Forbush decreases with AMS-02 data and 2D time-dependent modeling

Abstract

Galactic cosmic rays (GCRs) are high-energy charged particles originating outside the solar system. Their transport through the heliosphere is significantly influenced by solar activity, leading to both long-term modulation over the solar cycle and short-term disturbances such as Forbush decreases (FDs) caused by transient solar events like Interplanetary Coronal mass ejections (ICMEs). This thesis presents an integrated observational study of solar modulation, with a particular focus on FDs observed by the Alpha Magnetic Spectrometer (AMS-02). A detailed event-level analysis was performed using daily flux measurements of protons and helium nuclei, enabling the identification and characterization of FD properties such as amplitude, rigidity dependence, and correlation with solar wind parameters. To interpret these observations, a two-dimensional Stochastic Differential Equation (SDE) numerical model was developed to solve Parker’s transport equation under realistic heliospheric conditions. The model includes both diffusion and drift transport mechanisms, and the diffusion and drift coefficient scaling factors were set as free parameters and optimized using Bayesian optimization to match AMS-02 data for each month. The model was further extended to simulate short-term modulation effects from ICMEs by incorporating radially propagating diffusion barriers. The additionally introduced free parameters related to ICME geometry factors were obtained from satellite observation or tuned using Bayesian Optimization with the AMS daily flux on the ICME onset day. The result model was used to reproduce the daily fluxes during the recovery phase for the same FD. The combination of high-precision AMS-02 observations and physically grounded modeling offers new insight into the interaction between GCRs and transient solar phenomena. The methodology developed here provides a robust framework for studying space weather–driven modulation and supports future advancements in real-time forecasting and heliospheric transport modeling.