The Effects of Coupled Wave Power and Plasma Properties on Radiation Belt Dynamics

The Earth's radiation belts consist of energetic charged particles which surround the Earth like a ring doughnut. They were first discovered over 60 years ago, at the beginning of the space age, but many questions remain regarding the relative importance of the physical processes controlling their behaviour. The inner radiation belt, which typically lies at altitudes between 600 and 6,000 km in the magnetic equatorial plane, is relatively stable, but the outer radiation belt, which typically lies at altitudes between 12,000 and 45,000 km, is highly dynamic. Here the number of relativistic electrons can vary by orders of magnitude on timescales ranging from minutes to days. Understanding, modelling and ultimately predicting the behaviour of these so called "killer" electrons is critical because enhanced fluxes of these particles can damage satellites and pose a risk to humans in space.

A variety of plasma waves co-exist with the energetic charged particles in the Earth's radiation belts. They can interact strongly with the relativistic electrons and play a fundamental role in the dynamics of the belts, although their precise roles are yet to be determined. Two very important wave modes are whistler mode chorus and plasmaspheric hiss. Whistler mode chorus, so-called because it often resembles the twittering of birds in the dawn chorus when converted to sound, plays a dual role, contributing to both the acceleration and loss of energetic electrons. In contrast, plasmaspheric hiss, so-named because it resembles audible hiss when played back as sound, is primarily a loss mechanism. Our proposed project will assess the role of both wave modes to understand the basic physics and to improve radiation belt models and forecasts.

Current models for the interaction of plasma waves with electrons use models of the plasma waves based on spatial location and geomagnetic activity. The local plasma conditions in each location, which are also important for modelling the dynamics of the radiation belts, are modelled independently. However, recent studies have shown that it is important to incorporate co-located measurements of the local environment and wave spectra in radiation belt modelling. These new results mandate the development of new wave models binned not only by satellite location and geomagnetic activity, but also by the characteristics of the local environment.

The roles of chorus and plasmaspheric hiss using this new method are currently being investigated in a limited region of the radiation belts as part of the NERC-funded Space Weather Instrumentation Measurement Modelling and Risk (SWIMMR) project Sat-Risk. This study is mostly restricted to the region inside 28,000 km (in the magnetic equatorial plane) and absolute magnetic latitudes less than 21 degrees, excluding the important geostationary orbit region and beyond.

In this project we will use data from four additional satellites, THEMIS-A, -D, -E and Arase, to study how chorus and plasmaspheric hiss influence the behaviour of energetic electrons throughout the Earth's radiation belts. This will improve our understanding of the physics of the processes governing the behaviour of the belts and is essential for the accurate modelling and forecasting of space weather. Specifically, we will establish the importance of chorus at altitudes greater than 28,000 km on the acceleration and loss of energetic electrons in the Earth's outer radiation belt. We will also establish the importance of mid-latitude (21 < |MLAT| < 42 degrees) chorus and plasmaspheric hiss on radiation belt dynamics. Furthermore, we will run simulations with the outer radial boundary at the last closed drift shell to examine the roles of radial diffusion and chorus in the generation of MeV electrons throughout the outer radiation belt. The results will also be used to improve our radiation belt models and forecasts and, as such, will be of great value to satellite engineers, operators and insurers.