Large Solar Particle Events Research Articles

Radiations in space: risk estimates.

The complexity of radiation environments in space makes estimation of risks more difficult than for the protection of terrestrial populations. In deep space the duration of the mission, position in the solar cycle, number and size of solar particle events (SPE) and the spacecraft shielding are the major determinants of risk. In low-earth orbit missions there are the added factors of altitude and orbital inclination. Different radiation qualities such as protons and heavy ions and secondary radiations inside the spacecraft such as neutrons of various energies, have to be considered. Radiation dose rates in space are low except for short periods during very large SPEs. Risk estimation for space activities is based on the human experience of exposure to gamma rays and to a lesser extent X rays. The doses of protons, heavy ions and neutrons are adjusted to take into account the relative biological effectiveness (RBE) of the different radiation types and thus derive equivalent doses. RBE values and factors to adjust for the effect of dose rate have to be obtained from experimental data. The influence of age and gender on the cancer risk is estimated from the data from atomic bomb survivors. Because of the large number of variables the uncertainities in the probability of the effects are large. Information needed to improve the risk estimates includes: (1) risk of cancer induction by protons, heavy ions and neutrons: (2) influence of dose rate and protraction, particularly on potential tissue effects such as reduced fertility and cataracts: and (3) possible effects of heavy ions on the central nervous system. Risk cannot be eliminated and thus there must be a consensus on what level of risk is acceptable.

SEU measurements and predictions on MPTB for a large energetic solar particle event

Single-event upset (SEU) ground testing and rate calculations have been performed for 16-Mb DRAMs flown on the microelectronics and photonics testbed experiment (MPTB) using measured particle spectra and compared with space data from a period of high solar activity. The results show that with the measured spectra, the measurements agree quite well with the calculations. Using just modeled environments, the results do not agree as well.

Space Radiation Protection: Comparison of Effective Dose to Bone Marrow Dose Equivalent

In many instances, bone marrow dose equivalents averaged over the entire body have been used as a surrogate for whole-body dose equivalents in space radiation protection studies. However, career radiation limits for space missions are expressed as effective doses. This study compares calculations of effective doses to average bone marrow dose equivalents for several large solar particle events (SPEs) and annual galactic cosmic ray (GCR) spectra, in order to examine the suitability of substituting bone marrow dose equivalents for effective doses. Organ dose equivalents are computed for all radiosensitive organs listed in NCRP Report 116 using the BRYNTRN and HZETRN space radiation transport codes and the Computerized Anatomical Man (CAM) model. These organ dose equivalents are then weighted with the appropriate tissue weighting factors to obtain effective doses. Various thicknesses of aluminum shielding, which are representative of nominal spacecraft and SPE storm shelter configurations, are used in the analyses. For all SPE configurations, the average bone marrow dose equivalent is considerably less than the calculated effective dose. For comparisons of the GCR, there is less than a ten percent difference between the two methods. In all cases, the gonads made up the largest percentage of the effective dose.

Monte Carlo calculations of the influence on aircraft radiation environments of structures and solar particle events

Radiation transport codes are applied to calculate the influence of aircraft structure and loads on the internal environment from galactic cosmic rays. Significant shielding effects are predicted, while thermalization of the neutron flux can lead to enhanced single-event upset rates in devices containing boron. The enhanced environments due to large solar particle events are calculated and shown to be very important for both single-event effects in electronics and dose to crew and passengers.

Interplanetary Crew Dose Rates for the August 1972 Solar Particle Event

Parsons, J. L. and Townsend, L. W. Interplanetary Crew Dose Rates for the August 1972 Solar Particle Event. Using the coupled neutron-proton space radiation transport computer code (BRYNTRN), estimates of dose rates of protons in the skin, ocular lens and bone marrow, behind various thicknesses of aluminum shielding, for crews on space missions outside the Earth's magnetosphere, are made for the large solar particle event (SPE) of August 1972. Overall, the August 1972 dose rates are significantly higher than those estimated for any of the events that occurred in August-December 1989. The dose rates in the August 1972 SPE are not low dose rates as specified by the major national and international advisory bodies and committees.

3He Enhancements in Large Solar Energetic Particle Events.

We have measured the 3He abundance from approximately 0.5 to 2 MeV nucleon-1 in 12 large solar energetic particle (SEP) events during the period 1997 November-1999 June. In five of the events, the 3He time-intensity profile is similar to the 4He time-intensity profile, indicating a common acceleration and transport origin for the two species. The average 3He/4He ratio during these events is &parl0;1.9+/-0.2&parr0;x10-3, a factor of approximately 5 enhancement over the solar wind value. During this same survey period, we have also measured the low-energy ion intensities during quieter periods in between the large-particle events. We find 3He and Fe remnants from impulsive events present on a majority of the days, implying that they fill a substantial volume (>50%) of the in-ecliptic interplanetary medium during our survey. We suggest that these suprathermal ions may therefore be a source population that is available for further acceleration by interplanetary shocks that accompany large SEP events, thereby leading to the 3He enhancements in a significant fraction of large SEP events. This impulsive SEP event material might also account for recent observations of large solar particle events with energetic particle ionization states that have a wide range of ionization states that encompass values expected for both gradual and impulsive solar SEP events.

