Persistent Sub-MeV Ion Enhancement Downstream of an Interplanetary Shock: Multispacecraft Observations near L1

On 2026 February 4, an interplanetary shock crossed the L1 region and was followed about 7.7 hr later by a heliospheric current sheet/heliospheric plasma sheet (HCS/HPS), while energetic-ion intensities continued to evolve throughout the intervening downstream region. The shock was observed by IMAP, Wind, ACE, DSCOVR, and SOLAR-1, providing a multispacecraft view of its geometry. Multispacecraft timing and local shock-normal analyses show that the shock is highly quasiperpendicular but spatially nonplanar across L1. Energetic-ion intensities exhibit a pronounced energy-dependent response. Sub-MeV intensities increase well upstream of the shock and remain enhanced throughout the downstream region until approaching the HCS/HPS, whereas intensities above ∼1 MeV show a much weaker downstream enhancement and tend to peak near the shock. The energy spectra also soften downstream, with the fitted spectral break shifting from several MeV near the shock to lower energies farther downstream. Pitch-angle distributions evolve from pronounced field-aligned streaming upstream to enhanced intensities at oblique-to-perpendicular pitch angles downstream, with the anisotropy persisting through much of the downstream sheath and gradually broadening toward the HCS/HPS. A secondary current sheet embedded within the downstream HPS exhibits signatures consistent with a localized reconnection exhaust. Overall, the observations indicate that the sub-MeV ion population is strongly influenced by the shock, while its subsequent downstream evolution is shaped primarily by energy-dependent transport. Localized energization within the disturbed downstream sheath may provide an additional contribution to the sub-MeV population.