Extending the Energy Range of the SEPSTER Solar Energetic Particle Prediction Scheme

SEPSTER (SEP predictions based on STEReo observations) described in Richardson et al., Space Weather, 16, 1862, 2018, is an empirical model for the prediction of the peak intensity of solar energetic particle (SEP) events based on a relationship between SEP peak intensity and the speed and direction relative to the observer of the parent coronal mass ejection (CME) obtained from observations of SEP events detected by the STEREO spacecraft and at Earth. This original version predicts the peak intensity of 14-24 MeV protons. To extend the energy range of the predictions, one simple approach is to determine the typical numerical ratio of the intensities in the desired energy range and for 14-24 MeV protons in a sample of SEP events, then multiply the predicted 14-24 MeV proton intensity by this ratio to make a prediction for the desired energy range. For example, the GOES > 10 MeV (>100 MeV) integral flux (in pfu) is around 20 (0.2) times the intensity at 14-24 MeV (in (MeV s cm² sr)^-1). This version of SEPSTER is currently operational at the CCMC and is part of the Integrated Solar Energetic Proton Alert/Warning System (ISEP) under development to support missions such as Artemis.

Here we present a new version of SEPSTER that takes advantage of multi-point observations from the GOES, PAMELA and STEREO spacecraft to develop a new parameterization of the proton spatial distribution at 1 AU at energies ranging from 10 MeV to ∼130 MeV. This provides estimates of both peak and event-integrated intensity spectra in this energy range using near real-time CME information from the DONKI catalog as input. This analysis also provides insight into the longitudinal distribution of SEPs at 1 AU over this extended energy range, such as variations in the longitude of the intensity peak with proton energy that may be related to the energy-dependence of the contributions from particles accelerated early in the event near the Sun and those accelerated by the CME-driven shock as it moves out through the inner heliosphere. Validation of the two approaches to extending the SEPSTER energy range will also be discussed.