A Two-Population von Mises--Fisher Distribution of the Magnetic Field Direction in the Young Solar Wind
Year: 2026
Type: preprint
Abstract: The one-point distribution of the magnetic field direction in the young solar wind is found to be a superposition of two populations rather than a single exponential. Using Parker Solar Probe (PSP) Encounter 1 magnetometer data, we show that the distribution of the pitch-angle cosine mu(t) = bhat(t) . bhat_0 is well described by a two-component von Mises-Fisher (vMF) mixture: a broad, weakly confined component of concentration kappa_1 ~ 3.6 that accounts for the large-amplitude rotations and switchback tail, and a field-aligned component that reduces, in the small-angle limit, to a Gaussian in the polar angle theta of width sigma ~ 15 degrees (concentration kappa_2 ~ 14). The concentration of the field-aligned component is not a free parameter: it is set by the level of the transverse Alfvenic fluctuation through kappa_2 = 2<|B|^2>/ = 2/<1-mu^2> ~ 16.2, in agreement with the fitted value to within ~12%. The transverse fluctuations are weakly anisotropic (/ ~ 1.1), so the core is a weakly anisotropic Bingham distribution rather than a pure vMF. The broad component is exponential in mu, not in theta, confirming that switchbacks saturate toward mu = -1 rather than decaying away, consistent with the drift-diffusion framework of Bian et al. (2024, ApJ, 960, L15). This physics-based two-population framework connects macroscopic fluid parameters directly to those describing the directional statistics, offering a powerful diagnostic for probing the structural evolution of MHD turbulence and switchbacks in the inner heliosphere.
Source: arXiv (Cornell University)
Institutions: Macau University of Science and Technology, State Key Laboratory of Lunar and Planetary Sciences, Wuhan University, Adelaide University, The University of Adelaide, Instituto Nacional de Pesquisas Espaciais, Universidade de São Paulo, Instituto Tecnológico de Aeronáutica, Universidade de Brasília
Language: English
Subfield: Astronomy and Astrophysics
Field: Physics and Astronomy
Domain: Physical Sciences
Open Access status: green
OpenAlex ID: W7223053859