A Two-Population von Mises--Fisher Distribution of the Magnetic Field Direction in the Young Solar Wind
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.
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BibTeX
@misc{li2026twopopulation,
title = {{A Two-Population von Mises--Fisher Distribution of the Magnetic Field Direction in the Young Solar Wind}},
author = {Gang Li and N. H. Bian and S. Y. Huang and A. Chian and R. Santos-Lima and E. L. Rempel and R. A. Miranda and B. Raphaldini and Yasmmin Tamburus},
howpublished = {arXiv},
year = {2026},
month = oct,
doi = {10.48550/arxiv.2610.24057},
eprint = {2610.24057},
archivePrefix = {arXiv},
url = {https://arxiv.org/abs/2610.24057},
note = {CC BY 4.0},
}
RIS
TY - UNPB
TI - A Two-Population von Mises--Fisher Distribution of the Magnetic Field Direction in the Young Solar Wind
AU - Gang Li
AU - N. H. Bian
AU - S. Y. Huang
AU - A. Chian
AU - R. Santos-Lima
AU - E. L. Rempel
AU - R. A. Miranda
AU - B. Raphaldini
AU - Yasmmin Tamburus
PY - 2026
DA - 2026/10/16
PB - arXiv
DO - 10.48550/arxiv.2610.24057
UR - https://arxiv.org/abs/2610.24057
AB - 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.
ER -