Quantitative study of dusty plasma in the polar mesosphere

尘埃等离子体是许多自然环境的关键组成部分，存在于50至80公里高度之间的大气中间层区域。然而，人们对它仍然知之甚少。我们的最终目标是对极地中间层中的尘埃等离子体提供更定量的了解，这些尘埃等离子体的发生无疑与中间层温度的下降和通过增加甲烷浓度而增加的水蒸气含量有关，因此是气候变化和至少部分人类活动的直接后果。具体地说，我们的目标是回答几个关键问题，有关中间层区域，包括：有多少尘埃相对于等离子体密度？粉尘粒径分布如何？尘埃积聚多少电荷作为等离子体温度的函数？考虑到尘埃的大小应该根据浮力在重力作用下自行分类，那么尘埃密度与高度的关系是什么呢？由于流星的解体，尘埃自然地出现在上层大气中，典型的尺寸为~1-50纳米。在高纬度地区，在大气重力波的作用下，夏季半球大气的向上环流导致中层顶区域的极端冷却，温度降至~150K或更低，通常在80- 90km高度附近达到最低。在如此低的温度下，任何存在的水蒸气都会冻结在尘埃上，从而降低自由电子的流动性。此外，被冰覆盖的流星尘埃粒子从周围的等离子体中积累负电荷。剩余的自由低迁移率电子被准静态重尘粒子静电捕获。静电驱动的尘埃和自由电子间距引起雷达波的反射，即所谓的极地中间层夏季回波（PMSE）现象。PMSE很容易被雷达探测到，比如挪威北部的EISCAT设施，作为强大的回波。夜光云（NLC）是PMSE的视觉等效物，是在日落前后由PMSE层下方较大的尘埃颗粒散射阳光造成的。这两种现象都是气候变化的显著例证，即高层大气变冷，低层大气变热。此外，极地中间层冬季回波（PMWE）在雷达数据中出现在较低的高度，在冬季半球约50-80 km。然而，人们对PMWE知之甚少，但越来越多的证据表明它们也是一种尘埃等离子体现象。高频（HF）无线电波加热等离子体增加了自由电子迁移率，从而破坏了等离子体结构，降低了PMSE/PMWE雷达回波幅度。现在在EISCAT使用加热器进行常规操作。PMSE的时间演变和恢复是许多变量的函数，包括离子和电子温度、等离子体和尘埃密度、尘埃粒径和电荷。通过使用多个雷达波长，同时暂时改变尘埃周围的等离子体，我们可以独特地确定尘埃等离子体的特征，并回答上述关键问题。迄今为止，几次火箭发射只给我们提供了尘埃等离子体的简短快照，它们都与人工电离层加热无关。我们建议使用EISCAT上的4个雷达，以及电离层加热器，系统地确定PMSE尘埃等离子体的特征作为高度和时间的函数。我们还将确定同样的尘埃等离子体理论是否适用于PMWE。此外，我们非常幸运地邀请了带电气溶胶释放实验（美国CARE）火箭实验的合作者，这将为我们提供在EISCAT上产生人工尘埃等离子体的独特机会。这个控制实验将为我们对尘埃等离子体的理解提供一个明确的测试。

Dusty plasma is a key constituent in many natural environments and is found in the atmosphere's mesosphere region between 50 and 80 km altitude. However, it remains poorly understood. Our ultimate goal is to provide a greater quantitative understanding of the dusty plasma in the polar mesosphere, the occurrence of which is undoubtedly linked to decreasing mesospheric temperatures and increasing water vapour content via increased methane concentration, and therefore a direct consequence of climate change and at least partly human activity. Specifically, we aim to answer several key questions concerning the mesopause region, including: How much dust is there relative to the plasma density? What is the dust particle size distribution? How much charge does the dust accumulate as a function of plasma temperature? What is the dust density altitude dependence, if any, given that the dust size should sort itself out under gravity according to its buoyancy? Due to the disintegration of meteors, dust occurs naturally in the upper atmosphere with a typical size of ~1-50 nm. At high-latitudes, upward circulation of the atmosphere in the summer hemisphere under the action of atmospheric gravity waves causes extreme cooling in the mesopause region down to ~150K or less, typically minimising around 80-90 km altitude. At these low temperatures, any water vapour present freezes out onto the dust and reduces the mobility of the free electrons. In addition, the ice coated meteoric dust particles accumulate negative charge from the surrounding plasma. The remaining free low-mobility electrons are electrostatically trapped by the quasi-static heavy dust particles. The electrostatically driven dust and free electron spacing causes reflection of radar waves, i.e. the so-called Polar Mesospheric Summer Echoes (PMSE) phenomenon. PMSE are easily detected by radars, such as the EISCAT facility in northern Norway, as powerful echoes. Noctilucent clouds (NLC) are the visual equivalent of PMSE and are caused by scattering of sunlight around sunset by the larger dust particles, which occur below the PMSE layer. Both phenomena are striking examples of climate change where the upper atmosphere cools as the lower atmosphere heats up. In addition, Polar Mesospheric Winter Echoes (PMWE) appear at lower altitudes in the radar data, around 50-80 km in the winter hemisphere. However, much less is known about PMWE but there is increasing evidence that they are also a dusty plasma phenomenon.Heating the plasma by High Frequency (HF) radio waves increases the free electron mobility, which breaks down the plasma structuring and reduces the PMSE/PMWE radar echo amplitude. This is now done routinely at EISCAT using the Heater. The temporal evolution and recovery of the PMSE is a function of many variables, including ion and electron temperature, plasma and dust density, dust particle size and charge. By using multiple radar wavelengths whilst temporarily modifying the plasma surrounding the dust, we can uniquely determine the characteristics of the dusty plasma and answer the key questions given above. To date, a few rocket shots have provided us with only brief snapshots of the dusty plasma, none of them associated with artificial ionospheric heating. We propose to use the 4 radars present at EISCAT, along with the ionospheric Heater, to systematically determine the characteristics of the PMSE dusty plasma as a function of altitude and time. We will also determine whether the same dusty plasma theory is applicable to PMWE. In addition, we are very fortunate to be invited collaborators of the Charged Aerosol Release Experiment (CARE, USA) rocket experiment, which will provide us with the unique opportunity of generating an artificial dusty plasma over EISCAT. This control experiment will provide a definitive test for our understanding of dusty plasma.