Filamentary structure in the upper atmosphere

我们研究的对象是北极光，这是天空中一种令人惊叹的自然灯光秀，经常在北斯堪的纳维亚半岛等高纬度地区看到，但很少在英国的纬度地区看到。我们利用极光来诊断地球周围的环境，主要是在被称为电离层的高层大气区域。这种环境是由通常被称为物质第四态的等离子体(电离气体)组成的，这种物质占宇宙中直接可见物质的95%以上。然而，奇怪的是，在地球生物圈内维护和研究它是非常困难的。高层大气为其研究提供了理想的自然实验室，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，太阳是一种连续但非常可变的能源，以等离子流(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，以及其中的能量变化如何影响大尺度环境。为了研究极光，我们使用了一种特殊的仪器，它有三个摄像头同时拍摄不同的颜色。拟议的研究是为了在尽可能高的分辨率下研究非常动态和有结构的极光。该仪器被命名为Ask for Aural Structure and Kinetics。它被设计用来测量“磁天顶”上一个3度的小圆，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螺旋上升，当它们与大气中的氧气和氮气碰撞时会失去能量。光的确切颜色(或波长)取决于入射粒子开始时的能量，以及它击中的分子或原子。ASK相机通过在空间和时间上非常精确地测量三种不同物理来源的排放物，帮助解开这一复杂的过程。我们将把这些光学测量与特殊雷达实验的测量结合起来，这些实验旨在使用一种名为干涉测量的技术来测量小于波束宽度的结构，并以比目前可能的更好的位置和高度精度进行测量。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EISCAT_3D项目的基石之一。所采用的技术是孔径合成成像雷达(ASIR)。它与射电天文学家用来成像恒星物体的技术(VLBI，超长基线干涉测量)非常相似，也与飞机和卫星上用来绘制地球表面和其他行星表面的合成孔径雷达(SAR)技术有一些相似之处。在射电天文学的情况下，源本身自发地发射由许多无源天线收集的辐射。在ASIR中，雷达发射器的作用就像相机闪光灯，照亮目标(电离层或大气层)，一些天线收集散射的辐射，就像射电天文学中的情况(或像相机的镜头)。从这一点来看，这两个案例基本上是相同的。为了构建目标的图像，从所有不同的接收器对计算信号之间的互相关。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.