The great space weather washing machine : Examining the dynamics of high-latitude ionosphere-thermosphere coupling

Billett, Daniel and Wild, Jim and Grocott, Adrian (2019) The great space weather washing machine : Examining the dynamics of high-latitude ionosphere-thermosphere coupling. PhD thesis, Lancaster University.

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The Earth’s upper atmosphere at high latitudes is a complicated region that is under the influence of many competing forces. For one, it is where the atmosphere becomes partially ionised, and thus subjected to the electromagnetic influences of the coupled solar wind-magnetosphere system. In contrast, neutral particles strongly feel the effects of non-magnetic forces, such as those due to temperature gradients and the Coriolis effect. Once ion-neutral collisions are then taken into account, the result is a global scale “washing machine” of charged and neutral particles, moving through various different spin cycles of coupling strength. Firstly, it is the inherent differences between neutral and plasma flows that result in one of the largest atmospheric sinks of magnetospheric energy: Joule heating. It has sometimes been assumed in previous studies that the neutral wind is slow enough to be treated as if it was stationary. However, we show in Chapter 3 that this is not the case statistically using plasma velocity data from the Super Dual Auroral Radar Network (SuperDARN) and an empirical neutral wind model (HWM14). Overall, the inclusion of neutral winds can lead to global Joule heating estimates that differ by as much as 18% from calculations assuming they are stationary. We also present common scenarios by which the neutral wind can provide both a net increase or decrease to heating rates, depending on season and geomagnetic activity level. In Chapter 4, neutral wind velocity measurements from an all-sky Fabry-Perot Interferometer known as SCANDI are presented for a period where the plasma velocity varied greatly in magnitude. We show that over the relatively small region observed (about 1000 km in diameter) for the event in question, the time it took for the neutrals to be fully accelerated or decelerated by a change in the plasma varied by as much as 30 minutes. Compared to the average neutral wind response time of around 75 minutes, this is quite a large amount of variability for regions at mesoscale separations. This has implications for how closely coupled the ionosphere and thermosphere are on a global scale, and shows the importance of carefully taking into account the current states of both. Finally, an aurorally active period that occurred towards the end of the event pre- sented in Chapter 4 is examined more closely in Chapter 5. We saw that coinciding with the onset of poleward moving auroral forms, the neutrals rapidly accelerated in the direction of the plasma - much faster than prior to the aurora. We propose that due to the increased ionisation from particle precipitation, combined with the rapid transient plasma bursts in the poleward direction, the strength of ion-neutral coupling was en- hanced significantly. This also happened during a transition of the IMF By component, showing for the first time a pseudo coupling of the thermosphere directly to the solar wind.

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Thesis (PhD)
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30 Oct 2019 14:50
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16 Jul 2024 05:48