Planetary radiation belts are regions of space where the magnetic field of a planet can “trap” high-energy electrons and protons, forcing them to orbit the planet for days, months, or even years. In these extreme environments, a small fraction of electrons and protons can have velocities very close to the speed of light.
Radiation belts can vary enormously over timescales of hours or days in response to disturbances in the solar wind. However, the nature of their response – whether they intensify or weaken, expand or shrink in size – is currently unpredictable. There has been significant recent international scientific attention to this problem. The NASA Van Allen Probe mission (2012-2019) directly sampled the harsh environment with two heavily instrumented spacecraft. Large international teams of experts have used this enormous dataset, alongside advances in numerical modelling, to try to advance our understanding. Yet the reasons for large enhancements or depletions in the terrestrial radiation belt remain elusive.
The aim of this Large Grant is to determine why the terrestrial radiation belt environment is so unpredictable.
First, the radiation belt may be unpredictable because we do not fully understand the chain of subsystems in Earth’s magnetosphere that feed the radiation belt. Energy is transported from the solar wind to the nightside magnetotail, and then sporadically redirected Earthwards in a substorm injection, but we don’t know how much energy is injected or which factors control the process. In other words, the outer radiation belt may be unpredictable because we don’t understand the system well enough.
Second, the radiation belt may be unpredictable because the physical processes involved are highly sensitive to boundary conditions (e.g. the injection) or initial conditions (i.e. the time history of the system). The magnetosphere is a complex interdependent system with many different subsystems, and the sensitivity of each subsystem to energy throughput has not been studied or quantified. That is, the outer radiation belt may be unpredictable because parts of the system are chaotic.
This leads to two important and interconnected objectives:
What factors control the amount of energy injected from the nightside magnetotail to the radiation belt?
How sensitive are different subsystems of the magnetosphere in the chain of energy transfer from the solar wind to the outer radiation belt?
We will use the extensive legacy of spacecraft data from international missions to build the required statistical maps of energy transfer through the magnetosphere. We will also use a range of physics-based numerical models of subsystems of the magnetosphere to determine the sensitivity of each subsystem to different initial or boundary conditions. The ambitious combination of observations and numerical modelling, coupled with a focus on important subsystems in isolation and in concert, will ensure the success of the project.
The study of the terrestrial radiation belt is important as it is the only place in the universe where we can sample such a high-energy astrophysical environment directly. Our lack of understanding also severely impacts our ability to predict the space weather conditions for satellites in the harsh radiation environment of the outer radiation belt. This project will yield crucial information required to turn scientific numerical models into operational forecasting tools, including recommendations for where to include data directly in models to improve forecasting accuracy, and how to effectively use ensemble modelling to provide useful probabilistic forecasts.