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Corotating Shock Waves and the Solar-Wind Source of Energetic Ion Abundances: Power Laws in A/Q  

Donald V Reames   Submitted: 2018-08-30 12:52

We find that element abundances in energetic ions ccelerated by shock waves formed at corotating interaction regions (CIRs) mirror the abundances of the solar wind modified by a decreasing power-law dependence on the mass-to-charge ratio A/Q of the ions. This behavior is similar in character to the well-known power-law dependence on A/Q of abundances in large gradual solar energetic particles (SEP). The CIR ions reflect the pattern of A/Q, with Q values of the source plasma temperature or freezing-in temperature of 1.0 - 1.2 MK typical of the fast solar wind in this case. Thus the relative ion abundances in CIRs are of the form (A/Q)**a where a is nearly always negative and evidently decreases with distance from the shocks, which usually begin beyond 1 AU. For one unusual historic CIR event where a = 0, the reverse shock wave of the CIR seems to occur at 1 AU, and these abundances of the energetic ions become a direct proxy for the abundances of the fast solar wind.

Authors: Donald V. Reames
Projects: Wind

Publication Status: accepted by Solar Physics
Last Modified: 2018-08-31 11:22
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The "FIP Effect" and the Origins of Solar Energetic Particles and of the Solar Wind  

Donald V Reames   Submitted: 2018-01-19 11:43

We find that the element abundances in solar energetic particles (SEPs) and in the slow solar wind (SSW), relative to those in the photosphere, show different patterns as a function of the first ionization potential (FIP) of the elements. Generally, the SEP and SSW abundances reflect abundance samples of the solar corona, where low-FIP elements, ionized in the photosphere, are more efficiently conveyed upward to the corona than high-FIP elements that are initially neutral atoms. Abundances of the elements, especially C, P, and S show a crossover from low to high FIP at ~10 eV in the SEPs but ~14 eV for the solar wind, suggesting that cooler photospheric plasma, perhaps from cool sunspots beneath active regions, supply the coronal source material that eventually will be shock-accelerated as SEPs, while the source of the SSW may lie at the base of diverging open-field lines surrounding but outside active regions. Meanwhile, energetic particles accelerated from the solar wind itself by shock waves at corotating interaction regions (CIRs), generally beyond 1 AU, confirm the FIP pattern of the solar wind.

Authors: Donald V. Reames
Projects: None

Publication Status: submitted to Solar Phys.
Last Modified: 2018-01-23 11:53
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The Abundance of Helium in the Source Plasma of Solar Energetic Particles  

Donald V Reames   Submitted: 2017-09-13 14:39

Studies of patterns of abundance enhancements of elements, relative to solar-coronal abundances, in large solar energetic-particle (SEP) events, and of their power-law dependence on the mass-to-charge ratio A/Q of the ions, have been used to determine the effective source-plasma temperature T that defines the Q-values of the ions. We find that a single assumed value for the coronal reference He/O ratio in all SEP events is often inconsistent with the transport-induced power-law trend of the other elements. In fact, the coronal He/O actually varies rather widely from one SEP event to another. In the large Fe-rich SEP events with T = 3 MK, where shock waves, driven out by coronal mass ejections (CMEs), have reaccelerated residual ions from impulsive suprathermal events that occur earlier in solar active regions, He/O ~ 90, a ratio similar to that in the slow solar wind, which may also originate from active regions. Ions in the large SEP events with T < 2 MK may be accelerated outside active regions, and have values of 40 < He/O < 60. Mechanisms that determine coronal abundances, including variations of He/O, are likely to occur near the base of the corona (at ~ 1.1 RS) and thus to affect both SEPs (at ~2 - 3 RS) and the solar wind. Other than He, reference coronal abundances for heavier elements show little temperature dependence or systematic difference between SEP events; He, the element with the highest first ionization potential, is unique. The CME-driven shock waves probe the same regions of space, at ~2 RS near active regions, which are also likely sources of the slow solar wind, providing complementary information on conditions in those regions.

