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

We present an analytical diffusion-expansion Forbush decrease (FD) model ForbMod, which is based on the widely used approach of an initially empty, closed magnetic structure (i.e., flux rope) that fills up slowly with particles by perpendicular diffusion. The model is restricted to explaining only t...

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Bibliographic Details
Published in:The Astrophysical journal 2018-06, Vol.860 (1), p.71
Main Authors: Dumbovi, Mateja, Heber, Bernd, Vršnak, Bojan, Temmer, Manuela, Kirin, Anamarija
Format: Article
Language:English
Subjects:
Astrophysics
Boundary conditions
Case studies
Coronal mass ejection
Cylindrical shells
Diffusion
Diffusion coefficient
Diffusion rate
Fluctuations
Forbush decreases
Magnetic flux
Magnetic structure
Mathematical models
methods: analytical
Parameters
Properties (attributes)
solar-terrestrial relations
Sun: coronal mass ejections (CMEs)
Thermal expansion
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Summary:We present an analytical diffusion-expansion Forbush decrease (FD) model ForbMod, which is based on the widely used approach of an initially empty, closed magnetic structure (i.e., flux rope) that fills up slowly with particles by perpendicular diffusion. The model is restricted to explaining 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 expansion). A case study was performed to explain the FD observed on 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 an expected range of values. Our study shows that, in general, the model is able to explain the global properties of an FD caused by a flux rope and can thus be used to help understand the underlying physics in case studies.
ISSN:0004-637X
1538-4357
DOI:10.3847/1538-4357/aac2de