Dose uncertainties for large solar particle events: Input spectra variability and human geometry approximations

The true uncertainties in estimates of body organ absorbed dose and dose equivalent, from exposures of interplanetary astronauts to large solar particle events (SPEs), are essentially unknown. Variations in models used to parameterize SPE proton spectra for input into space radiation transport and shielding computer codes can result in uncertainty about the reliability of dose predictions for these events. Also, different radiation transport codes and their input databases can yield significant differences in dose predictions, even for the same input spectra. Different results may also be obtained for the same input spectra and transport codes if different spacecraft and body self-shielding distributions are assumed. Heretofore there have been no systematic investigations of the variations in dose and dose equivalent resulting from these assumptions and models. In this work we present a study of the variability in predictions of organ dose and dose equivalent arising from the use of different parameters to represent the same incident SPE proton data and from the use of equivalent sphere approximations to represent human body geometry. The study uses the BRYNTRN space radiation transport code to calculate dose and dose equivalent for the skin, ocular lens and bone marrow using the October 1989 SPE as a model event. Comparisons of organ dose and dose equivalent, obtained with a realistic human geometry model and with the oft-used equivalent sphere approximation, are also made. It is demonstrated that variations of 30–40% in organ dose and dose equivalent are obtained for slight variations in spectral fitting parameters obtained when various data points are included or excluded from the fitting procedure. It is further demonstrated that extrapolating spectra from low energy (⩽30 MeV) proton fluence measurements, rather than using fluence data extending out to 100 MeV results in dose and dose equivalent predictions that are underestimated by factors as large as 2–3. Finally, it is also demonstrated that the use of equivalent sphere approximations to represent body organ self-shielding distributions results in organ doses and dose equivalent predictions that are 2–3 times larger than values obtained with anthropomorphic shielding configurations.

Radiation effects on the proportional counter X-ray detectors on board the NEAR spacecraft

The X-ray proportional counters on board the Near Earth Asteroid Rendezvous (NEAR) spacecraft have exhibited a resolution degradation and recovery phenomenon several times during the long cruise phase of the mission. The resolution is checked periodically by commanding an 55Fe source into the window area. The degradation is seen as a low energy tailing of the 5.9 keV photopeak. Two events have occurred which provided good spectral data for better understanding the degradation phenomenon. In November 1997 a large solar particle event occurred that degraded the resolution and excited copper in the collimator. Eventually the detectors returned to normal. In January 1998 the spacecraft performed an Earth swingby gravity assist maneuver. The near Earth environment excited the magnesium and aluminum in the filter elements. The copper line was also produced. The NEAR spacecraft was launched in February 1996 and will rendezvous and orbit the asteroid 433 Eros in early 1999.

Advanced Composition Explorer provides new insights into particle sources

The January 15th issue of Geophysical Research Letters (GRL) includes a special section on results from Advanced Composition Explorer (ACE). ACE was launched in August 1997, and is now in orbit around the L1 Lagrangian point. ACE is instrumented to measure the elemental, isotopic, and ionic charge state composition of the heavy nuclei in the solar wind, solar energetic particles, and galactic cosmic rays.As reported in the special section of GRL, a large solar particle event occurred on November 6, 1997. The energetic particle composition, as measured by ACE, was surprising with significant effects due to mass‐dependent fractionation. The isotopic abundance ratios of 13C/12C, 22Ne/20Ne, and 26Mg/24Mg were about twice as large as typical solar system values, implying charge‐to‐mass dependent acceleration. Data from ACE and the Solar, Anomalous and Magnetospheric Particle Explorer (SAMPEX) show the ionic charge states of Mg, Si, and Fe increased with increasing energy for this event. This indicates that the higher energy nuclei may have come from a source millions of degrees hotter than typical coronal temperatures (∼ 1 million degrees). Several papers in the special section conclude that more than one acceleration process may have been present during the November 6 event.

Studies of modern and ancient solar energetic particles

Modern solar energetic particles (SEPs) have been studied for about 50 years by satellites and groundbased observations. These measurements indicate much about the nature of SEPs but cover too short a period to quantify the probabilities of very large solar particle events. Many SEPs have high enough energies to make nuclides in material in which they interact. Radionuclides measured in lunar samples have been used to extend the record about SEPs back several million years. Some new measurements of modern SEPs during the last solar cycle and new results for nuclides made by SEPs in lunar samples are presented and their implications discussed. Both the modern and ancient records need to be improved, and methods to get a better understanding of solar energetic particles discussed. The fluxes of SEPs during the last million years show an increasing trend when averaged over shorter radionuclide half-lives.

