The Impact of Particle Drifts on the Flux of High-energy Solar Protons and He Ions Arriving at Earth from Compact Solar Sources

We present a numerical model to investigate the role of the ion mass-charge ratio (m/q) in determining the flux of MeV-GeV solar energetic particles (SEPs), released impulsively from a compact source near the Sun at 1 au. The equations of motion are numerically integrated for 0.1-1 GeV n⁻¹ H⁺³He²⁺, and ⁴He²⁺ test ions as they move in the Parker spiral interplanetary magnetic field (IMF) carried by the solar wind. Pitch-angle scattering is included by using an ad hoc scattering operator, resulting in a parallel mean free path corresponding to “weak” scattering (λ = 1 au). Our modeled field also includes a flat heliospheric current sheet (HCS) at the ecliptic plane. We consider both A- and A+ IMF polarities and determine the time-flux profile at 1 au, at multiple observation locations, for each ion species and energy. We find that the proximity of the observer to the HCS, magnetic connectivity, and the drifts that depend on kinetic energy, m/q, and IMF polarity strongly impact the resulting flux-time profile. We find significant differences between the onset time, decay time, peak flux, and initial flux of the three ion species at 1 au. This contrast in the decay phase of the three species is evident in the four edges of the magnetically well-connected areas at 1 au. For magnetically poorly connected observers to the source, the onset of the proton flux with respect to that of ³He²⁺, and ⁴He²⁺ can be delayed for several hours depending on the observer’s relative heliographic latitude and the SEPs' energy.