William F. Dietrich

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.

The mean ionic charge state of solar energetic Fe ions above 200 MeV per nucleon

We have analyzed the geomagnetic transmission of solar energetic Fe ions at approximately 200-600 MeV per nucleon during the great solar energetic particle (SEP) events of 1989 September-October. By comparing fluences from the Chicago charged-particle telescope on IMP-8 in interplanetary space and from NRLs Heavy Ions in Space (HIIS) experiment aboard the Long Duration Exposure Facility (LDEF) in low-Earth orbit, we obtain a mean ionic charge (Q(sub 3)) = 14.2 +/- 1.4. This result is significantly lower than (Q) observed at approximately 1 MeV per nucleon in impulsive, He-3 rich SEP events, indicating that neither acceleration at the flare site nor flare-heated plasma significantly contributes to the high-energy Fe ions we observe. But it agrees well with the (Q) observed in gradual SEP events at approximately 1 MeV per nucleon, in which ions are accelerated by shocks driven by fast coronal mass ejections, and hence shows that particles are accelerated to very high energies in this way. We also note apparent differences between solar wind and SEP charge state distributions, which may favor a coronal (rather than solar wind) seed population or may suggest additional ionization in the ambient shock-region plasma.

Evidence for Remnant Flare Suprathermals in the Source Population of Solar Energetic Particles in the 2000 Bastille Day Event

The energy spectra of Fe in the very large solar energetic particle (SEP) event of 2000 July 14 are strikingly different from those of lighter species. We show that this difference can be explained by shock acceleration from a two-component source population, comprising solar wind suprathermals and a small (∼5%) admixture of remnant flare particles, as previously proposed to explain enhanced ^3He/^4He in some gradual SEP events. Flare remnants can also account for several previously unexplained features of high-energy solar heavy ions as well as important aspects of SEP event-to-event variability. These results offer a new perspective on the enduring controversy over the relative roles of flares and coronal mass ejections (CMEs) in producing SEPs. Flare activity clearly makes a unique and critical contribution to the source population. But the predominate accelerator in large gradual SEP events is the CME-driven shock, and many spectral, compositional, and charge state characteristics of highenergy heavy ions can be understood without invoking other acceleration mechanisms.

Probability distributions of high-energy solar-heavy-ion fluxes from IMP-8: 1973-1996

We present probability distributions for the fluxes and event-integrated fluences of solar He, CNO, and Fe ions at high energies relevant to space-system design, based on observations from the University of Chicagos Cosmic Ray Telescope on IMP-8 in 1973-96. We compare the observed distributions to CREME96, whose new solar models are shown to produce realistic, extreme-worst-case (/spl sim/99/sup +/% confidence level) environments. The probability distributions show that a modest reduction in the reliability requirement (from /spl sim/99% to /spl sim/90-95% confidence level, for example) can significantly reduce the severity of the heavy-ion hazard. This reduction factor is larger for solar heavy ions than for solar protons, and the exact amount of the reduction will depend critically upon other factors, such as device sensitivity, shielding, and orbit. We also show predictions of mission-accumulated solar-heavy-ion fluences as functions of confidence level and mission duration.

Flare- and Shock-accelerated Energetic Particles in the Solar Events of 2001 April 14 and 15

We report heavy-ion composition and spectra for the solar energetic particle (SEP) events of 2001 April 14 and 15, using the combined capabilities of the Advanced Composition Explorer (ACE), Wind, and the Interplanetary Monitoring Platform 8 (IMP-8) to cover the energy range from ∼30 keV nucleon^(-1) to ∼400 MeV nucleon^(-1). These two events are, respectively, the largest impulsive event and the largest ground-level event observed so far in solar cycle 23. These events arose from the same active region and launched into similar interplanetary conditions. Both were associated with large western flares and fast coronal mass ejections (CMEs). However, the two events are distinctly different, thereby providing useful reminders of the fundamental differences between flare- and shock-accelerated SEPs. The detailed observations present challenges for our theoretical understanding of SEP production. Of particular note is the fact that iron has a harder power-law energy spectrum than oxygen above ∼3 MeV nucleon^(-1) in the shock-dominated April 15 event. This spectral difference, which is seen in many other gradual events of various sizes and heliolongitudes, leads to enhanced Fe/O at high energies. Simple shock acceleration models predict the same power-law index for all species. Thus, understanding the origin of this spectral difference will significantly contribute to the resolution of the ongoing debate about the relative roles of CME-driven shocks and flares in producing high-energy solar heavy ions.

The Solar Flare Heavy Ion Environment for Single-Event Upsets: A Summary of Observations over the Last Solar Cycle, 1973-1983

A summary of observations of the flux of 25 to 400 MeV/nucleon heavy ions from solar flares is presented covering the period from late 1973 to early 1984. Distributions of flare occurrence frequency versus fluence, energy spectra, and composition are presented for the 30 events observed during this period, to quantify the variability of the heavy ion environment. A comparison of these data to a model environment suggests some refinements to the model. LET spectra based on the worst case flare observed are presented to illustrate the significance of the flare ion distribution and the importance of accurate shielding estimates.

