Definition of Solar energetic particles (SEP)

Solar energetic particles (SEP) are high-energy particles coming from the Sun as a result of the very efficient acceleration mechanisms of either magnetic reconnection and/or shock waves. They consist of protons, electrons, helium ions, and HZE ions with energy ranging from a few tens of keV to GeV

Introduction

Solar energetic particle (SEP) events, are among the most important elements of space weather. They consist of protons, electrons, helium ions, and HZE ions with energy ranging from a few tens of keV to GeV (the fastest particles can reach speed up to 80% of the speed of light). They are of particular interest and importance because they can endanger life in outer space (especially particles above 40 MeV). Solar energetic particles can originate from two processes: energetization at a solar-flare site or by shock waves associated with coronal mass ejections (CMEs). However, only about 1% of the CMEs produce strong SEP events.

Energetic particles are produced intermittently due to particle acceleration processes in the solar atmosphere associated with solar flares, and in coronal and interplanetary shocks created by the interaction of coronal mass ejections (CMEs) with the solar wind. An evolving CME may generate an interplanetary shock which propagates in space, deforming the interplanetary magnetic field (IMF) lines and draping them around the driver downstream of the shock. Energetic particles may escape from their acceleration sites and propagate along interplanetary magnetic field (IMF) lines into the interplanetary space enabling their detection by spacecraft.

The intensity-time profiles of SEP events result from the evolution of the particle population in a set of flux tubes that sweep over the observer. The motion of charged energetic particles from their source to the observer is constrained by the Parker spiral pattern of the IMF. This causes an asymmetry in the intensity-time profiles depending upon the observer’s longitude (and latitude).

Classification

SEP events are conventionally classified into two loosely defined categories: impulsive and gradual. This classification was originally based on the decay time scale of the associated soft X-ray flare, but nowadays the naming relates to the duration of the SEP event itself. Impulsive SEP events have durations from hours to a day and are related to short-duration soft X-ray emission (less than 10 minutes). These SEP events, often associated with flare acceleration processes, are characterized by small interplanetary ion intensities, a high electron to proton intensity ratio, enhanced abundances of heavy elements, and enhancements of 3He relative to 4He by up to a factor of 10^4. Given the various uncertainties of the IMF including the solar wind speed variations, impulsive SEP events observed at near geospace are generally limited to within a 30° longitude band about the footpoint of the nominal field line magnetically connected to the flaring region, ~W55º.

Gradual SEP events have durations of days and are related to long-duration (more than 10 minutes) soft X-ray emission
and to interplanetary shocks driven by fast CMEs (vCME> 750 km/s). These events are characterized by large interplanetary ion intensities, small electron to proton ratios, average elemental abundances and ionic charge states in consistency with solar coronal abundances and temperatures. Gradual SEP events have a longitude distribution of the associated flare that is much wider than for impulsive events, essentially spread over the whole solar disk. The associated CME-driven shock waves may extend a large range of heliolongitudes —even up to 180° in extreme cases— and latitudes, regardless of the associated solar flare location.