The acceleration and propagation of solar energetic particles

Abstract

Solar Energetic Particles (SEPs) are an important component of Space Weather, including radiation hazard to humans and electronic equipment, and the ionisation of the Earth's atmosphere. We review the key observations of SEPs, our current understanding of their acceleration and transport, and discuss how this knowledge is incorporated within Space Weather forecasting tools. Mechanisms for acceleration during solar flares and at shocks driven by Coronal Mass Ejections are discussed, as well as the timing relationships between signatures of solar eruptive events and the detection of SEPs in interplanetary space. Evidence on how the parameters of SEP events are related to those of the parent solar activity is reviewed and transport effects influencing SEP propagation to near-Earth locations are examined. Finally, the approaches to forecasting Space Weather SEP effects are discussed. We conclude that both flare and CME shock acceleration contribute to Space Weather relevant SEP populations and need to be considered within forecasting tools.

Energetic Particle Propagation in Three Dimensions

“…Explanation of the transport behaviors is still challenging. There clearly is a need for a mission to make measurements out-of-the-ecliptic plane during a large sample of SEP events to fully characterize particle transport in the inner heliosphere and understand the physical causes of key differences (e.g., significant delays in the arrival of the ions and in reaching peak intensities) with respect to the near-ecliptic measurements (e.g., Dalla, 2003;2004).…”

“…With the notable exception of Ulysses, our current understanding of SEP transport is essentially confined to observations near the ecliptic plane, and is hence inherently 2-dimensional. However, Ulysses data obtained during its rapid transit over the polar regions of the Sun showed several key differences with respect to the near-ecliptic measurements (e.g., Lario et al 2003;Dalla, 2004).…”

“…In particular, significant delays in the arrival of the ions and in reaching their peak intensity was inconsistent with standard models of SEP acceleration and propagation, and was instead attributed to efficient cross-field diffusion of energetic particles across helio-latitudes. Clearly, SEP observations from an out-of-the-ecliptic plane mission during a large sample of events are needed to fully characterize particle transport in the inner heliosphere and understand the physical causes of the differences with respect to the near-ecliptic measurements (e.g., Dalla, 2003;2004).…”

“…Figure1. Time-intensity profiles of an SEP event observed simultaneously by Ulysses at a heliolatitude of 71 ° and by the Solar and Heliospheric Observatory (SoHO) near the ecliptic plane at 1 au, taken fromDalla (2004).…”