Deterministic Propagation of Suprathermal Ions Through the Heliosphere

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Abstract
The PEPSSI instrument onboard New Horizons is sampling the energy range that bridges pickup and suprathermal ions for the first time in the outer heliosphere. New Horizons repeatedly encountered structures beyond 30 au that we interpret as the spiral of a corotating interaction region (CIR) for an extended period from 2015 to 2016. Ion intensities abruptly increase at each CIR crossing, followed by a gradual, energy‐dependent decrease until the next passage. PEPSSI also has been able to constrain the instantaneous pitch angle distribution of pickup ions, suggesting the intensities in the hemispheres parallel and anti‐parallel to the assumed field to differ by an order of magnitude. Such observations are traditionally explained through transport equations involving diffusion coefficients, which are difficult to measure directly. Here, we are using an entirely different approach and make the Ansatz that after acceleration at the CIR ions are propagating deterministically without critical reliance on diffusive processes. We develop a comprehensive theoretical framework and find that it yields promising results. The new framework is indeed able to fit examples of the observed spectral evolution, while also naturally maintaining the observed hemispheric asymmetry.