Determining the geomagnetic and heliophysical parameters that control increases and decreases in Earth's outer radiation belt

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Earth's geospace environment incorporates a dynamic population of near-relativistic electrons trapped on magnetic field lines that extend out to geosynchronous orbit known as the Outer Radiation Belt (ORB). These electrons can damage spacecraft components and even cause terminal spacecraft failures thus forecasting and nowcasting the conditions in the ORB are critical to spacecraft operations. The number of energetic electrons in the ORB determined from a combination of acceleration and loss processes. Electromagnetic waves can cause electrons to diffuse inwards across magnetic field-lines, causing the electrons to gain energy. At the same time, other electromagnetic wave populations can scatter the electrons into the atmospheric loss cone, causing the particles to precipitate into the upper atmosphere. Current physics-based models of the radiation belts require global, statistical maps of electromagnetic waves to drive the dynamics of the ORB. However, these maps are commonly parameterised by geomagnetic indices which can take the same values during periods of net electron loss or acceleration within the ORB, such as during the main and recovery phases of storms respectively. As such, the models can be using the same wave populations to attempt to model different net changes in the ORB. This leads to two fundamental questions: "what are the electromagnetic wave populations during different net changes in the ORB and what dictates these changes?" and "are the wave populations or gradients in the particle populations the dominant factor in radiation belt dynamics?". Observations of the total number of energetic electrons in the radiation belt from the NASA Van Allen Probes mission hint that the changes in the radiation belt can be categorised as either rapid loss, rapid acceleration, or steady loss. Using a combination of in-situ and ground-based measurements, we will challenge the common parameterisation of the wave populations by re-casting the wave and particle distributions in terms of whether the ORB is undergoing rapid loss, steady loss or acceleration and revealing statistically significant differences between them. We will also examine whether categorising the waves in the same wave can improve physical models of the radiation belts.