The Solar Isotope Spectrometer for the Advanced Composition Explorer

The Solar Isotope Spectrometer (SIS), one of nine instruments on the Advanced Composition Explorer (ACE), is designed to provide high-resolution measurements of the isotopic composition of energetic nuclei from He to Zn (Z = 2 to 30) over the energy range from ∼ 10 to ∼ 100 MeV nucl-1. During large solar events SIS will measure the isotopic abundances of solar energetic particles to determine directly the composition of the solar corona and to study particle acceleration processes. During solar quiet times SIS will measure the isotopes of low-energy cosmic rays from the Galaxy and isotopes of the anomalous cosmic-ray component, which originates in the nearby interstellar medium. SIS has two telescopes composed of silicon solid-state detectors that provide measurements of the nuclear charge, mass, and kinetic energy of incident nuclei. Within each telescope, particle trajectories are measured with a pair of two-dimensional silicon-strip detectors instrumented with custom, very large-scale integrated (VLSI) electronics to provide both position and energy-loss measurements. SIS was especially designed to achieve excellent mass resolution under the extreme, high flux conditions encountered in large solar particle events. It provides a geometry factor of ∼ 40 cm2 sr, significantly greater than earlier solar particle isotope spectrometers. A microprocessor controls the instrument operation, sorts events into prioritized buffers on the basis of their charge, range, angle of incidence, and quality of trajectory determination, and formats data for readout by the spacecraft. This paper describes the design and operation of SIS and the scientific objectives that the instrument will address.

Effects of material and/or structure on shielding of electronic devices

In this paper, we have pioneered a new direction concerning the shielding of electronic devices (at a microcircuit-packaging or a larger level) in a radiative environment. As a matter of fact, this work not only considers the role of the material itself on the shielding efficiency but also the effect of the structure of the shield. By simulations with the NOVICE Code, we have explored the possibility of combining the specific attenuation properties of given materials in an optimized multilayer (and multimaterial) structure. This study is based on a brief theoretical overview of interaction of space radiation with material. Then, the results of simulations are presented. The workframe of these calculations is a worst-case geosynchronous orbit (160/spl deg/ West) for a period of 15 years (trapped electrons and protons from 4 anomalously large solar particle events). We found that it is possible to improve the shielding properties of a monomaterial absorber by using an optimized multilayer, multimaterial shield.

Proton spectra of the four remarkable gle in the 22nd solar cycle

The proton spectra of several large solar particle events in the 22nd solar cycle (namely for the enhancements on 29 September 1989, 19 October 1989, 24 May 1990 and 15 June 1991) were obtained within the wide energy range directly by experimental data without any advance model. The data from spacecraft measurements and the world network of neutron monitors have been used to analyse these events. All the obtained spectra indicate a similar behaviour in the low energy range but differ at high energies > 1 GeV. The proposed method allows us to define the times of the maximum increase depending n the particle energy. The time-energy dependencies together with the proton flux profiles at different energies and the obtained spectra might be used as a predictor for estimation of the radiation dose in the maximum of the event, as well as its behaviour.

The energy spectra and other properties of the great proton events during 22-nd solar cycle

The proton spectra and their variations during the several large solar particle events in 22-nd solar cycle (29.09.89, 19.10.89, 24.10.89, 24.05.90 and 15.06.91) were obtained within the wide energy range. The data of spacecraft measurements and neutron monitors of the world-wide network have been used to analyse the energy and temporal dependencies of proton increases. All peak spectra reveal the similar behaviour in the low energy range and significantly differ at energy > 1 GeV.

Solar particle induced upsets in the TDRS-1 attitude control system RAM during the October 1989 solar particle events

The three large solar particle events, beginning on October 19, 1989 and lasting approximately six days, were characterized by high fluences of solar protons and heavy ions at 1 AU. During these events, an abnormally large number of upsets (243) were observed in the random access memory of the attitude control system (ACS) control processing electronics (CPE) on-board the geosynchronous TDRS-1 (Telemetry and Data Relay Satellite). The RAR I unit affected was composed of eight Fairchild 93L422 memory chips. The Galileo spacecraft, launched on October 18, 1989 (one day prior to the solar particle events) observed the fluxes of heavy ions experienced by TDRS-1. Two solid-state detector telescopes on-board Galileo designed to measure heavy ion species and energy, were turned on during time periods within each of the three separate events. The heavy ion data have been modeled and the time history of the events reconstructed to estimate heavy ion fluences. These fluences were converted to effective LET spectra after transport through the estimated shielding distribution around the TDRS-1 ACS system. The number of single event upsets (SEU) expected was calculated by integrating the measured cross section for the Fairchild 93L422 memory chip with average effective LET spectrum. The expected number of heavy ion induced SEUs calculated was 176. GOES-7 proton data, observed during the solar particle events, were used to estimate the number of proton-induced SEUs by integrating the proton fluence spectrum incident on the memory chips, with the two-parameter Bendel cross section for proton SEUs.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Composition of energetic particles from solar flares

