Modelling solar energetic particle events: the shock-and-particle model approach.

Author: Angels Aran
Affiliation: Departament de Física Quàntica i Astrofísica, Institut de Ciències del Cosmos. Universitat de Barcelona.

Abstract:

Europe is working towards developing its own infrastructure for the monitoring and prediction of space weather. This includes the study of solar energetic particle (SEP) events, with the ultimate goal of predicting the particle radiation environment in the interplanetary space, and particularly at near-Earth. The most important contribution to radiation doses comes from protons > 10 MeV, although ions > 1 MeV/nuc may traverse typical shielding. We know that, per solar cycle, about 100 SEP events with > 10 MeV protons are produced, and also that < 10 large events occur with fluences accounting for a significant fraction of the cycle’s accumulated fluence. Hence, at present, we need both the study of individual SEP event periods, and statistical SEP models for long-term (years) predictions.

Currently, 3-dimensional models describing the propagation of interplanetary shocks (IP) driven by coronal mass ejections are routinely run to estimate IP shocks arrivals at Earth. The combination of such models with SEP models of particle acceleration and transport is the next big step to be undertaken for the near-real time forecast of large SEP events. The main difficulty is that the current knowledge of the mechanisms involved in the generation of these SEP events is not complete yet. In addition to the models describing SEP events episodes, particle radiation models are used to estimate the fluence, peak intensity and worse case scenarios that interplanetary missions may encounter during their orbit. These models are based on the statistical treatment of SEP data measured during the last 40 years, and they make assumptions of the variation of the SEP event size with the heliocentric radial distance from the Sun.

In the University of Barcelona, and in the recent years in collaboration with KU Leuven, we have developed the shock-and-particle (SaP) model for the description of gradual SEP events. This model combines the simulation of the propagation of a CME-driven shock (from ~ 4 solar radii) and the simulation of the transport of particles along the interplanetary magnetic field (IMF) line connecting the shock front and the observer. We assume that the shock-accelerated particles escaping from the shock are injected at the point in the shock front intersected by this IMF line, i.e. at the cobpoint. The method developed permits us to perform simulations of < 300 MeV proton intensity-time profiles to be measured by a virtual armada of spacecraft scanning different regions of the expanding interplanetary shock front. A semi-empirical relation between the injection rate of shock-accelerated particles, Q, and the jump in speed across the shock, VR, was derived from the modelling of several SEP events. Such a relation -the Q(VR) relation- is the basis of the ESA’s SEPEM/SOLPENCO2 tool (http://dev.sepem.oma.be). SOLPENCO2 provides the SEPEM statistical analysis tools for interplanetary missions with heliocentric radial distance scaling parameters of the SEP events peak intensities and fluences. We present here the latest developments of the SaP model and of the SOLPENCO2 tool.