Analytic modeling of recurrent Forbush decreases caused by corotating interaction regions

Abstract

On scales of days, the galactic cosmic ray (GCR) flux is affected by coronal mass ejections and corotating interaction regions (CIRs), causing so-called Forbush decreases and recurrent Forbush decreases (RFDs), respectively. We explain the properties and behavior of RFDs recorded at about 1 au that are caused by CIRs generated by solar wind high-speed streams (HSSs) that emanate from coronal holes. We employed a convection-diffusion GCR propagation model based on the Fokker-Planck equation and applied it to solar wind and interplanetary magnetic field properties at 1 au. Our analysis shows that the only two effects that are relevant for a plausible overall explanation of the observations are the enhanced convection effect caused by the increased velocity of the HSS and the reduced diffusion effect caused by the enhanced magnetic field and its fluctuations within the CIR and HSS structure. These two effects that we considered in the model are sufficient to explain not only the main signatures of RFDs, but also the sometimes observed "over-recovery" and secondary dips in RFD profiles. The explanation in terms of the convection-diffusion GCR propagation hypothesis is tested by applying our model to the observations of a long-lived CIR that recurred over 27 rotations in 2007-2008. Our analysis demonstrates a very good match of the model results and observations.

15 pages, 9 figures

References in corpus (6)

• Study of the 27-day variations in GCR fluxes during 2007-2008 based on PAMELA and ARINA observations
• Generic profile of a long-lived corotating interaction region and associated recurrent Forbush decrease
• On the relationship of the 27-day variations of the solar wind velocity and galactic cosmic ray intensity in minimum epoch of solar activity
• Evolution of coronal mass ejections and the corresponding Forbush decreases: modelling vs multi-spacecraft observations
• Galactic cosmic ray hydrogen spectra and radial gradients in the inner heliosphere measured by the HELIOS Experiment 6
• On the Interaction of Galactic Cosmic Rays with Heliospheric Shocks During Forbush Decreases

Cited by in corpus (2)

• Generic profile of a long-lived corotating interaction region and associated recurrent Forbush decrease
• Scaler rates from the Pierre Auger Observatory: a new proxy of solar activity