The Two Sources of Solar Energetic Particles

By D. Reames. 2012

487 citations recorded by the source corpus.

Evidence for two different physical mechanisms for acceleration of solar energetic particles (SEPs) arose 50 years ago with radio observations of type III bursts, produced by outward streaming electrons, and type II bursts from coronal and interplanetary shock waves. Since that time we have found that the former are related to “impulsive” SEP events from impulsive flares or jets. Here, resonant stochastic acceleration, related to magnetic reconnection involving open field lines, produces not only electrons but 1000-fold enhancements of 3He/4He and of (Z>50)/O. Alternatively, in “gradual” SEP events, shock waves, driven out from the Sun by coronal mass ejections (CMEs), more democratically sample ion abundances that are even used to measure the coronal abundances of the elements. Gradual events produce by far the highest SEP intensities near Earth. Sometimes residual impulsive suprathermal ions contribute to the seed population for shock acceleration, complicating the abundance picture, but this process has now been modeled theoretically. Initially, impulsive events define a point source on the Sun, selectively filling few magnetic flux tubes, while gradual events show extensive acceleration that can fill half of the inner heliosphere, beginning when the shock reaches ∼2 solar radii. Shock acceleration occurs as ions are scattered back and forth across the shock by resonant Alfvén waves amplified by the accelerated protons themselves as they stream away. These waves also can produce a streaming-limited maximum SEP intensity and plateau region upstream of the shock. Behind the shock lies the large expanse of the “reservoir”, a spatially extensive trapped volume of uniform SEP intensities with invariant energy-spectral shapes where overall intensities decrease with time as the enclosing “magnetic bottle” expands adiabatically. These reservoirs now explain the slow intensity decrease that defines gradual events and was once erroneously attributed solely to slow outward diffusion of the particles. At times the reservoir from one event can contribute its abundances and even its spectra as a seed population for acceleration by a second CME-driven shock wave. Confinement of particles to magnetic flux tubes that thread their source early in events is balanced at late times by slow velocity-dependent migration through a tangled network produced by field-line random walk that is probed by SEPs from both impulsive and gradual events and even by anomalous cosmic rays from the outer heliosphere. As a practical consequence, high-energy protons from gradual SEP events can be a significant radiation hazard to astronauts and equipment in space and to the passengers of high-altitude aircraft flying polar routes.

Subfields: Astronomy and Astrophysics

View Paper: https://www.semanticscholar.org/paper/81e46eadd26f750715b02170af957a24022fb35f

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Citations in the corpus

• Solar flares, coronal mass ejections and solar energetic particle event characteristics (2016, background): “The classical paradigm divides SEP events into two categories: the impulsive and the gradual ones based on their parent solar events (i.e., SFs, CMEs) (Reames 1999, 2013).”
• Advances in Observing Various Coronal EUV Waves in the SDO Era and Their Seismological Applications (Invited Review) (2014, unknown): “Solar energetic particles (SEPs) are generally considered to be accelerated by CME shocks and/or flares (see Reames, 2013 for a review).”
• The Physical Processes of CME/ICME Evolution (2017, methodology): “A connection between type II radio bursts and CME-driven shocks has been established by many studies (e.g., Reiner et al. 1998; Liu et al. 2009, 2013; Reames 2013; Gopalswamy et al. 2013; Nitta et al. 2014; Gopalswamy et al. 2016) MHD simulations also provide predictions of the appearance of CME-dri”
• The Origin, Early Evolution and Predictability of Solar Eruptions (2018, background): “The impact of a CME extends beyond its direct interaction with the magnetosphere as shocks driven by fast CMEs (roughly above around 800 km s−1, Kahler, 2001) can accelerate particles, producing an additional space weather effect when these solar energetic particles (SEPs) arrive at the Earth (Reame”
• The Interaction of Successive Coronal Mass Ejections: A Review (2016, background): “CME-driven shocks are overwhelmingly thought to be the main accelerator of gradual solar energetic particles (SEPs) (Kahler et al., 1984; Reames, 2013).”
• The Solar Orbiter mission (2020, background): “Two main physical mechanisms are thought to energise SEPs: diffusive shock acceleration (also known as Fermi or stochastic acceleration, Jones & Ellison 1991) and acceleration by solar flares or jets (Reames 2013).”
• Integrated Science Investigation of the Sun (ISIS): Design of the Energetic Particle Investigation (2016, background): “2, it is now widely accepted that the Sun accelerates charged particles (ions and electrons) from ∼few keV up to near-relativistic energies in at least two ways (e.g., Reames 2013).”
• Relationship between Solar Energetic Particles and Properties of Flares and CMEs: Statistical Analysis of Solar Cycle 23 Events (2014, background): “One of the outcomes of such studies has been the suggestion, in the 1990s, of the so-called two-class paradigm for SEPs (Cane, McGuire, and von Rosenvinge, 1986; Reames, 1988, 1999, 2013).”