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 data base, we identified multistep FD profiles, with substructure boundaries from the Richardson & Cane ICME catalogue (timing uncertainty ±1 h). FD amplitudes range from ∼2.5 to ∼5.8 per cent, 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 d)fall within isolated-ICME ranges,so interacting events differ mainly in substructure number and sequencing. FD onsetrelative to shock arrival varies, with early onsets up to ∼11 h occurring preferentially forinteractions closer to the Sun, based on a small sample. A lag of 1–17 h 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.