Coupled Kinetic Physics of Protons and Electrons in the Solar Wind

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The problem of solar wind modeling has been of historical significance. The related research is ongoing, and most models rely on macroscopic (fluid) models of the solar wind. When kinetic effects, such as wave-particle interaction, are considered, however, the approach is usually not self-consistent in that certain types of wave turbulence is presupposed, and the wave spectra are modeled. In contrast, the theoretical model to be developed as part of this three-year project will be fully kinetic and self-consistent. This will significantly advance the knowledge in the field of heliospheric physics. The model to be developed as part of this project will predict a particular state in which the solar wind plasma exists given a set of parameters, allowing the observed quantities to be tested against historic spacecraft data as well the planned measurements of the near-Sun environment that will be obtained by the Parker Solar Probe. Although the present project is a theoretical one, rather than one involving data analysis, the resulting prediction may help experimentalists better interpret the past and future spacecraft data. This has the potential to advance knowledge in a related, but different field of expertise, namely experimental heliophysics.This three-year research project will investigate theoretically how the kinetic coupling of electrons and ions (protons) via collisional and instability processes affect the large-scale dynamics in the expanding solar wind. The specific problem to be investigated relates to how the dynamically coupled solar wind electrons and protons contribute to the near isotropization of their temperatures -- an observed phenomena which has not yet been satisfactorily explained theoretically. The PI's preliminary work based upon the assumption of bi-Maxwellian particle distribution shows that collisional process may be the dominant mechanism for the isotropization of temperatures, but the instabilities are important too for limiting the upper/lower bounds of temperature anisotropies. This project aims to extend the preliminary work by relaxing the assumption of bi-Maxwellian model in both instability analysis and collisional transport calculation. The full kinetic equations for particles and waves, as well as the dispersion relation, will be solved by grid-based numerical scheme. The collisional transport equation will also be solved by relaxing the assumption of bi-Maxwellian models. Finally, the effects of inhomogeneities will be rigorously investigated by solving the macro-microscopic kinetic equation.The research on the theoretical modeling of the solar wind is important from the broader perspective of understanding and characterizing fundamental processes that take place within the heliosphere and throughout the universe. Discovering and understanding the basic physical processes in the immediate near-Earth environment and in the wider universe is essential for future human explorations of the universe, and it also has a space environmental impact, as the modern civilization is increasingly dependent on electromagnetic conditions in outer space. Developing an accurate model of the solar wind contributes to such a wider goal. From the perspective of education and training, the present proposal involves a young post-doctoral research associate, which is a major strength in terms of Broader Impacts. It is important for the healthy scientific future of our nation -- and the entire human society -- that a steady stream of young scientists are educated and trained in STEM disciplines. The PI has in the past always strived to educate and widely disseminate the scientific knowledge acquired during his career to younger generation of scientists both from the U.S. and worldwide. The present proposal will thus have a broader impact on the education and training of a young scientist. The research and EPO agenda of this project supports the Strategic Goals of the AGS Division in discovery, learning, diversity, and interdisciplinary research.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.