This project will investigate the generation, propagation and dissipation of magnetosonic waves in the inner region of the magnetosphere as well as the energization of radiation belt electrons interacting with them. In the space environment near Earth, the plasma is so tenuous that plasma particles (ions and electrons) rarely collide. However despite this, some portion of the electrons in the magnetosphere reach dangerously high energies exceeding a million electron volts. This energy is high enough to pose a risk to satellites orbiting within the inner regions of the magnetosphere, a preferred location for communications, navigation, search-and-rescue, weather prediction and national security satellites. Plasma waves are mainly responsible for the acceleration of radiation belt electrons to these high energies but they can also contribute to the scattering of radiation belt particles into the Earth's atmosphere. Many types of plasma waves are generated in a collisionless plasma in the presence of a magnetic field and each has its own set of interactions with the charged particles in the plasma. These waves play a significant role in the dynamics of the inner magnetosphere and radiation belts. Magnetosonic waves, the focus of the proposed investigation, are found in the vicinity of the magnetic equatorial plane in a wide region near the dayside portion of the magnetosphere. The component of the wave electric field, which is aligned along a magnetic field line, can continuously accelerate electrons traveling near the speed of the wave in much the same way that a surfer is pushed forward by an ocean wave. This is termed Landau damping. Magnetosonic waves are thought to be generated by ring current protons, and dissipated in accelerating radiation belt electrons, in effect acting as intermediaries in the collisionless transport of energy between different particle populations. As a broader impact, this project is led by an early-career scientist thus contributing to the training of the next generation of scientists.
The primary goal of this research is to use linear theory and kinetic particle-in-cell (PIC) simulations to explore the excitation and propagation of fast magnetosonic waves driven by observed types of ring-like proton velocity distributions and to use insights gathered to interpret the observed wave measurements, both datasets from the twin Van Allen Probes. Furthermore, scattering of radiation belt electrons will be quantified using test-particle computations, providing clear criteria as to whether and under what conditions the conventional quasi-linear approach can be applied. Two fundamental science questions will be addressed: (1) how does the excitation and propagation of fast magnetosonic waves produce the complex pattern of the observed wave frequency spectra? and (2) how important to the evaluation of radiation belt electron scattering are the complex kinetic dispersion properties of the waves compared to the commonly assumed cold plasma dispersion used in quasi-linear theory? The generalization of the full kinetic PIC code to account for the dipole magnetic field in an inhomogeneous medium will be an additional valuable resource for the radiation belt community enabling a whole spectrum of new studies not previously possible.
