Diffusion Coefficients, Short-Term Cosmic Ray Modulation, and Convected Magnetic Structures

John J Quenby,J Bernard Blake,Tamitha Mulligan,Diana N A Shaul

doi:10.1155/2013/429303

John J Quenby, J Bernard Blake + Show 2 more

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https://doi.org/10.1155/2013/429303

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Abstract

Three cases of large-amplitude, small spatial-scale interplanetary particle gradients observed by the anticoincidence shield (ACS) aboard the INTEGRAL spacecraft in 2006 are investigated. The high data rates provided by the INTEGRAL ACS allow an unprecedented ability to probe the fine structure of GCR propagation in the inner Heliosphere. For two of the three cases, calculating perpendicular and parallel cosmic ray diffusion coefficients based on both field and particle data results in parallel diffusion appearing to satisfy a convection gradient current balance, provided that the magnetic scattering of the particles can be described by quasi-linear theory. In the third case, perpendicular diffusion seems to dominate. The likelihood of magnetic flux rope topologies within solar ejecta affecting the local modulation is considered, and its importance in understanding the field-particle interaction for the astrophysics of nonthermal particle phenomena is discussed.

Highlights

A Forbush Decrease (FD) is a global transient decrease in Galactic Cosmic-Ray (GCR) intensity followed by a substantially slower recovery
The authors in [5] investigate four simple magnetic field models for explaining short-term reductions in the GCR intensity and associated energetic particle propagation concluding that only a magnetic flux rope topology similar to that found in magnetic cloud interplanetary CMEs (ICMEs) provides the magnetic conditions most likely to explain the overall depth of an FD
We have set out to demonstrate the likely validity of a model for significant short-term reduction of the GCR intensity and the use of this model to obtain information on energetic particle diffusion

Summary

Introduction

A Forbush Decrease (FD) is a global transient decrease in Galactic Cosmic-Ray (GCR) intensity followed by a substantially slower recovery. Detailed observations of coronal mass ejections (CMEs) and in situ observations of the solar wind and energetic particles have greatly increased understanding of the underlying physics of FDs (see review articles by [1,2,3]) This investigation focuses on small amplitude and highfrequency variability in the GCR corresponding to timescales less than a few hours, much shorter than that described by the classical FD. Computational models solve the transport equation describing three-dimensional long-term GCR modulation by employing empirically justified diffusion coefficients, based on the goodness of fit to the overall spatial, temporal, and energy dependence of the modulation (e.g., [8]) These approaches do not attempt to relate the coefficients to in situ field data.

Particle and Field Data

Observations

Derivation of the Quasi-Linear Diffusion Coefficients

Findings

Conclusions