Modulation of cosmic-ray ground-level enhancements by solar-wind stream interfaces: a case study

Abstract. Ground-level enhancements (GLEs) provide crucial insights into the acceleration and transport of solar energetic particles (SEPs). We present a comprehensive analysis of GLE 72, which occurred on 10 September 2017, coinciding with a solar-wind stream interaction region (SIR) impacting Earth's magnetosphere. By combining multi-station neutron monitor observations with a focused transport model constrained by solar-wind data, we investigate how the SIR modulates the observed GLE pulse shape. Our analysis reveals that the turbulent magnetic field within the SIR significantly enhances pitch angle scattering rates, with the diffusion coefficient increasing by up to 200 % during the 6 h SIR crossing. This leads to a 60 % increase in the particle mean free path across the SIR. Our model successfully reproduces the observed gradual rise phase (>8 h) and prolonged decay, demonstrating that even moderate interplanetary disturbances can substantially alter SEP transport conditions. Our results challenge the traditional impulsive–gradual classification of GLEs, highlighting the need to consider interplanetary transport effects when interpreting these events. The findings of this study highlight the importance of integrating multi-point observations and advanced modelling to disentangle particle acceleration and transport processes in the complex medium of solar wind.