Schematic illustration of bidirectional SEP events within a magnetic flux rope structure. The observer (magenta circle) is embedded inside the flux rope (solid blue line), which is rooted at the Sun with both legs. The black arrows indicate the directions of SEP propagation associated with different possible sources: the anti-sunward SEPs primarily associated with flare-acceleration (red asterisk) and the sunward SEPs associated with particles accelerated by the CME-driven shock (black arc).