Allan J. Tylka

Evidence for trapped anomalous cosmic ray oxygen ions in the inner magnetosphere

A series of measurements of 5–30 MeV/nucleon oxygen ions made with track detector stacks on Cosmos satellites show isotropic angular distributions during solar energetic particle events. Solar-quiet times, on the other hand, have highly anisotropic distributions suggestive of a trapped-particle component. Detailed Monte Carlo simulations confirm this interpretation and allow us to measure the trapped and cosmic-ray contributions to the observed fluxes. Our data are fully consistent with anomalous cosmic-ray ions, rather than radial diffusion from the outer zone, as the source of the trapped particles.

Solar energetic particles: Shock acceleration and transport through self-amplified waves

This article reviews our work on the powerful influence of self-amplified Alfven waves on the interplanetary (IP) transport and shock acceleration of solar energetic particles (SEPs). In large gradual events, a huge number of shock-accelerated protons stream through the IPmediumand amplify ambient Alfven waves by orders of magnitude. Nonlinear models that take account of selfamplified waves semi-quantitatively explain many intriguing SEP observations at 1 AU: (a) upper limits to early SEP intensities, (b) flat intensity energy spectra up to ~ 30 MeV/amu before shock arrival, and (c) complex temporal, energy, and event-to-event variations of elemental abundances. Streaming limit complicates estimation of the number and energy of SEPs accelerated in a solar event but provides a safety window for astronauts to seek shelter before a potential hazardous intensity rise at shock passage. Self-amplified waves help bootstrap shock acceleration and the high near-shock SEP intensity predicted at ≤ 20r⊙. is relevant ...

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.

LET Spectra in Low Earth Orbit

The Trapped Ions in Space (TRIS) experiment was flown on the Space Shuttle in October, 1984. It contained a stack of plastic track detectors that recorded linear energy transfer (LET) spectra behind various thicknesses of shielding in a 57 degree inclination orbit. A preliminary analysis of the data shows that behind shielding of at least 40 mils Al equivalent, the LET spectrum is dominated by galactic cosmic rays and the direct ionization of trapped protons and that it is reasonably well predicted by the CREME model. For a minimum shielding of approximately 4 mils Al equivalent, the measured LET flux exceeds the CREME predictions, especially at high LET. This is probably due to contributions from the products of proton-induced nuclear reactions in the detector and helium ions trapped in the inner radiation belt.

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.

The charge state of the anomalous component

The Trapped Ions in Space (TRIS) experiment1 was flown in a 57° orbit at an average altitude of 254 km in October 1984 aboard the Space Shuttle. The detector was a stack of Lexan and CR‐39 plastic track detectors. We have increased our statistics and extended our previously reported results2 to lower energies. As shown in Fig. 1, the observed orbit‐averaged oxygen flux greatly exceeds what is expected from galactic3,4 and solar5,6,7 cosmic rays; the observed carbon flux agrees with galactic and solar expectations. If the excess oxygen flux is attributed entirely to fully‐ionized nuclei coming from outside the magnetosphere, the inferred exomagnetospheric flux exceeds contemporaneous upper limits on the oxygen flux at 1 AU5,7. The excess flux is consistent, however, with a singly‐ionized anomalous component (AC).

Radiation Exposure Assessments for Solar Proton Ground Level Enhancements

Solar proton events (SPEs) represent the single-most important source of acute space radiation exposure to humans and radio-sensitive spacecraft systems for lunar and Mars missions. In this paper, a review of the major sola r proton events for the past 5 solar cycles is presented with a focus on those SPEs that produced ground level enhancements (GLEs) as observed/measured by high latitude neutron monitor (NM) stations. Using NM GLE data one can infer the upper energy portion of the SPE e nergy spectrum, which extends up to several GeV (10 3 MeV). Historically, the scientific community has u sed solar proton data in the 10-100 MeV range from early IMP (Interplanetary Monitoring Platform) and, more recently, GOES (Geostationary Operational Environmental Satellite) spacecraft to make radiation exposure estimates. These SPE data were usually fitted using an exponential in rigidity (momentum) assumption (Malitson and Webber, 1963): φ(>P) = No * Exp(-P/Po) where φ is the integral fluence in protons/cm 2 , No is the normalization constant, P is the proton rigidity in million volts (MV), and Po is th e characteristic rigidity in MV. More recently, Xapsos, et al (2000) have determined that a better fit to the SPE data in the 10-100 MeV range is obtained by using a Weibull function. For GLEs, the proton energies extend to several GeV. Recently, Tylka and Dietrich (2008) ha ve utilized NM data and developed methods (Band [1993] function) to extend the solar particle data over the entire particle energy spectrum. SPE radiation assessments can be grossly underestimated if the entire proton energy spectrum is not taken into considerat ion. In this paper, we compare the various fitting methods to estimate the absorbed do se and dose equivalent for six (6) classic GLEs. Finally, detailed radiation exposure analyses are presented using several different radiation-mitigating shielding materials.

The cosmic ray environment of tactical ABMs

The battlespace of tactical antiballistic missiles (TABMs) is mostly exoatmospheric and includes regions with negligible geomagnetic shielding, especially during geomagnetic storms. It is therefore necessary to harden TABMs against galactic cosmic ray (GCR) and solar particle event (SPE) induced single event effects (SEEs) and star tracker and focal plane array (FPA) glitches. The variability of the worldwide GCR and SPE exoatmospheric TABM environment is described.

Assessment of F200 fluence for major solar energetic particle events on the multi-millennial time scale

Solar energetic particle (SEP) fluxes are typically quantified in the F30 units (integrated fluence of particles with energy above 30 MeV) and their direct measurements are available only for the last several decades. On the other hand, a reconstruction of major SEP events in the distant past (centennial-millennial time scales) is possible using data on the cosmogenic isotopes such as 14C and 10Be measured in stratified and independently datable terrestrial archives (tree trunks or polar ice cores). While such events (or at least candidates for those) can be identified in the cosmogenic records with a degree of confidence, the estimate of the F30 flux/fluence is ambiguous depending on the proposed SEP energy spectrum. Here we introduce a new index, the integral fluence of an SEP above 200 MeV, F200, which is related to the effective energy of the production of the cosmogenic isotopes by SEP in the Earth atmosphere. This index is robust against the assumptions on the exact shape of the energy spectrum of the event in a wide range of parameters. Using several records of cosmogenic isotopes, we present a reconstruction of the F200 fluence for major SEP events in the past, and also assess the occurrence probability density function for extreme events. In particular, we evaluate that extreme SPEs with F200 > 1010 cm−2 may occur no more frequently than once per 10 000 years or even rarer up to never.

Preliminary Radiation Analysis of the Total Ionizing Dose for the Resource Prospector Mission

NASAs Resource Prospector (RP) is a collaborative project between multiple centers and institutions to search for volatiles at the polar regions of the Moon as a potential resource for oxygen and propellant production. The mission is rated Class D and will be the first In-Situ Resource Utilization (ISRU) demonstration on the lunar surface and at the lunar poles. Given that this mission is rated Class D, the project is considering using commercial off the shelf (COTS) electronics parts to reduce cost. However, COTS parts can be more susceptible to space radiation than typical aerospace electronic parts and carry some additional risk. Thus, prior to parts selection, having a better understanding of the radiation environment can assist designers in the parts selection process. The focus of this paper is to provide a preliminary analysis of the radiation environment from launch, through landing on the surface, and some surface stay as an initial step in determining worst case mission doses to assist designers in screening out electronic parts that would not meet the potential dose levels experienced on this mission.