Introduction

In comparison to the universal cosmic ray spectrum that extends to energies well over 1010 GeV, particles emitted by the Sun, even in the most violent solar processes, are of very low energy. Their spectrum extends at most up to about 10 GeV, in some cases particles having energies as high as 50 GeV were reported (e.g. Karpov et al., 1997). However, with respect to the bulk of the radiation from the Sun, corpuscular or other, particles that are associated with energetic solar events, such as solar flares (SF) or coronal mass ejections (CME), that may reach energies as high as 10 GeV and more, are of comparatively high energy for the solar regime. Thus, the term energetic solar particles (ESP) or solar energetic particles (SEP), i.e., energetic with respect to solar wind particles. In the following we will use the first term.

Not all solar flare events produce particle fluxes that are observable on Earth. Whether or not a solar flare causes an observable flux of particles on Earth depends on its strength and on its location at the Sun with respect to the position of the Earth. The accepted view that energetic solar particles are accelerated in association with solar flares or coronal mass ejections is based on the fact that they are time correlated with these phenomena. However, the recently observed association of particle fluxes in the MeV range with interplanetary shocks suggests that fast coronal mass ejections could be another significant contributor of MeV particles in space (Smart, 1996). The mechanisms by which particles are accelerated to the energies observed in a solar flare are not yet fully understood although various models have been proposed.

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