Authors: L. Y. Khoo (Princeton University)

We present observations of a large, gradual solar energetic particle (SEP) event detected by Parker Solar Probe (PSP) during its inbound trajectory toward perihelion (Encounter 22). The event, observed across multiple species—including protons, helium, electrons, and heavy ions—persisted for approximately seven days (17–25 December 2024). This sustained activity is consistent with a series of eruptions that continuously injected energetic particles into the inner heliosphere. The initial SEP onset is likely associated with a fast CME (~3161 km/s) launched around 16:00 UT on 17 December. PSP did not observe a sharp SEP increase until it crossed a magnetic sector boundary near 22:30 UT, suggesting SEPs were confined behind this magnetic sector-boundary during their propagation to PSP. Notably, PSP’s crossing of the heliospheric current sheet (HCS) near perihelion did not coincide with any significant change in SEP intensity, indicating that the HCS was not a dominant boundary for particle transport at this time. Instead, the SEP intensities dropped to background levels upon PSP’s encounter with a fast solar wind stream at <15 solar radii, suggesting a disruption in magnetic connectivity and particle transport conditions. This event offers new insights into the influence of evolving heliospheric conditions close to the Sun on the SEP transport and confinement mechanisms.