Filamentary structure in the upper atmosphere

我们研究的主题是北极光，这是天空中令人惊叹的自然光秀，经常在斯堪的纳维亚半岛北部等高纬度地区看到，但很少在英国的纬度地区看到。我们利用极光作为一种诊断手段来发现许多关于地球周围环境的事情，主要是在高层大气中被称为电离层的区域。这种环境是由“等离子体”（电离气体）组成的，通常被称为物质的第四种状态，占宇宙中直接可观测物质的95%以上。然而奇怪的是，在地球生物圈内维持和研究它是非常困难的。高层大气为其研究提供了理想的天然实验室，因为不需要考虑等离子体与容器壁的碰撞。极光的故事始于太阳，太阳是一个连续但非常多变的能量来源，以等离子体流（“太阳风”）的形式影响地球。我们感兴趣的是了解最小尺度的极光结构，以及它们内部的能量变化如何影响大尺度的环境。为了研究极光，我们使用了一种特殊的仪器，它有三台相机同时观察不同的“颜色”。提议的研究是在尽可能高的分辨率下研究非常动态和结构化的极光。该仪器被命名为ASK极光结构和动力学。它被设计用来测量“磁天顶”上一个3度的小圆圈，即沿着地球磁场垂直向上。来自太阳的粒子沿着这些假想的磁力线旋转，当它们与大气中的氧和氮相撞时，就会失去能量。确切的颜色（或光的波长）取决于入射粒子的能量，以及它撞击的分子或原子。ASK相机通过非常精确地测量具有不同物理起源的三种发射的空间和时间，帮助解开这个复杂的过程。我们将把这些光学测量与特殊雷达实验的测量结果结合起来，这些实验旨在使用一种称为干涉测量的技术来测量小于光束宽度的结构，并且位置和高度的精度比迄今为止可能的要高。该雷达成像技术是非相干散射雷达领域的新技术，将成为未来一个名为EISCAT_3D的项目的基石之一。所采用的技术是孔径合成成像雷达（ASIR）。它与射电天文学家（VLBI，甚长基线干涉测量法）用于成像恒星物体的技术非常相似，并且与飞机和卫星上用于绘制地球表面和其他行星表面的SAR（合成孔径雷达）技术也有一些相似之处。在射电天文学的情况下，源本身自发地发射辐射，由许多无源天线收集。在ASIR中，雷达发射器像照相机闪光灯一样照亮目标（电离层或大气），许多天线收集散射辐射，就像射电天文学中的情况一样（或像照相机的镜头一样）。从这一点上讲，这两种情况本质上是相同的。为了构造目标图像，从所有不同的接收器对计算信号之间的相互关系。通过使用雷达成像技术，我们将成为这项新技术在欧洲的先驱。

The subject of our study is the aurora borealis, or northern lights, which is an amazing natural lightshow in the sky, seen regularly at high latitudes such as northern Scandinavia, but rarely at the latitudes of the UK. We use the aurora as a diagnostic to find out many things about the environment around the Earth, mainly in the region of upper atmosphere called the ionosphere. That environment is made up of 'plasma' (ionised gas) often called the fourth state of matter, which makes up over 95% of the directly observable material in the cosmos. Yet it is strangely difficult to maintain and study within Earth's biosphere. The upper atmosphere provides an ideal natural laboratory for its study since there is no need to consider collisions of the plasma with container walls. The story of the aurora begins at the Sun, which is a continuous but very variable energy source, in the form of a plasma stream (the 'solar wind') which impacts on the Earth. We are interested in understanding the smallest scale auroral structures, and how the energy changes within them influence the large scale environment. To study the aurora, we use a special instrument which has three cameras looking at different 'colours' simultaneously. The proposed research is for studies of very dynamic and structured aurora at the highest possible resolution. The instrument is named ASK for Auroral Structure and Kinetics. It was designed to measure a small circle of 3 degrees in the 'magnetic zenith' i.e. straight up along the Earth's magnetic field. Particles from the Sun spiral along these imaginary magnetic field lines, and lose energy when they collide with atmospheric oxygen and nitrogen. The exact colour (or wavelength of the light) depends on how much energy the incoming particle started with, and what molecule or atom it hits. The ASK cameras help to unravel this complicated process by making very precise measurements in space and time of three emissions which have different physical origins. We will combine these optical measurements with measurements from special radar experiments, which are designed to use a technique known as interferometry to measure structures smaller than the beam width, and with accuracy of position and height better than has been possible to date. The radar imaging technology is new in the field of incoherent scattering radar and will be one of the cornerstones of a future project that is called EISCAT_3D. The technology employed is Aperture Synthesis Imaging Radar (ASIR). It is very similar to the technology used by radio astronomers (VLBI, Very Long Baseline Interferometry) to image stellar objects, and also has some similarity with the SAR (Synthetic Aperture Radar) technique used onboard airplanes and satellites to map the Earth's surface and other planetary surfaces. In the radio astronomy case the source itself spontaneously emits radiation that is collected by a number of passive antennas. In ASIR, the radar transmitter acts like a camera flash to illuminate the target (the ionosphere or atmosphere) and a number of antennas collect the scattered radiation exactly as in the radio astronomy case (or like the lens of a camera). From this point on, the two cases are essentially identical. To construct the image of the target, the cross-correlation between the signals is calculated from all different pairs of receivers. By using the radar imaging technique we will become the pioneers of this new technique in Europe.