EPREM

The Energetic Particle Radiation Environment Module (EPREM) models acceleration and transport of ions throughout the heliosphere by numerically solving the focused transport equation on a Lagrangian grid in a frame co-moving with the solar wind plasma. EPREM manages a set of surfaces on which it places nodes for calculation. Each set of linked nodes (a “stream”) defines a velocity path line that tracks the trajectory of the distribution of energetic particles. If, and only if, the frozen-in condition of magnetohydrodynamics holds, these lines also represent the magnetic field lines. The EPREM streams therefore form a Parker spiral in the case of steady state solar wind. This steady state solar wind is what is what is provided in the uncoupled version of the model.

The model allows for the grid of nodes to be specified, which will dictate the total number of streams covering the heliosphere. For locations between streams, interpolation must be used to calculated particle and plasma quantities. The model simulates in three dimensions, and allows observers to be placed anywhere within that volume. Observers will return particle and plasma measurements at their location, allowing for the simulation of Solar Energetic Particle (SEP) events at locations chosen by the user.

In this uncoupled, open source version of the model, an ideal cone shock with parameters defined by the user can be included. Using a source particle spectrum (which can also be tuned by the user), the ideal shock will accelerate particles and those energetic particles will propagate out into the heliosphere.

The model computes the particle distribution using a Lagrangian frame of reference, which means that the coordinates used in the simulation will move and deform with the solar wind. However, the output can be organized and viewed in coordinates that are fixed in space (for example, centered on the Sun).

EPREM requires values of the magnetic field, velocity field, and density in the ambient plasma in order to update the ion distribution function via the focused transport equation. The currently available model is not coupled to another model to produce complex heliosphere conditions. Therefore, the model produces a simple Parker spiral structure perturbed by a tunable ideal shock.

EPREM uses a simple text file as an input. It accepts user arguments to define a wide variety of simulation parameters. The number of nodes can be defined, along with the time duration and time step size. Many physical parameters – including, but not limited to, solar wind parameters, ideal shock parameters, source particle spectrum parameters, and particle transport parameters – can be tuned by the user. There are a number of other inputs that can also be specified by the user to customize the simulation run. Model input parameters can be easily accessed through the CCMC model input page.

The raw output of EPREM is a number of netCDF files, one for each stream of nodes and one for each observer. These output files contain the particle and plasma outputs from the simulation run. A companion python package “eprempy” has been developed for processing this data (linked below). In addition, a model run at the CCMC includes a number of quicklook graphics as output. These will include plots to show particle and solar wind parameters at each observer chosen. In addition, there are interactive html files of the full simulated portion of the heliosphere for selected simulation parameters. There are also .gif images produced to show the particle and solar wind output simulated by the run.