Filamentary structure in the upper atmosphere

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.