Advancing interplanetary magnetohydrodynamic models through solar energetic particle modelling: Insights from the 2013 March 15 SEP event

Aims. This study utilises a modelling approach to investigate the impact of perturbed solar wind conditions caused by multiple interplanetary coronal mass ejections (ICMEs) on the evolution of solar energetic particle (SEP) distributions. Furthermore, we demonstrate the utility of SEP models in evaluating the performance of solar wind and coronal mass ejection (CME) models. To illustrate these concepts, we focussed on modelling the gradual SEP event that occurred on 2023 March 15. Methods. We utilised the 3D magnetohydrodynamic model EUHFORIA (EUropean Heliospheric FORecasting Information Asset) to simulate the various ICMEs that caused the highly perturbed solar wind conditions observed during the March 15 event. We conducted three separate EUHFORIA simulations, employing both non-magnetised and magnetised models for these ICMEs. To analyse the behaviour of energetic particles in the simulated solar wind environments, we employed the energetic particle transport and acceleration model PARADISE (PArticle Radiation Asset Directed at Interplanetary Space Exploration). Results. In the vicinity of Earth, the three EUHFORIA simulations exhibit strong similarities and closely match the observed in situ data. Nevertheless, when incorporating these distinct solar wind configurations into PARADISE, notable disparities emerge in the simulated SEP intensities. This discrepancy can be attributed to the different magnetic enhancements and closed magnetic structures introduced by the different CME models within the EUHFORIA simulations. These variations strongly impact the transport mechanisms of SEPs, leading to significant deviations in the particle intensities simulated by PARADISE. Furthermore, our findings highlight the significance of cross-field diffusion even in scenarios with reduced perpendicular mean free path. This effect becomes particularly prominent when SEPs are trapped within the inner heliosphere due to the presence of ICMEs. In these scenarios, the extended duration of confinement allows the slower cross-field diffusion process to become more pronounced and exert a greater influence on the spatial distribution of SEPs, especially near and within the boundaries of ICMEs. Conclusions. Solar energetic particle models enable us to indirectly validate the accuracy of the underlying solar wind and CME models across significant portions of the heliosphere, rather than solely relying on discrete points where spacecraft are situated. This broader validation provides valuable insights into the reliability and effectiveness of the CME models on a global scale.