Abstract
Solar energetic particles, or SEPs, from suprathermal (few keV) up to relativistic (sim few GeV) energies are accelerated near the Sun in at least two ways: (1) by magnetic reconnection-driven processes during solar flares resulting in impulsive SEPs, and (2) at fast coronal-mass-ejection-driven shock waves that produce large gradual SEP events. Large gradual SEP events are of particular interest because the accompanying high-energy ({>}10s MeV) protons pose serious radiation threats to human explorers living and working beyond low-Earth orbit and to technological assets such as communications and scientific satellites in space. However, a complete understanding of these large SEP events has eluded us primarily because their properties, as observed in Earth orbit, are smeared due to mixing and contributions from many important physical effects. This paper provides a comprehensive review of the current state of knowledge of these important phenomena, and summarizes some of the key questions that will be addressed by two upcoming missions—NASA’s Solar Probe Plus and ESA’s Solar Orbiter. Both of these missions are designed to directly and repeatedly sample the near-Sun environments where interplanetary scattering and transport effects are significantly reduced, allowing us to discriminate between different acceleration sites and mechanisms and to isolate the contributions of numerous physical processes occurring during large SEP events.
Highlights
1.1 Historical perspective: pre-space age Motivated by the discovery of the sunspot cycle by Schwabe (1844) and an apparent connection between variations in sunspots and geomagnetic activity by Sabine (1852), Richard Carrington embarked on a comprehensive study of sunspots over an ∼8-year period from November 9, 1853 to March 24, 1861
The particles observed in interplanetary space and near Earth are commonly referred to as solar energetic particles or SEPs: those accelerated at flares are known as impulsive SEP events, particle populations accelerated by near-Sun coronal mass ejections (CMEs)-shocks are termed as gradual SEPs, and those associated with CME shocks observed near Earth are known as energetic storm particles or ESP events
The considerable disparity in observational aspects of ST populations has led to two basic categories of competing theories: (1) ST tails result from continuous acceleration in interplanetary space (e.g., Fisk and Gloeckler 2008, 2012, 2014; Zhang 2010; Fahr et al 2012; Drake et al 2012; Zank et al 2014), or (2) ST tails are lower energy portions of material accelerated in energetic particle events such as corotating interaction regions (CIRs), CME shocks, flares, etc. (e.g., Livadiotis and McComas 2009; Jokipii and Lee 2010; Schwadron et al 2010a)
Summary
1.1 Historical perspective: pre-space age Motivated by the discovery of the sunspot cycle by Schwabe (1844) and an apparent connection between variations in sunspots and geomagnetic activity by Sabine (1852), Richard Carrington embarked on a comprehensive study of sunspots over an ∼8-year period from November 9, 1853 to March 24, 1861. In an eloquent article, entitled “Description of a Singular Appearance seen in the Sun on September 1, 1859,” Carrington (1859) describes his observations of that day (quoted and paraphrased): While engaged in the forenoon of Thursday, September 1, in taking his customary observation of the form and positions of the solar spots, an appearance was witnessed which he believed to be exceedingly rare Describing it as the break out of two patches of intensely white light (identified as A and B in Figure 1), Carrington’s first impression was that by some chance a ray of light had penetrated a hole in the screen attached to the object-glass. Carrington noted that the outburst lasted less than ∼5 minutes, from ∼1118 to ∼1123 Greenwich mean time (GMT)
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