Authors: Adolfo Santa Fe Dueñas, Noé Lugaz, Bin Zhuang, Nada AlHaddad

We investigate longitudinal variations in energetic storm particle (ESP) intensities and shock compression properties using multi-spacecraft observations from STEREO-A and L1 spacecraft (primarily ACE) during Solar Cycle 25. For each individual ESP event observed at both spacecraft, we verify that the signatures are associated with the same CME-driven shock. ESP intensity ratios (Avg(+3h/–3h), Peak/Avg(–3h), and I_ESP) are calculated at ~0.09, 0.25, and 0.67 MeV and examined as a function of the spacecraft–flare longitudinal angle. We find consistent longitudinal trends, with peak intensities near 0° separation, especially for fast CMEs (v > 1000 km/s). Events simultaneously observed by ACE and STEREO-A confirm this trend. We also analyze the relationship between ESP intensities and local shock compression ratios, using upstream/downstream measurements of solar wind density and magnetic field. Interestingly, higher shock compression ratios in density often correlate with lower ESP intensities, in contrast with earlier expectations. For magnetic field compression, the trend is clearer and generally positive, especially for fast events. These results provide new insights into the longitudinal distribution and local shock conditions that govern ESP production.