Theory of Kinetic Alfven Waves and Auroral Particle Acceleration

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This proposal is focused on developing an entirely new paradigm for auroral arc formation that, if correct, will be groundbreaking for auroral physics. The dynamic auroras that appear in the polar regions are visual indicators of energy flowing into the atmosphere from disturbances raging in near-Earth space. Electrons and ions, that power the auroral emissions, travel down magnetic field lines from these disturbances and must be further accelerated along the way. For several decades, the majority of the acceleration was thought to be the result of electrons and ions traversing quasi-static potential drops while moving down magnetic field lines. These potential drops are created when two parallel oppositely-charged layers form in the plasma setting up an electric field and an associated voltage drop. The auroral electrons are accelerated through the voltage drop creating a single energy peak. Recently the significance of time dependent acceleration of electrons by kinetic Alfven waves has become clear. Kinetic Alfven waves are formed under certain circumstances when Alfven waves (traveling along the magnetic field communicating the effects of changes elsewhere in the field) develop an electric field parallel to the background magnetic field. In contrast to the previous mechanism, the parallel wave electric fields accelerate electrons over a broad energy range. However, neither of these processes is well understood. To complicate things, both processes are influenced by coupling and feedbacks with the ionosphere. The PI has developed a compelling new theory that the two processes are related (i.e., that kinetic Alfven waves transition to a quasi-steady state forming parallel potential drops). The focus here is on testing this theory. Broader impacts of the work include the training of a graduate student and communication of results through public outreach. The research is expected to be of broad interest to other plasma physics communities. In the longer-term, it will contribute to better understanding of the space environment, resulting in improved forecasts, and the development of efficient mitigation strategies to protect vulnerable technologies of value to society. The approach combines theory and modeling with the goal of including a large range of temporal and spatial scales involved in the transition from time-dependent to quasi-steady conditions, a large parameter range, feedbacks through ionospheric reflection of the Alfven waves and by modifications of the ionospheric conductivity due to the precipitating electrons, as well as by the formation and effects of density cavities along the auroral field lines. The primary tool is an already developed linearized two-fluid model, which will be modified to include: nonlinear electron dynamics along the magnetic field line and a realistic dipole magnetic field geometry. This model combined with a kinetic electron model will be used to investigate the acceleration of electrons and their self-consistent interactions with the kinetic Alfven waves.