Abstract
Sixty years of study of energetic particle abundances have made a major contribution to our understanding of the physics of solar energetic particles (SEPs) or solar cosmic rays. An early surprise was the observation in small SEP events of huge enhancements in the isotope 3He from resonant wave–particle interactions, and the subsequent observation of accompanying enhancements of heavy ions, later found to increase 1000-fold as a steep power of the mass-to-charge ratio A/Q, right across the elements from H to Pb. These “impulsive” SEP events have been related to magnetic reconnection on open field lines in solar jets; similar processes occur on closed loops in flares, but those SEPs are trapped and dissipate their energy in heat and light. After early controversy, it was established that particles in the large “gradual” SEP events are accelerated at shock waves driven by wide, fast coronal mass ejections (CMEs) that expand broadly. On average, gradual SEP events give us a measure of element abundances in the solar corona, which differ from those in the photosphere as a classic function of the first ionization potential (FIP) of the elements, distinguishing ions and neutrals. Departures from the average in gradual SEPs are also power laws in A/Q, and fits of this dependence can determine Q values and thus estimate source plasma temperatures. Complications arise when shock waves reaccelerate residual ions from the impulsive events, but excess protons and the extent of abundance variations help to resolve these processes. Yet, specific questions about SEP abundances remain.
Highlights
Differential rotation of the ionized plasma in the Sun generates the solar magnetic field, and solar activity is driven by energy released in sudden changes and reconnections of that field. This energy release appears in the form of solar flares and jets, and in the detachment of coronal mass ejections (CMEs) that expand into interplanetary space
Solar cosmic rays or solar energetic particles (SEPs), as they are commonly known, can be associated with all of these processes, from the direct reconnection and wave–particle resonances in flares and jets to the shock waves driven by fast CMEs
The CMEs from jets that are fast enough to drive shock waves can reaccelerate the SEPs produced in the reconnection event, essentially maintaining much of the increase in heavy-ion abundance with A/Q that is in the impulsive component, but these shock waves can sample the ambient coronal plasma
Summary
Differential rotation of the ionized plasma in the Sun generates the solar magnetic field, and solar activity is driven by energy released in sudden changes and reconnections of that field. This energy release appears in the form of solar flares and jets, and in the detachment of coronal mass ejections (CMEs) that expand into interplanetary space. As the study of SEPs has advanced, we have found that the particles themselves carry information on conditions in their coronal source in addition to the physical processes of their acceleration. The purpose of this article is to acknowledge sixty years of progress in this field, to review the controversies and accomplishments, and to highlight remaining questions
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