The Importance of Nonlinear Physics in Radiation Belt Modelling

Space is not a vacuum, but is permeated with electrically charged particles. This fourth state of matter is called plasma, and is not familiar to us on Earth since it is created at very high temperatures. The most significant sources of plasma are stars such as our sun, with plasma both fuelling and being created by self-sustaining thermonuclear fusion. There is an ever-present 'solar wind' composed of plasma that flows out of the sun in all directions and into interstellar space at hundreds of kilometres per second. Whilst less well-known and understood to us than the first three states of matter, more than 99% of the material in the standard model of the universe is plasma.

The motion of solids, liquids and gases is dominated by the familiar forces of gravity and pressure. In contrast, and due to the presence of charged particles, plasma dynamics are dominated by electric and magnetic (electromagnetic) forces. The Earth has a magnetic field similar to that of a bar magnet. This magnetic field forms a protective boundary that prevents the majority of the otherwise dangerous solar wind plasma from streaming directly towards the Earth's surface. In addition to its main function as a protective barrier, the Earth's magnetic field interacts with the plasma-filled solar wind via many complex and dynamic interactions. These different processes operate on a range of timescales from years to millionths of a second. One of the dominant global-scale processes is known as the 'Dungey Cycle'. Via the Dungey Cycle, plasma originating in the solar wind can be transported past the outermost protective barriers of the Earth's magnetic field by entering at the nightside of the Earth. Plasma originating from this, and other, mechanisms then proceeds to surround the Earth from altitudes ranging from the outer reaches of the atmosphere, up to around 60,000km on the dayside and beyond 1,000,000km on the nightside.

The magnetic field and plasma surrounding the Earth are together known as a magnetosphere. As suggested above, the Earth's magnetosphere plays host to many highly energetic dynamics, and these dynamics are ultimately driven by the solar wind. Plasma sourced via the Dungey Cycle can itself be unstable, and these instabilities can generate electromagnetic waves (e.g. radio waves) that propagate throughout the magnetosphere. These radio waves can then go on to interact with other charged particles within the plasma and change their velocity. These particles can be accelerated close to the speed of light via so-called 'resonant interactions'. The regions of the Earth's magnetosphere containing these energetic particles are known as the radiation belts.

Satellite technologies underpin much of our modern society: navigation, communication, defense and Earth observation. Hundreds of operational satellites orbit the Earth and must traverse the hazardous radiation environment in the radiation belts. Highly energetic particles pose many operational and financial risks to orbiting spacecraft, including total loss. These risks, and other associated ground-based effects, have led to the inclusion of Space Weather in the UK Cabinet Office National Risk Register of Civil Emergences.

Recent satellite observations have revealed that electromagnetic waves can have significantly higher amplitudes (i.e. carry more energy) than previously thought. This also means that they can energise plasma particles to higer energies much more rapidly than previously thought. Numerous Space Weather forecasting models exist around the world, but none of them include these effects. The British Antarctic Survey hosts one world leading model, which is licenced to the UK Met Office. The ultimate objective of this Fellowship is to improve forecasting accuracy of this operational model by understanding and including the effects high amplitude waves have on particle dynamics. This is crucial as society becomes more and more dependent on satellite technologies.