IMP-8 observations of the spectra, composition, and variability of solar heavy ions at high energies relevant to manned space missions

In more than 25 years of almost continuous observations, the University of Chicagos Cosmic Ray Telescope (CRT) on IMP-8 has amassed a unique database on high-energy solar heavy ions of potential relevance to manned spaceflight. In the very largest particle events, IMP-8/CRT has even observed solar Fe ions above the Galactic cosmic ray background up to approximately 800 MeV/nucleon, an energy sufficiently high to penetrate nearly 25 g/cm2 of shielding. IMP-8/CRT observations show that high-energy heavy-ion spectra are often surprisingly hard power laws, without the exponential roll-offs suggested by stochastic acceleration fits to lower energy measurements alone. Also, in many solar particle events the Fe/O ratio grows with increasing energy, contrary to the notion that ions with higher mass-to-charge ratios should be less abundant at higher energies. Previous studies of radiation hazards for manned spaceflight have often assumed heavy-ion composition and steeply-falling energy spectra inconsistent with these observations. Conclusions based on such studies should therefore be re-assessed. The significant event-to-event variability observed in the high-energy solar heavy ions also has important implications for strategies in building probabilistic models of solar particle radiation hazards.

Spectral Analyses and Radiation Exposures from Several Ground-Level Enhancement (GLE) Solar Proton Events: A Comparison of Methodologies

Several methods for analyzing the particle spectra from extremely large solar proton events, called Ground-Level Enhancements (GLEs), have been developed and utilized by the scientific community to describe the solar proton energy spectra and have been further applied to ascertain the radiation exposures to humans and radio-sensitive systems, namely electronics. In this paper 12 GLEs dating back to 1956 are discussed, and the three methods for describing the solar proton energy spectra are reviewed. The three spectral fitting methodologies are EXP [an exponential in proton rigidity (R)], WEIB [Weibull fit: an exponential in proton energy], and the Band function (BAND) [a double power law in proton rigidity]. The EXP and WEIB methods use low energy (MeV) GLE solar proton data and make extrapolations out to approx.1 GeV. On the other hand, the BAND method utilizes low- and medium-energy satellite solar proton data combined with high-energy solar proton data deduced from high-latitude neutron monitoring stations. Thus, the BAND method completely describes the entire proton energy spectrum based on actual solar proton observations out to ~10 GeV. Using the differential spectra produced from each of the 12 selected GLEs for each of the three methods, radiation exposures are presented and discussed in detail. These radiation exposures are then compared with the current 30-day and annual crew exposure limits and the radiation effects to electronics.

Radiation Exposures from Several Ground Level Enhancements During the 23rd Solar Cycle

Solar proton events (SPEs) represent the single-most significant source of acute radiation exposure to humans and space systems during lunar and deep-space missions. In this paper several unique solar particle emissions that occurred during solar cycle 23 were investigated. In particular, there were four (4) events that exhibited both a ground level enhancement (GLE) and an energetic solar particle (ESP) feature. These four SPEs occurred on 1) 6 November 1997, 2) 14-18 July 2000, 3) 4-8 November 2001, and 4) 28-29 October 2003. Each event is discussed in detail. Proton integral and differential energy and corresponding rigidity (momentum) spectra were generated based on GOES MEPAD and GOES HEPAD satellite data and ground-based neutron monitor (NM) data. The differential proton spectra are utilized with the NASA Langley Research Center high energy particle transport/dose code, HZETRN 2005, to compute simulated human exposures behind several shielding materials. We have previously shown that a Band function (double power law in particle rigidity) fit more correctly describes the complete event spectra when compared with an exponential rigidity extrapolation that has been in use by the scientific community for several decades, i.e., the exponential extrapolation method consistently under-estimates the human exposure. Comparisons of simulated human radiation exposures are presented for both the Band function fit and the exponential extrapolation methods.

HIIS results on the mean ionic charge state of SEP Fe above 200 MeV per nucleon

We have analyzed the geomagnetic transmission of solar energetic Fe ions at ∼200–600 MeV/nuc during the great solar energetic particle (SEP) events of 1989 September–October. By comparing fluences from the Chicago charged‐particle telescope on IMP‐8 in interplanetary space and from NRL’s Heavy Ions in Space (HIIS) experiment aboard LDEF in low‐Earth orbit, we obtain a mean ionic charge 〈Q〉=14.2±1.4. This result is significantly lower than 〈Q〉 observed at ∼1 MeV/nuc in impulsive events, and suggests that neither acceleration at the flare site nor flare‐heated plasma significantly contributes to the high‐energy Fe ions we observe. But it agrees well with the 〈Q〉 observed in gradual SEP events at lower energies, demonstrating that acceleration by CME‐driven shocks is the primary SEP production mechanism in gradual events even at these very high energies.