Magnetic flux tube evolution in solar wind anisotropic magnetic turbulence

Abstract

The magnetic turbulence in the solar wind causes a magnetic field line transport that is reflected in the propagation in space of charged particles. Assuming a small localized source, the distribution in space of energetic particles is determined, in part, by the shape of the magnetic flux tube. The spatial evolution of a magnetic flux tube is studied here by means of a numerical realization of three‐dimensional magnetic turbulence that takes into account the anisotropy of the solar wind turbulence and is quantified by correlation lengths in the three spatial directions. Several diagnostics of flux tube evolution are shown, such as patterns of the flux tube cross sections and histograms representing possible energetic particle intensity profiles. We show that flux tube evolution can be assessed by the Kubo number R = (δB/B0)(lz/lx), where δB/B0 is the turbulence level and lz (lx) is the correlation length parallel (transversal) to the background magnetic field B0. We find that when lz/lx (i.e., R) is large, the flux tube evolves very quickly, forming very fine, diffusive structures. These diffusive structures would correspond to a nearly Gaussian envelope for the energetic particle time profile detected by a spacecraft. On the other hand, when lz/lx is small, the flux tube evolves slowly, executing large coherent transverse displacements, and thereby forms well resolved (i.e., detached) intermittent high particle fluxes in observed energetic particle profiles. Hence an accurate study of the morphology of impulsive energetic particle events, when compared with our simulation results, can give information on the microphysical evolution of flux tubes in the solar wind and on the turbulence anisotropy. A first comparison indicates that lx ≃ 3–10lz is appropriate for the solar wind. Our study also allows us to reconcile fast field line transport with the observation of sharp composition or energetic particle gradients in the solar wind, since lx ≫ lz implies considerable transverse elongation of the flux tube cross section, with the possibility of non‐Gaussian, superdiffusive transport regimes.

Trapping and Diffusive Escape of Field Lines in Two‐Component Magnetic Turbulence

“…Similar sharp features are seen in observations Gosling et al 2004) and simulations of particle motion Zimbardo et al 2004;Pommois et al 2005) in turbulent fields. Therefore, we aim to understand over what distance the field lines are trapped, and whether there are sharp trapping boundaries, for the simple model of a single Gaussian 2D island plus slab turbulence.…”

“…Similar features have also been observed in solar electron bursts (at less than 1.4 keV) by ACE, often in coincidence with the ion dropouts (Gosling et al 2004). The dropouts are generally attributed to filamentation of magnetic connection from small regions of the solar corona to Earth orbit as the filaments convect past the spacecraft at the solar wind speed, an interpretation that is supported by computer simulations based on a random distribution of transverse fluctuations at the Sun , or anisotropic turbulence in the interplanetary medium (independently by Ruffolo et al [2003] and by Zimbardo et al [2004] and Pommois et al [2005]), distinct physical descriptions that are nevertheless mathematically similar (Giacalone et al 2006).…”

Nonclassical Transport and Particle-Field Coupling: from Laboratory Plasmas to the Solar Wind

“…The axes of the constant amplitude ellipsoids in phase space are given by the inverse of the correlation lengths l and l ⊥ (Zimbardo et al 2000a), with l /l ⊥ 1 ( 1) corresponding to the quasi-2D (quasi-slab) anisotropy. Zimbardo et al (2004) have shown that indeed the complexity of the magnetic flux tubes grows with l /l ⊥ and with the Kubo number, see Fig. 5.…”

“…The influence of turbulence anisotropy on the structure of magnetic flux tubes has been studied by Zimbardo et al (2004) with a fully 3D spectrum where quasi-slab turbulence is represented by a cigar-shaped distribution of wave vectors along B 0 , while the quasi-2D turbulence is represented by a pancake-shaped distribution of wave vectors in the plane perpendicular to B 0 . The axes of the constant amplitude ellipsoids in phase space are given by the inverse of the correlation lengths l and l ⊥ (Zimbardo et al 2000a), with l /l ⊥ 1 ( 1) corresponding to the quasi-2D (quasi-slab) anisotropy.…”

High energy particle transport in stochastic magnetic fields in the solar corona

“…2). This motion is not truly random, but chaotic, and can give rise to directed transport (Zimbardo et al 2004;Zimbardo 2005). The non uniform, "intermittent" distribution of protons in Fig.…”