High-energy Solar Energetic Particle Research Articles

Analysis on H spectral shape during the early 2012 SEPs with the PAMELA experiment

The satellite-borne PAMELA experiment has been continuously collecting data since 2006. This apparatus is designed to study charged particles in the cosmic radiation. The combination of a permanent magnet, a silicon strip tracker and a silicon-tungsten imaging calorimeter, and the redundancy of instrumentation allow very precise studies on the physics of cosmic rays in a wide energy range and with high statistics. This makes PAMELA a very suitable instrument for Solar Energetic Particle (SEP) observations. Not only does it span the energy range between the ground-based neutron monitor data and the observations of SEPs from space, but PAMELA also carries out the first direct measurements of the composition for the highest energy SEP events, including those causing Ground Level Enhancements (GLEs). In particular, PAMELA has registered many SEP events during solar cycle 24, offering unique opportunities to address the question of high-energy SEP origin. A preliminary analysis on proton spectra behaviour during this event is presented in this work.

Geomagnetic influence on aircraft radiation exposure during a solar energetic particle event in October 2003

[1] We present initial results from the Nowcast of Atmospheric Ionizing Radiation for Aviation Safety (NAIRAS) model during the Halloween 2003 superstorm. The objective of NAIRAS is to produce global, real-time, data-driven predictions of ionizing radiation for archiving and assessing the biologically harmful radiation exposure levels at commercial airline altitudes. We have conducted a case study of radiation exposure during a high-energy solar energetic particle (SEP) event in October 2003. The purpose of the case study is to quantify the important influences of the storm time and quiet time magnetospheric magnetic field on high-latitude SEP atmospheric radiation exposure. The Halloween 2003 superstorm is an ideal event to study magnetospheric influences on atmospheric radiation exposure since this event was accompanied by a major magnetic storm which was one of the largest of solar cycle 23. We find that neglecting geomagnetic storm effects during SEP events can underestimate the high-latitude radiation exposure from nearly 15% to over a factor of 2, depending on the flight path relative to the magnetosphere open-closed boundary.

History of research on solar energetic particle (SEP) events: the evolving paradigm

AbstractForbush initiated research on solar energetic particle (SEP) events in 1946 when he reported ionization chamber observations of the first three ground level events (GLEs). The next key development was the neutron monitor observation of the GLE of 23 February 1956. Meyer, Parker and Simpson attributed this high-energy SEP event to a short time-scale process associated with a solar flare and ascribed the much longer duration of the particle event to scattering in the interplanetary medium. Thus “flare particle” acceleration became the initial paradigm for SEP acceleration at the Sun. A more fully-developed picture was presented by the Australian radio astronomers Wild, Smerd, and Weiss in 1963. They identified two distinct SEP acceleration processes in flares: (1) the first phase accelerated primarily ~100 keV electrons that gave rise to fast-drift type III emission as they streamed outward through the solar atmosphere; (2) the second phase was produced by an outward moving (~1000 km s−1) magnetohydrodynamic shock, occurring in certain (generally larger) flares. The second phase, manifested by slow-drift metric type II emission, appeared to be required for substantial acceleration of protons and higher-energy electrons. This two-stage (or two-class) picture gained acceptance during the 1980s as composition and charge state measurements strengthened the evidence for two distinct types of particle events which were termed impulsive (attributed to flare-resident acceleration process(es)) and gradual (shock-associated). Reames championed the two-class picture and it is the commonly accepted paradigm today. A key error made in the establishment of this paradigm was revealed in the late 1990s by observations of SEP composition and charge states at higher energies (>10 MeV) than previously available. Specifically, some large and therefore presumably “gradual” SEP events looked “impulsive” at these energies. One group of researchers attributes these unusual events to acceleration of high-energy SEPs by flares and another school favors acceleration of flare seed particles by quasi-perpendicular shocks. A revised SEP classification scheme is proposed to accommodate the new observations and to include ideas on geometry and seed particle composition recently incorporated into models of shock acceleration of SEPs.

Shock Geometry, Seed Populations, and the Origin of Variable Elemental Composition at High Energies in Large Gradual Solar Particle Events

Above a few tens of MeV per nucleon, large, gradual solar energetic particle (SEP) events are highly variable in their spectral characteristics and elemental composition. The origin of this variability has been a matter of intense and ongoing debate. In this paper, we propose that this variability arises from the interplay of two factors—shock geometry and a compound seed population, typically comprising both solar-wind and flare suprathermals. Whereas quasi-parallel shocks generally draw their seeds from solar-wind suprathermals, quasi-perpendicular shocks—by requiring a higher initial speed for effective injection—preferentially accelerate seed particles from flares. Solar-wind and flare seed particles have distinctive compositional characteristics, which are then reflected in the accelerated particles. We first examine our hypothesis in the context of particles locally accelerated near 1 AU by traveling interplanetary shocks. We illustrate the implications of our hypothesis for SEPs with two very large events, 2002 April 21 and 2002 August 24. These two events arise from very similar solar progenitors but nevertheless epitomize extremes in high-energy SEP variability. We then test our hypothesis with correlation studies based on observations of 43 large SEP events in 1997-2003 by the Advanced Composition Explorer, Wind, the Interplanetary Monitoring Platform 8, and GOES. We consider correlations among high-energy Fe/O, event size, spectral characteristics, the presence of GeV protons, and event duration at high energies. The observed correlations are all qualitatively consistent with our hypothesis. Although these correlation studies cannot be construed as proof of our hypothesis, they certainly confirm its viability. We also examine the alternative hypothesis in which a direct flare component—rather than flare particles subsequently processed through a shock—dominates at high energies. This alternative would produce compositional characteristics similar to those of our hypothesis. However, the observed longitude distribution of the enhanced Fe/O events, their spectral characteristics, and recent timing studies all pose serious challenges for a direct flare component. We also comment on measurements of the mean ionic charge state of Fe at high energies. We conclude that shock geometry and seed population potentially provide a framework for understanding the overall high-energy variability in large SEP events. We suggest additional studies for testing this hypothesis.

