Kelvin-Helmholtz versus Gradient drift instability: which one "wins" in the high latitude ionosphere?

Abstract

The high latitude ionosphere is highly irregular, which is caused by plasma patches, flow shears and electron precipitation. These causes are not well understood, but it is thought to be three sources to these irregularities: density gradients, arcs/flow channels and electron precipitation. It is speculated that there are two main mechanisms for the formation of these irregularities. These mechanisms are Kelvin-Helmholtz Instability (KHI), a instability with flow shears as a source and Gradient Drift Instability (GDI), which have plasma density gradient in plasma patches as a source. The reason for the interest in these irregularities, are the technological problems they cause. Signals from satellites can be shifted away from intended targets or reflected back from the atmosphere completely. The goal of the thesis is to compute the linear growth rate of these instabilities to be able to see which one is more likely to be the dominant one. I used the Swarm satellites to access data for 2015 and 2020 as they are at approximately solar maximum and minimum respectively. The data was accessed from 60 degrees latitude and upwards, as the area of interest were the auroral region and the polar cap. I used both latitude and Kp as means of restriction for the datasets, so I could analyse and compare differences for the auroral region and polar cap, geomagnetic activity and seasonal variations. I also compare the differences between the solar maximum and minimum. The results indicate that the GDI is the more dominant mechanism from the assumptions made in the thesis, as it had a consistently larger growth rate than KHI. The GDI growth rate also seems to be affected by the seasonal variations as there is more GDI growth rate in the winter months. The KHI growth rate however seems to be invariant of the seasons. The GDI growth rate also reaches larger values in 2015 than in 2020, which might have to do with the solar maximum, while the KHI growth rate is pretty consistent throughout all the results.