Authors: Donald V. Reames
Projects: Wind

Publication Status: accepted for Solar Physics
Last Modified: 2017-09-15 10:14
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Temperature of the Source Plasma for Impulsive Solar Energetic Particles  

Donald V Reames   Submitted: 2015-05-19 09:03

The steep power-law dependence of element abundance enhancements on the mass-to-charge ratios [A/Q] of the ions in impulsive solar energetic-particle (SEP) events causes these enhancements to reflect the temperature-dependent pattern of Q of the ions in the source plasma. We searched for SEP events from coronal plasma that is hotter or cooler than the limited region of 2.5 - 3.2 MK previously found to dominate 111 impulsive SEP events. Fifteen new events were found, four (three) originated in 2-MK (4-MK) plasma, but none from outside this temperature range. Although the impulsive SEP events are strongly associated with flares, this result indicates that these ions are not accelerated from flare-heated plasma, which can often exceed 10 MK. Evidently the ions of 2 - 20 MeV/amu that we observe in space are accelerated from active-region plasma on open magnetic-field lines near the flare, but not from the closed loops of the flare. The power-law dependence of the abundance enhancements on A/Q of the ions is expected from theoretical models of acceleration from regions of magnetic reconnection.

Authors: D. V. Reames, E.W. Cliver, and S. W. Kahler
Projects: Wind

Publication Status: accepted for publication by Solar Physics
Last Modified: 2015-05-20 09:44
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Variations in Abundance Enhancements in Impulsive Solar Energetic-Particle Events and Related CMEs and Flares  

Donald V Reames   Submitted: 2014-07-31 09:15

We study event-to-event variations in the abundance enhancements of the elements He through Pb for Fe-rich impulsive solar energetic-particle (SEP) events, and their relationship with properties of associated coronal mass ejections (CMEs) and solar flares. Using a least-squares procedure we fit the power-law enhancement of element abundances as a function of their mass-to-charge ratio A/Q to determine both the power and the coronal temperature (which determines Q) in each of 111 impulsive SEP events identified previously. Individual SEP events with the steepest element enhancements, e.g. ~(A/Q)^6, tend to be smaller, lower-fluence events with steeper energy spectra that are associated with B- and C-class X-ray flares, with cooler (~2.5 MK) coronal plasma, and with narrow (<100 deg), slower (<700 km s-1) CMEs. On the other hand, higher-fluence SEP events have flatter energy spectra, less-dramatic heavy-element enhancements, e.g. ~(A/Q)^3, and come from somewhat hotter coronal plasma (~3.2 MK) associated with C-, M- and even X-class X-ray flares and with wider CMEs. Enhancements in 3He/4He are uncorrelated with those in heavy elements. However, events with 3He/4He > 0.1 are even more strongly associated with narrow, slow CMEs, with cooler coronal plasma, and with B- and C-class X-ray flares than are other Fe-rich impulsive SEP events with smaller enhancements of 3He.

Authors: Donald V. Reames, Edward W. Cliver, and Stephen W. Kahler
Projects: Wind

Publication Status: accepted by Solar Physics
Last Modified: 2014-07-31 17:36
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Abundance Enhancements in Impulsive Solar Energetic-Particle Events with Associated Coronal Mass Ejections  

Donald V Reames   Submitted: 2014-05-28 13:33

We study the abundances of the elements He through Pb in Fe-rich impulsive solar energetic-particle (SEP) events with measurable abundances of ions with atomic number Z>2 and their relationship with coronal mass ejections (CMEs). On average the element abundances in these events are similar to coronal abundances at low Z but, for heavier elements, enhancements rise as a power law in the mass-to-charge ratio A/Q of the ions (at coronal temperatures of 2.5-3 MK) to a factor of 3 at Ne, 9 at Fe, and 900 for Z>76. The 111 of these Fe-rich impulsive SEP events we found, between November 1994 and August 2013 using the Wind spacecraft, have a 69% association rate with CMEs. The CMEs are narrow with a median width of 75 deg, are characteristically from western longitudes on the Sun, and have a median speed of ~600 km s-1. Nearly all SEP onsets occur within 1.5-5 h of the CME onset. The faster (>700 km s-1), wider CMEs in our sample are related to SEPs with coronal abundances indicating hot coronal plasma with fully ionized He, C, N and O and moderate enhancements of heavier elements, relative to He, but slower (<700 km s-1), narrower CMEs emerge from cooler plasma where higher SEP mass-to-charge ratios, A/Q, yield much greater abundance enhancements, even for C/He and O/He. Apparently, the open magnetic-reconnection region where the impulsive SEPs are accelerated also provides the energy to drive out CME plasma, accounting for a strong, probably universal, impulsive SEP-CME association.

