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

Abstract. Ground level enhancements (GLEs) result from transient intensity increases in secondary cosmic ray particles detected by ground-based neutron monitors. Characterizing the temporal evolution of GLEs provides insight into particle acceleration mechanisms and interplanetary transport processes. The present study investigates the moderate intensity GLE 72 event on 10 September 2017, which fortuitously occurred during a solar wind stream interface (SI) region impacting the Earth's magnetosphere. We quantify how transient solar wind structures modulate the observed GLE pulse shape by combining multi-station neutron monitor observations with Monte Carlo particle transport models. Based on this analysis, we find that the turbulent magnetic field within the SI significantly enhances pitch angle scattering rates for energetic particles. In comparison to typical impulsive events, particle mean free paths declined by approximately 35 % during the 6-hour SI crossing. The stochastic acceleration caused by interactions with the disturbed magnetic fields resulted in higher particle intensities later in the event. According to these results, even moderate interplanetary disturbances can significantly alter transport conditions and alter the intensity-time profiles for GLEs. This further corroborates previous findings that the traditional classification of GLEs solely based on temporal characteristics, which may be obscured by transient propagation effects is superfluous. The study highlights the need to integrate multi-spacecraft solar wind observations into the interpretation of GLEs in order to disentangle intrinsic acceleration mechanisms from interplanetary modulation processes.