Contributions and Timing Characteristics of Forbush Decreases from Substructures of Interacting ICMEs

Abstract Forbush decreases (FDs) are short-term reductions in galactic cosmic ray (CR) intensity associated with interplanetary coronal mass ejections (ICMEs). We analyze a diverse sample of eight interacting ICME events (2010-2015), spanning merged and separate ejecta, with and without an interaction region (IR), and interactions at different heliocentric distances, to examine how ICME substructures modulate CR intensity and to inter-compare the events for patterns linked to their configuration. Using in-situ solar wind and magnetic field data with CR intensity at 10 GV from the FEID database, we identified multi-step FD profiles, with substructure boundaries from the Richardson & Cane ICME catalog (timing uncertainty ±1 hr). FD amplitudes range from ~2.5 to ~5.8%, showing only a moderate, non-robust correlation with geomagnetic storm intensity. A three-step profile, not possible for an isolated single-ejecta ICME, occurs in two events, while four others show two ejecta-related steps with weak or absent sheath; yet amplitudes, decline rates, and recovery durations (2-4 days) fall within isolated-ICME ranges, so interacting events differ mainly in substructure number and sequencing. FD onset relative to shock arrival varies, with early onsets up to ~11 hr occurring preferentially for interactions closer to the Sun, based on a small sample. A lag of 1-17 hr separates the peak field and FD minimum, reflecting cross-field diffusion in the sheath or diffusion-expansion in the ejecta. The dominant CR-modulating substructure varies between the preceding ICME, following ICME, or IR. Given the limited sample, results are illustrative, and a larger matched sample is needed to confirm them.

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