Authors: Donald V. Reames, Edward W. Cliver, Stephen W. Kahler
Projects: Wind

Publication Status: Solar Physics (in press)
Last Modified: 2014-05-29 11:24
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The Two Sources of Solar Energetic Particles  

Donald V Reames   Submitted: 2013-06-15 14:51

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.

Authors: Donald V. Reames
Projects: Wind

Publication Status: accepted by Space Science Reviews
Last Modified: 2013-06-17 19:53
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Element Abundances in Solar Energetic Particles and the Solar Corona  

Donald V Reames   Submitted: 2013-06-10 10:58

This is a study of abundances of the elements He, C, N, O, Ne, Mg, Si, S, Ar, Ca, and Fe in solar energetic particles (SEPs) in the 2 - 15 MeV amu-1 region measured on the Wind spacecraft during 54 large SEP events occurring between November 1994 and June 2012. The origin of most of the temporal and spatial variations in abundances of the heavier elements lies in rigidity-dependent scattering during transport of the particles away from the site of acceleration at shock waves driven out from the Sun by coronal mass ejections (CMEs). Variation in the abundance of Fe is correlated with the Fe spectral index, as expected from scattering theory but not previously noted. Clustering of Fe abundances during the ''reservoir'' period, late in SEP events, is also newly reported. Transport-induced enhancements in one region are balanced by depletions in another, thus, averaging over these variations produces SEP abundances that are energy independent, confirms previous SEP abundances in this energy region, and provides a credible measure of element abundances in the solar corona. These SEP-determined coronal abundances differ from those in the solar photosphere by a well-known function that depends upon the first ionization potential (FIP) or ionization time of the element.

Authors: Donald V. Reames
Projects: Wind

Publication Status: accepted for Solar Physics
Last Modified: 2013-06-11 11:32
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Particle Acceleration at the Sun and in the Heliosphere  

Donald V Reames   Submitted: 1999-11-04 16:04

Energetic particles are accelerated in rich profusion at sites throughout the heliosphere. They come from solar flares in the low corona, from shock waves driven outward by coronal mass ejections (CMEs), from planetary magnetospheres and bow shocks. They come from corotating interaction regions (CIRs) produced by high-speed streams in the solar wind, and from the heliospheric termination shock at the outer edge of the heliospheric cavity. We sample all these populations near Earth, but can distinguish them readily by their element and isotope abundances, ionization states, energy spectra, angular distributions and time behavior. Remote spacecraft have probed the spatial distributions of the particles and examined new sources in situ. Most acceleration sources can be “seen” only by direct observation of the particles; few photons are produced at these sites. Wave-particle interactions are an essential feature in acceleration sources and, for shock acceleration, new evidence of energetic-proton-generated waves has come from abundance variations and from local cross-field scattering. Element abundances often tell us the physics of the source plasma itself, prior to acceleration. By comparing different populations, we learn more about the sources, and about the physics of acceleration and transport, than we can possibly learn from one source alone.

Authors: Reames, D. V.
Projects:

Publication Status: Space Science Revs. (in press)
Last Modified: 1999-11-04 16:04
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Abstracts by Author
Corotating Shock Waves and the Solar-Wind Source of Energetic Ion Abundances: Power Laws in A/Q
The ?FIP Effect? and the Origins of Solar Energetic Particles and of the Solar Wind
The Abundance of Helium in the Source Plasma of Solar Energetic Particles
Temperature of the Source Plasma for Impulsive Solar Energetic Particles
Variations in Abundance Enhancements in Impulsive Solar Energetic-Particle Events and Related CMEs and Flares
Abundance Enhancements in Impulsive Solar Energetic-Particle Events with Associated Coronal Mass Ejections
The Two Sources of Solar Energetic Particles
Element Abundances in Solar Energetic Particles and the Solar Corona
Particle Acceleration at the Sun and in the Heliosphere

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