Coronal Mass Ejection Shock and Sheath Structures Relevant to Particle Acceleration

Most high-energy solar energetic particles are believed to be accelerated at shock waves driven by coronal mass ejections (CMEs). The acceleration process strongly depends on the shock geometry and the structure of the sheath that forms behind the shock. In an effort to understand the structure and time evolution of such CME-driven shocks andtheirrelevancetoparticleacceleration,weinvestigatetheinteractionofafastCMEwiththeambientsolarwind by means of a three-dimensional numerical ideal MHD model. Our global steady state coronal model possesses high-latitudecoronalholesandahelmetstreamerstructurewithacurrentsheetneartheequator,reminiscentofnear solar minimum conditions. Fast and slow solar winds flow at high and low latitude, respectively, and the Archimedean spiral geometry of the interplanetary magnetic field is reproduced by solar rotation. Within this model system, we drive a CME to erupt by introducing a Gibson-Low magnetic flux rope that is embedded in the helmet streamer in an initial state of force imbalance. The flux rope rapidly expands and is ejected from the corona with maximum speeds in excess of 1000 km s � 1 , driving a fast-mode shock from the inner corona to a distance of 1 AU. We find that the ambient solar wind structure strongly affects the evolution of the CME-driven shocks, causing deviations of the fast-mode shocks from their expected global configuration. These deflections lead to substantial compressions of the plasma and magnetic field in their associated sheath region. The sudden postshock increase in magneticfieldstrengthonlow-latitudefieldlinesisfoundtobeeffectiveforacceleratingparticlestotheGeVrange. Subject heading gs: acceleration of particles — MHD — shock waves — Sun: coronal mass ejections (CMEs)

Transport and Acceleration of Solar Energetic Particles from Coronal Mass Ejection Shocks

After a brief overview of solar energetic particle (SEP) emission from coronal mass ejection (CME) shocks, we turn to a discussion of their transport and acceleration. The high energy SEP are accelerated near the Sun, and because of their well-known source location, their transport can be modeled quantitatively to obtain precise information on the injection function (number of particles emitted vs. time), including a determination of the onset time to within 1 min. For certain events, transport modeling also indicates magnetic topology with mirroring or closed field loops. Important progress has also been made on the transport of low energy SEP from very strong events, which can display exhibit interesting saturation effects and compositional variations. The acceleration of SEP by CME-driven shocks in the interplanetary medium is attributed to diffusive shock acceleration, but the spectrum of SEP production is typically modeled empirically. Recent progress has largely focused on using detailed composition measurements to determine fractionation effects of shock acceleration and even to clarify the nature of the seed population. In particular, there are many indications that the seed population is suprathermal (pre-energized) and the injection problem is not relevant to acceleration at interplanetary CME-driven shocks. We argue that the finite time available for shock acceleration provides the best explanation of the high-energy rollover.To search for other articles by the author(s) go to: http://adsabs.harvard.edu/abstract_service.html

The He-3/He-4 ratios for solar energetic particle events during the Combined Release and Radiation Effects Satellite Mission

view Abstract Citations (19) References (56) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS The 3He/ 4He Ratios for Solar Energetic Particle Events during the Combined Release and Radiation Effects Satellite Mission Chen, Jiasheng ; Guzik, T. Gregory ; Wefel, John P. Abstract Helium data measured by the University of Chicago instrument, ONR-604, are employed to determine the ratio of He-3 to He-4 for solar energetic particle (SEP) events over an energy range 50-110 MeV/nucleon during the 1990/1991 Combined Release and Radiation Effects Satellite mission. The Sun in this period is extremely active. A total of 29 separate SEP events have been identified; among them 16 major events have been analyzed to obtain He-3/He-4 ratios, with a mass resolution of 0.10 amu. Thirteen events have a He-3/He-4 ratio larger than 0.005, one order of magnitude greater than the solar coronal value. The He-3/He-4 ratio at energies of 50-110 MeV/nucleon is independent of the size of the SEP event, for the moderately large flares analyzed here. The helium energy spectra are represented by power laws. During the 1991 June flare period, different large-particle injections associated with different solar flares, but occurring from the same active region, have a similar average spectral index and a similar He-3/He-4 ratio. The spectral index of He-4 varies from event to event, i.e., from as small as 1.5 to as large as 7.5. A correlation is found between the inferred spectral index from gamma-ray measurements and our measured spectral indices for the 1991 June 11 and June 15 events, suggesting that the high-energy SEPs may come from the same acceleration event as the particles that interact at the Sun and produce the gamma rays. The implications of these results for particle acceleration and propagation at the flare site and in the solar corona are discussed. Publication: The Astrophysical Journal Pub Date: April 1995 DOI: 10.1086/175490 Bibcode: 1995ApJ...442..875C Keywords: Energetic Particles; Gamma Rays; Helium Isotopes; Mass Ratios; Solar Flares; Solar Radiation; Abundance; Crres (Satellite); Particle Acceleration; Particle Density (Concentration); Particle Energy; Particle Mass; Radiation Counters; Solar Physics; SUN: ABUNDANCES; SUN: FLARES; SUN: PARTICLE EMISSION full text sources ADS |