Earth's radiation belts are “donut-shaped” regions surrounding the Earth between ~2 – 10 Earth radii that contain energetic charged particles trapped by the Earth’s magnetic field. The flux of the trapped electron population can vary by several orders of magnitude during geomagnetically active periods that can pose serious threats to spacecraft orbiting in this region of space. As our modern society is becoming increasingly reliant on space-based technology, understanding the physical processes that control the dynamics of this region of space is therefore of paramount importance to the entire space science community so that we can mitigate space weather hazards and provide better protection to the satellites.

A useful approach to studying the radiation belt electron flux variability is to examine their pitch angle distribution. Pitch angle, in simple words, describes the orientation of the electrons in the background geomagnetic field, and different processes can alter their orientation producing different pitch angle distributions. In the radiation belts, there are primarily three types of pitch angle distributions, namely, pancake, butterfly, and flat top. These are named after the shapes of the distribution in plots of flux against pitch angle. The processes impacting these pitch angle distributions can be of both external origin, such as the Sun-Earth interaction, or internal origin, such as wave-particle interactions. Studying the evolution of pitch angle distribution provides useful information about these underlying physical processes, and hence, several past studies have rigorously used this approach to study the radiation belts. 

In this project, our main aim was to examine the impact of the external driver, i.e., Sun-Earth interaction on the radiation belt electron pitch angle distribution. For this, we calculated a pitch angle anisotropy index using a very simplified approach, where we defined the anisotropy index as the ratio of electron fluxes in directions perpendicular to parallel to the background magnetic field. We used electron flux data from the Relativistic Electron-Proton Telescope (REPT) instrument onboard the Van Allen Probe-B spacecraft during the period 2013 – 2018 and performed all the calculations for the 1.8 MeV electron energy channel in the outer radiation belt (L ≥ 3, where L is the measure of distance from Earth's core in units of Earth radii). We procured solar wind parameters (solar wind velocity, dynamic pressure, and the z-component of interplanetary magnetic field which is particularly important in the Sun-Earth interaction) and geomagnetic indices (SYM-H and AL) from the OMNI database during the same period. We merged both datasets and tried to find any existing correlation between the calculated radiation belt electron pitch angle anisotropy index and the solar wind parameters. The main findings from this project can be summarized as follows:

1. Pancake distributions are mostly observed on the dayside between 6 – 18 MLT and L ≥ 5.

2. Butterfly distributions are mostly observed on the nightside between 19 – 5 MLT and L ≥ 6.

3. Flattop distributions are mostly observed around dawn (6 MLT) and dusk (18 MLT) outside L = 4.

4. Pancake distributions exhibit a strong correlation with the different solar wind parameters, the best correlation being with solar wind dynamic pressure and SYM-H (Figure 1).

5. Butterfly distributions exhibit a strong correlation with solar wind dynamic pressure (Figure 2).

The results suggest that even using a very simplified approach, all three primary pitch angle distributions in the outer radiation belt could be resolved. Their spatial distribution also agrees well with previous studies, further validating the methodology used in the project. The results further suggest that the sudden magnetospheric compression following shock impact is a significant factor in driving the anisotropy of the radiation belt electrons.