Determining optical-flow for turbulent motions in the aurora borealis

Abstract

In dynamic aurora there are structures that shows shear-flow and rotations. These flows have a locally varying vorticity-field, which can be observed in small scale aurora. Vorticity in plasma-flows have a direct relation to field-aligned currents. An effective method to determine these flows would be a great tool in research on magnetosphere-ionosphere coupling and auroral physics. With the current observations of aurora with high resolution and fps, it should be possible to determine these auroral motions with the use of optical-flow techniques. By using already existing methods, we show how well they work for turbulent motions in aurorae, and highlights advantages and disadvantages. A 20min recording with 32 fps have been studied, where a small image sequence containing several kinds of motions have been worked further on. By comparing differential techniques with phase-based techniques, we found that the differential techniques displayed better accuracy in turbulent flow. The phase-based provided good results for stable motions lasting a couple of frames, but there are still some adjustment thats should be done to optimize this technique for these kinds of motions.

The technique developed by Nagel provided the best results. This method maintained velocity gradients to a greater extent than other methods without the cost of significantly more noise. Although most methods provided reliable results, there are still work that must be done for a robust technique to use on aurorae in general.