Mean Cross‐Field Displacement of Magnetic Field Lines in Slow Solar Wind: A Confirmation of the Supradiffusion Predicted by the Generalized Quasilinear Theory

Abstract

A quantitative comparison of the magnetic field-line mean cross-field displacements predicted by the generalized quasilinear theory and of those deduced from Helios 2 in situ measurements is presented for the quiet slow solar wind (SW) at 0.3 and 1 AU. While the supradiffusivity of the magnetic field lines, X2 ∝ (Δz)μ with 2 ≥ μ > 1, is confirmed on all field-aligned scales Δz < 0.1 AU, the precise amplitude of the mean cross-field displacement X may not be obtained without some correction of the in situ measured spectra at frequencies <10-4 Hz. The SW cross flow and resulting transverse decorrelation and spectral flattening would cause the global amplitude of X to be underestimated by up to a factor 2. The value of the transport exponent μ below 0.01 AU is little affected by the uncertainty at very low frequencies; it is very close to the value determined by the spectrum around (Δz)-1. By plotting segments of wandering magnetic field lines computed from Helios 2 data as perturbations to Parker spiral fields, a lower limit of the order of 20° is also established for the median angular deviation of the wandering field lines. The cumulative angular deviation of a magnetic field line wandering from the source region of impulsive solar energetic particles (SEPs) out to 1 AU may, in some instances, be a few times larger. It occurs in longitude and latitude and could explain the relatively low magnetic connectivity of impulsive SEPs. The variability of the slow SW turbulence is also demonstrated.