Authors: Gabriel Muro (Caltech), Christina Cohen (Caltech), Rick Leske (Caltech), Zigong Xu (Caltech)
Solar energetic particle (SEP) events enriched in the rare isotope 3He remain one of the most unusual composition signatures in heliophysics. In the solar wind, the 3He/4He ratio is ~5 x 10−4, and yet SEP observations have measured the ratio far higher, in many cases making 3He comparable to or more abundant than the normally dominant 4He population. This remarkable isotope selectivity cannot be explained by standard coronal abundances or simple transport effects, and is generally interpreted as evidence for specialized acceleration processes involving wave-particle interactions, ion cyclotron resonance, magnetic reconnection, and active-region jets. Because 3He-rich periods are often short-lived, low in total particle intensity, and associated with Type III radio bursts and energetic electrons, they provide a sensitive tracer of small-scale particle acceleration and magnetic connectivity in the low corona and inner heliosphere.
We present a mission-wide survey of 3He-enhanced periods observed by Parker Solar Probe’s IS⊙IS/EPI-Hi/LET instrument from July 2019 through June 2026. Using hourly helium mass-bin histogram data, we construct a database of 3He flux, 4He flux, 3He/4He ratio, energy, time, telescope direction, and PSP heliocentric position across ~1 to 60 MeV/nuc. Unlike traditional event catalogs, this survey is not limited to isolated 3He-rich SEP events, but instead includes all hourly 3He-enhanced periods that satisfy “valley” criteria within the mass-bin data. After scaling for PSP’s unequal radial dwell time, we find that the summed 3He flux does not follow a smooth radial trend. Instead, four radial regimes emerge: the innermost heliosphere shows roughly twice as much 3He flux as any other radial range, 0.15 to 0.35 AU maintains relatively steady levels, 0.40 to 0.55 AU contains a pronounced depletion, and 0.60 to 0.75 AU shows increasing flux dominated largely by large SEP events observed near PSP’s aphelion.
This radial partitioning suggests that 3He-enhanced particles observed by PSP may represent more than a simple population of rapid-onset SEP events diluted outward from compact sources. These results imply that 3He is not only a marker of rare 3He-rich events, but may also act as a statistical tracer of how small-scale coronal acceleration, magnetic connection, and heliospheric transport combine to shape SEP composition throughout the inner heliosphere.