An Analytical Diffusion-Expansion Model for Forbush Decreases Caused by Flux Ropes

TL;DR

This paper introduces an analytical model for Forbush decreases caused by flux ropes, incorporating CME expansion and diffusion, and demonstrates its effectiveness through a case study matching observed data.

Contribution

The paper presents a new diffusion-expansion model for Forbush decreases that accounts for CME flux rope evolution and validates it with observational data.

Findings

Model qualitatively agrees with observations

Quantitative fit depends on diffusion coefficient and expansion mode

Diffusion coefficient aligns with expected physical values

Abstract

We present an analytical diffusion-expansion Forbush decrease (FD) model ForbMod which is based on the widely used approach of the initially empty, closed magnetic structure (i.e. flux rope) which fills up slowly with particles by perpendicular diffusion. The model is restricted to explain only the depression caused by the magnetic structure of the interplanetary coronal mass ejection (ICME). We use remote CME observations and a 3D reconstruction method (the Graduated Cylindrical Shell method) to constrain initial boundary conditions of the FD model and take into account CME evolutionary properties by incorporating flux rope expansion. Several flux rope expansion modes are considered, which can lead to different FD characteristics. In general, the model is qualitatively in agreement with observations, whereas quantitative agreement depends on the diffusion coefficient and the expansion properties (interplay of the diffusion and the expansion). A case study was performed to explain the FD observed 2014 May 30. The observed FD was fitted quite well by ForbMod for all expansion modes using only the diffusion coefficient as a free parameter, where the diffusion parameter was found to correspond to expected range of values. Our study shows that in general the model is able to explain the global properties of FD caused by FR and can thus be used to help understand the underlying physics in case studies.