³He Enrichment in SEP Events: Observational Constraints and Evidence from Isotopic Fractionation via Resonant Wave–Particle Interactions and Turbulent Acceleration

Extremely high-energy SEPs, which attain energies in the GeV range, are accelerated during impulsive solar flares and in association with CMEs. These relativistic particles represent a primary driver of hazardous space weather phenomena. They pose significant risks to spacecraft operations and crew safety during deep-space missions, while also threatening ground-based infrastructure, particularly power grids and other critical components of the electromagnetic environment, through geomagnetic disturbances and induced currents. Spectral observations indicate that the acceleration of SEPs to GeV energies involves highly complex, multi-component processes characterized by diverse ion compositions. The particle population predominantly comprises hydrogen (protons and electrons, approximately 73%), helium (approximately 25%), and heavier ions. Notably, the mass-to-charge ratio (A/Q) differs markedly between helium isotopes: 1.5 for ³He²⁺ and 2.0 for ⁴He²⁺.

 

In the present study, we examine the acceleration processes and ³He enrichment in SEP events within a statistical plasma-physics framework integrated with turbulence theory. Our methodology explicitly accounts for the realistic proton-to-electron mass ratio, the distinct mass-to-charge ratios of relevant ion species, and the effects of turbulence resistivity and viscosity. These elements are incorporated into a fully coupled hydrodynamics–magnetohydrodynamics–kinetic model that bridges continuous spatial and temporal scales, thereby circumventing the traditional separation of micro-kinetic and macro-dynamic regimes.

 

Numerical simulations are conducted on a supercomputer using our newly developed relativistic hybrid particle-in-cell and lattice-Boltzmann method (RHPIC-LBM) code. This advanced computational approach facilitates self-consistent modeling of the multi-scale, multiphase dynamics underlying the extreme ³He enhancements observed in impulsive solar SEP events. The simulations reveal that high-frequency magnetic fluctuations at kinetic scales play a pivotal role in generating self-organized perturbations. Concurrently, plasma velocity fluctuations sustain and amplify these waves through self-feeding mechanisms. When the frequency of these perturbations approaches the Langmuir oscillation frequency, resonant wave–particle interactions become particularly efficient. Langmuir turbulence, driven by nonlinear resonant wave–particle coupling, preferentially accelerates ions whose resonance conditions match those of ³He. This selective resonance renders the acceleration of ³He substantially more efficient than that of ⁴He, thereby providing a compelling explanation for the extreme ³He enrichment characteristic of impulsive SEP events.