Quantifying the Effects of Parameters in Widespread SEP Events with EPREM

TL;DR

The paper investigates how key parameters in EPREM influence the morphology of widespread SEP events by using an uncoupled cone-shock model to generate MHD context for proton acceleration and transport. Through eight simulation runs (baseline plus seven parameter variations) on a Lagrangian grid with 294 streams and 16 fixed observers, it demonstrates that SEP flux profiles are highly sensitive to perpendicular diffusion, mean free-path scaling, and shock profile, producing substantial longitudinal spread but sometimes strong suppression beyond $\pm90^{\circ}$ from the shock origin. The results illuminate how transport physics and connectivity govern SEP timing and energy spectra, offering a framework to interpret multi-spacecraft observations and infer the solar-wind state along CME-driven paths. The work also underscores the potential of EPREM as a development platform for coupling-agnostic SEP modeling and future observational validation.

Abstract

The Energetic Particle Radiation Environment Model (EPREM) solves the focused transport equation (FTE) on a Lagrangian grid in a frame co-moving with the solar wind plasma and simulates the acceleration and transport of solar energetic particles (SEP) in the heliosphere. When not coupled to an external magnetohydrodynamic model, EPREM functions in an uncoupled mode where an ideal cone-shock is injected into a homogeneous background solar wind. We carried out an analysis of the effects of multiple physical parameters in producing widespread SEP events simulated by the uncoupled EPREM using a relatively simple model of a strong magnetized shock propagating radially outward through the inner heliosphere to produce the requisite MHD quantities for EPREM's sophisticated model of proton acceleration and transport. We compared a baseline simulation with seven variations in which the value of a single parameter differed from its baseline value. All simulations exhibit complex profiles of SEP flux as a function of time and energy, with clear dependence on parameters related to diffusion, mean free path, and shock profile. Moreover, while all simulations exhibit significant longitudinal spread in SEP flux, for certain parameter values there exists a decrease or absence in SEP flux at observers located $\geq 90^\circ$ from the shock origin. Relating the differences in SEP flux to the specific values of each parameter in the simulations provides insight into the morphology of observed SEP events and the state of the solar wind through which the driving CME propagates.

Figures (20)

• Figure 1: Positions of point observers. Dotted circles indicate observer radii. The small circle at the center represents the Sun and the arrow protruding from the Sun indicates the origin and direction of the ideal shock. Dashed lines show the region of influence of the ideal shock.
• Figure 2: MHD quantities as functions of time during the baseline simulation run. The top set of panels shows components of the magnetic field, the velocity field, and the density at 0.5 au. The bottom set of panels show the same quantities at 1.0 au. Black dashed lines bounding the magnetic-field components represent $\pm |\vec{B}|$.
• Figure 3: Flux at each point observer during the baseline simulation run. Observer locations have been adjusted to avoid overlap and to condense the figure. Refer to Figure \ref{['fig:observer-positions']} for correspondence between observer number and location. Vertical dotted lines indicate the time of shock passage, where applicable. All panels share a common x axis, labeled below panel 08, and y axis, labeled to the left of panel 13.
• Figure 4: Fluxes of protons with energies (top-to-bottom) 1 MeV, 10 MeV, and 100 MeV at simulation days (left-to-right) 2, 3, 4, and 5. Point observer locations are indicated by black circles. Note that the streams shown are a subset of the 294 streams used in for the simulation run.
• Figure 5: Flux at each point observer during the simulation run with no perpendicular diffusion. The layout is identical to that of Figure \ref{['fig:observer-flux-baseline']}.