; (b) g vs. p; (c) μ vs. p; (d) A vs. p Figure6. Relationship between THz wave attenuation and water content of coastal sand and dust storms: (a) ω vs. p; (b) g vs. p; (c) μ vs. p; (d) A vs. p.
在能见度Vb = 1 km的条件下, 选取1—10 THz范围的太赫兹波频率, 采用与上类同的方法, 计算得到含水量不同时(5%, 10%, 20%, 30%), 两种湿沙模态沙尘暴对太赫兹波消光参量、衰减率与频率的关系如图7和图8所示, 为了与干沙相比较, 还同时在图中呈现了干沙(0%)的计算结果. 图 7 不同含水量的黄土沙尘暴对太赫兹波的衰减与频率的关系 (a) ω vs. f ; (b) g vs. f ; (c) μ vs. f ; (d) A vs. f Figure7. Relationship of THz wave attenuation caused by loess sand and dust storms with different water content to frequency: (a) ω vs. f ; (b) g vs. f ; (c) μ vs. f ; (d) A vs. f .
图 8 不同含水量的海岸沙尘暴对太赫兹波的衰减与频率的关系 (a) ω vs. p; (b) g vs. p; (c) μ vs. p; (d) A vs. p Figure8. Relationship between THz wave attenuation caused by coastal sand and dust storms with different water content to frequency: (a) ω vs. p; (b) g vs. p; (c) μ vs. p; (d) A vs. p.