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.