Probabilistic model for fluences and peak fluxes of solar energetic particles

Space Radiation and Plasma Effects on Satellites and Aviation: Quantities and Metrics for Tracking Performance of Space Weather Environment Models

The Community Coordinated Modeling Center has been leading community-wide space science and space weather model validation projects for many years. These efforts have been broadened and extended via the newly launched International Forum for Space Weather Modeling Capabilities Assessment (https://ccmc.gsfc.nasa.gov/assessment/). Its objective is to track space weather models' progress and performance over time, a capability that is critically needed in space weather operations and different user communities in general. The Space Radiation and Plasma Effects Working Team of the aforementioned International Forum works on one of the many focused evaluation topics and deals with five different subtopics (https://ccmc.gsfc.nasa.gov/assessment/topics/radiation-all.php) and varieties of particle populations: Surface Charging from tens of eV to 50-keV electrons and internal charging due to energetic electrons from hundreds keV to several MeVs. Single-event effects from solar energetic particles and galactic cosmic rays (several MeV to TeV), total dose due to accumulation of doses from electrons (>100 keV) and protons (>1 MeV) in a broad energy range, and radiation effects from solar energetic particles and galactic cosmic rays at aviation altitudes. A unique aspect of the Space Radiation and Plasma Effects focus area is that it bridges the space environments, engineering, and user communities. The intent of the paper is to provide an overview of the current status and to suggest a guide for how to best validate space environment models for operational/engineering use, which includes selection of essential space environment and effect quantities and appropriate metrics.

Variation in lunar neutron dose estimates

The radiation environment on the Moon includes albedo neutrons produced by primary particles interacting with the lunar surface. In this work, HZETRN2010 is used to calculate the albedo neutron contribution to effective dose as a function of shielding thickness for four different space radiation environments and to determine to what extent various factors affect such estimates. First, albedo neutron spectra computed with HZETRN2010 are compared to Monte Carlo results in various radiation environments. Next, the impact of lunar regolith composition on the albedo neutron spectrum is examined, and the variation on effective dose caused by neutron fluence-to-effective dose conversion coefficients is studied. A methodology for computing effective dose in detailed human phantoms using HZETRN2010 is also discussed and compared. Finally, the combined variation caused by environmental models, shielding materials, shielding thickness, regolith composition and conversion coefficients on the albedo neutron contribution to effective dose is determined. It is shown that a single percentage number for characterizing the albedo neutron contribution to effective dose can be misleading. In general, the albedo neutron contribution to effective dose is found to vary between 1-32%, with the environmental model, shielding material and shielding thickness being the driving factors that determine the exact contribution. It is also shown that polyethylene or other hydrogen-rich materials may be used to mitigate the albedo neutron exposure.

Prediction of frequency and exposure level of solar particle events

For future space missions outside of the Earth's magnetic field, the risk of radiation exposure from solar particle events (SPEs) during extra-vehicular activities (EVAs) or in lightly shielded vehicles is a major concern when designing radiation protection including determining sufficient shielding requirements for astronauts and hardware. While the expected frequency of SPEs is strongly influenced by solar modulation, SPE occurrences themselves are chaotic in nature. We report on a probabilistic modeling approach, where a cumulative expected occurrence curve of SPEs for a typical solar cycle was formed from a non-homogeneous Poisson process model fitted to a database of proton fluence measurements of SPEs that occurred during the past 5 solar cycles (19-23) and those of large SPEs identified from impulsive nitrate enhancements in polar ice. From the fitted model, we then estimated the expected frequency of SPEs at any given proton fluence threshold with energy >30 MeV (Phi(30)) during a defined space mission period. Analytic energy spectra of 34 large SPEs observed in the space era were fitted over broad energy ranges extending to GeV, and subsequently used to calculate the distribution of mGy equivalent (mGy-Eq) dose for a typical blood-forming organ (BFO) inside a spacecraft as a function of total Phi(30) fluence. This distribution was combined with a simulation of SPE events using the Poisson model to estimate the probability of the BFO dose exceeding the NASA 30-d limit of 250 mGy-Eq per 30 d. These results will be useful in implementing probabilistic risk assessment approaches at NASA and guidelines for protection systems for astronauts on future space exploration missions.

A temporal forecast of radiation environments for future space exploration missions

The understanding of future space radiation environments is an important goal for space mission operations, design, and risk assessment. We have developed a solar cycle statistical model in which sunspot number is coupled to space-related quantities, such as the galactic cosmic radiation (GCR) deceleration potential (phi) and the mean occurrence frequency of solar particle events (SPEs). Future GCR fluxes were derived from a predictive model, in which the temporal dependence represented by phi was derived from GCR flux and ground-based Climax neutron monitor rate measurements over the last four decades. These results showed that the point dose equivalent inside a typical spacecraft in interplanetary space was influenced by solar modulation by up to a factor of three. It also has been shown that a strong relationship exists between large SPE occurrences and phi. For future space exploration missions, cumulative probabilities of SPEs at various integral fluence levels during short-period missions were defined using a database of proton fluences of past SPEs. Analytic energy spectra of SPEs at different ranks of the integral fluences for energies greater than 30 MeV were constructed over broad energy ranges extending out to GeV for the analysis of representative exposure levels at those fluences. Results will guide the design of protection systems for astronauts during future space exploration missions.

Errors in the particle flux measurement data relevant to solar energetic particle spectra

Systematical errors of the spacecraft measured high-energy particle fluxes are analyzed. The errors are shown to be inherent to most of the measurements made to be the monitoring of the high-energy radiation in the space. The level of the systematic errors of the measurements varies with energy, thus resulting in distortions of the solar energetic particle spectra based on the measurement data. The erroneous experimental data have resulted in spurious estimates of space radiation environment and give rise to erroneous physical conclusions.

The high-energy heavy-particle fluences in the orbits of manned space stations

The results are presented of measurements high-energy particles in a customary manned space station orbit (a 350-450-km altitude, a 51.6 degrees inclination; Salyut-6 and 7, MIR). The particles were recorded by the chambers composed of the Lavsan (polyethyleneterephtalate) solid-state nuclear track detector layers mounted outside a spacecraft for 1-3 years. A high resolution has been attained in the charge and energy spectra of 30-200 MeV/n Fe group particles. The results of measuring the particle fluxes in the space station orbits are used to restore the initial particle energy spectra in terms of the models that describe the galactic and solar cosmic rays and their penetration to the Earth's magnetosphere. The analysis demonstrates a high effectiveness of the described methods when applied to quite a number of space physics problems.

Estimates of radiation effect for a spacecraft on the Earth-Mars-Earth route

The space radiation environment predicted for a spacecraft on the Earth-Mars-Earth route at different solar activity levels is analyzed in terms of the Russian-devised models.

The dependence of solar energetic particle fluxes in the Earth-Mars-Earth route on solar activity period

This report presents the results of analyzing the relative importance of particle fluxes of different origin in the Earth-Mars-Earth route during different solar activity periods. The analysis has been made in terms of the galactic cosmic ray and solar energetic particle flux models developed at Moscow State University. The results demonstrate the extreme importance of the high-energy solar particle fluxes in interplanetary space even during the years of "quiet" Sun.

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 Chicago's 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.