We present a model for composition of heavy ions in the solar energetic particles (SEP). The SEP composition in a typical large solar particle event reflects the composition of the Sun, with adjustments due to fractionation effects which depend on the first ionization potential (FIP) of the ion and on the ratio of ionic charge to mass (Q/M). Flare-to-flare variations in composition are represented by parameters describing these fractionation effects and the distributions of these parameters are presented.

Risk from relativistic heavy ions on manned space missions.

The risk from exposure to radiation posed to space travelers outside the magnetic shielding provided by the geomagnetosphere will come from two sources: the slowly varying but low intensity high-energy galactic cosmic rays and the more intense predominantly low-energy protons from large solar particle events associated with magnetic disturbances originating sporadically on or within the solar surface during the active period of the 11-year solar cycle. The energy spectra of the protons in solar particle events are quite soft, with large numbers of low-energy protons and a rather steep decrease of the energy spectra with increasing energy. This allows for the possibility to provide, within the space vehicle or habitat, a well-shielded area sometimes called a "storm shelter" or "safe haven" where the travelers could gather during the largest particle events. Intensity risetimes on the order of half an hour or more and overall event durations of 1 to 2 days would make actively seeking a well-shielded shelter for the duration a distinct possibility. The high-energy and penetrating nature and relative constancy of the galactic cosmic rays, on the other hand, do not allow the use of highly shielded areas as a means of protection against them. The first question to answer becomes: what is the risk to human health from the galactic cosmic rays? We need to have a good idea of the answer to this question before we can address the problem of how to best protect human health or, indeed, whether any specific measures need to be taken.

The abundances of hydrogen, helium, oxygen, and iron accelerated in large solar particle events

Energy spectra measured in 10 large flares with the University of Maryland/Max-Planck-Institut sensors on ISEE I and Goddard Space Flight Center sensors on IMP 8 allowed us to determine the average H, He, O, and Fe abundances as functions of energy in the range of about 0.3-80 MeV/nucleon. Model fits to the spectra of individual events using the predictions of a steady state stochastic acceleration model with rigidity-dependent diffusion provided a means of interpolating small portions of the energy spectra not measured with the instrumentation. Particles with larger mass-to-charge ratios were relatively less abundant at higher energies in the flare-averaged composition. The Fe/O enhancement at low SEP energies was less than the Fe/O ratios observed in He-3-rich flares. Unlike the SEP composition averaged above 5 MeV/nucleon, the average SEP abundances above 0.3 MeV/nucleon were similar to the average solar wind.

Solar particle composition: Measurements in the March 1991 event at 2.5AU

The time evolution of the very large solar particle event occurring on days 82–90 (March 23–31), 1991, as measured at 2.5 AU by the instrumentation on the Ulysses spacecraft was quite complex. Measurements by the HI‐SCALE instrument of the nuclear composition (emphasizing Z≥6) of the interplanetary particles at a time resolution of two hours provides information on the different interplanetary regions which swept over the spacecraft. The Fe/O abundance ratio is found to differ slightly in the regions before and after two tangential discontinuities. The Fe/O abundance ratio is also found to depend strongly on the energy/nucleon of the particle, with values of ∼0.7 for energies of ∼0.5–1.0 MeV/nucl. to values of∼0.2 for energies ∼8–16 MeV/nucl.

Human exposure to large solar particle events in space.

Whenever energetic solar protons produced by solar particle events traverse bulk matter, they undergo various nuclear and atomic collision processes which significantly alter the physical characteristics and biologically important properties of their transported radiation fields. These physical interactions and their effect on the resulting radiation field within matter are described within the context of a recently developed deterministic, coupled neutron-proton space radiation transport computer code (BRYNTRN). Using this computer code, estimates of human exposure in interplanetary space, behind nominal (2 g/cm2) and storm shelter (20 g/cm2) thicknesses of aluminum shielding, are made for the large solar proton event of August 1972. Included in these calculations are estimates of cumulative exposures to the skin, ocular lens, and bone marrow as a function of time during the event. Risk assessment in terms of absorbed dose and dose equivalent is discussed for these organs. Also presented are estimates of organ exposures for hypothetical, worst-case flare scenarios. The rate of dose equivalent accumulation places this situation in an interesting region of dose rate between the very low values of usual concern in terrestrial radiation environments and the high dose rate values prevalent in radiation therapy.