Jasmine Kaur Sandhu
The radiation belts are populations of highly energetic particles that are situated in the Earth’s space environment and trapped by our global geomagnetic field. During active periods, the radiation belts experience dramatic structural changes and intense energisation, that can be hazardous to local spacecraft. Therefore, understanding how and why the radiation belts change during these periods is a key goal of the Space Weather community. We use direct observations of magnetic and electric field fluctuations from the Van Allen Probes spacecraft to explore how electromagnetic waves can scatter particles towards and away from the Earth, where this process is termed radial diffusion. The results show that radial diffusion is significantly enhanced during active conditions, and also that there is a huge range of variability between events. Estimates are compared to an existing model and analysis shows that the model tends to underestimate radial diffusion during these active conditions. The results show that radial diffusion may be more important than previously thought and provides estimates of radial diffusion for direct use in modelling radiation belt dynamics.
We conducted a statistical analysis of Van Allen Probes observations. Firstly, storm periods were selected using an automated algorithm that identified characteristic signatures in ground magnetometer data. Secondly, the power spectral density of electromagnetic perturbations were calculated from magnetic and electric field observations. Field observations were provided by the Van Allen Probes for a time period covering 2012 to 2019, and we considered a frequency band from 1 to 15 mHz. We then calculated radial diffusion coefficients, which are parameters describing the magnitude of radial diffusion and are used as inputs in radiation belt simulations. The coefficients are directly estimated from the observed power using existing formulations.
An analysis of the observed wave power sheds light on the storm time sources of wave power. Results show that power is enhanced during the main phase of storms across radial distances from 3 to 7 Earth radii. Enhancements on the dayside demonstrate that external sources of wave power from elevated solar wind conditions are important. Enhancements are also observed on the nightside and duskside sectors, which show that there are also strong internal sources of wave power. These arise from drift-bounce resonance with ring current ions and substorm dynamics. The internal sources are often neglected by previous work, but our analysis shows that they are important contributors to wave power and thus radial diffusion during storms.
The calculated diffusion coefficients are assessed to quantify storm time variations. We observe huge variability in diffusion coefficients throughout storms, with values typically spanning 6 orders of magnitude, and with positively skewed distributions. Values peak in the main phase, coincident with enhanced solar wind conditions, and remain elevated during the early recovery phase. The main phase enhancements are observed across all radial distances considered (3 to 7 Earth radii).
We then compare the observed diffusion coefficients to existing empirical models. Many commonly used existing empirical modes are simply parameterised by activity indices. We assessed whether these models, specifically the Ozeke et al. [2014] model, provides realistic descriptions during storms. We find that although the Ozeke et al. [2014] model accurately reproduces the electric field component, on average, the modelled magnetic field component is significantly underestimated during all storm phases. Therefore, existing models may underestimate the magnitude of radial diffusion effects.
In conclusion, we demonstrate that radial diffusion can be significantly enhanced during storms, and this enhancement is due to both external and internal sources of wave power. We provide the community with new and improved storm time diffusion coefficients that can be directly inputted into existing radiation belt models and enable more accurate radiation belt forecasting capabilities.