Engineering topics

Tylka, Allan J.

Publications and source records attributed to Tylka, Allan J..

13 records

NASA NTRS2016

Probability Estimates of Solar Particle Event Doses During a Period of Low Sunspot Number for Thinly-Shielded Spacecraft and Short Duration Missions

In an earlier paper (Atwell, et al., 2015), we investigated solar particle event (SPE) radiation exposures (absorbed dose) to small, thinly-shielded spacecraft during a period when the sunspot number (SSN) was less than 30. These SPEs contain Ground Level Events (GLE), sub-GLEs, and sub-sub-GLEs (Tylka and Dietrich, 2009, Tylka and Dietrich, 2008, and Atwell, et al., 2008). GLEs are extremely energetic solar particle events having proton energies extending into the several GeV range and producing secondary particles in the atmosphere, mostly neutrons, observed with ground station neutron monitors. Sub-GLE events are less energetic, extending into the several hundred MeV range, but do not produce secondary atmospheric particles. Sub-sub GLEs are even less energetic with an observable increase in protons at energies greater than 30 MeV, but no observable proton flux above 300 MeV. In this paper, we consider those SPEs that occurred during 1973-2010 when the SSN was greater than 30 but less than 50. In addition, we provide probability estimates of absorbed dose based on mission duration with a 95% confidence level (CL). We also discuss the implications of these data and provide some recommendations that may be useful to spacecraft designers of these smaller spacecraft.

Atwell, William↗

NASA NTRS2016

Probability Estimates of Solar Proton Doses During Periods of Low Sunspot Number for Short Duration Missions

In an earlier paper presented at ICES in 2015, we investigated solar particle event (SPE) radiation exposures (absorbed dose) to small, thinly-shielded spacecraft during a period when the monthly smoothed sunspot number (SSN) was less than 30. Although such months are generally considered "solar-quiet", SPEs observed during these months even include Ground Level Events, the most energetic type of SPE. In this paper, we add to previous study those SPEs that occurred in 1973-2015 when the SSN was greater than 30 but less than 50. Based on the observable energy range of the solar protons, we classify the event as GLEs, sub-GLEs, and sub-sub-GLEs, all of which are potential contributors to the radiation hazard. We use the spectra of these events to construct a probabilistic model of the absorbed dose due to solar protons when SSN < 50 at various confidence levels for various depths of shielding and for various mission durations. We provide plots and tables of solar proton-induced absorbed dose as functions of confidence level, shielding thickness, and mission-duration that will be useful to system designers.

Atwell, William↗

NASA NTRS2015

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

NASA's 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.

Rojdev, Kristina↗

NASA NTRS2015

Sub-GLE Solar Particle Events and the Implications for Lightly-Shielded Systems Flown During an Era of Low Solar Activity

Many of the large space missions must be very rigorous in their designs to reduce risk from radiation damage as much as possible. Some ways of reducing this risk have been to build in multiple redundancies, purchase/develop radiation hardened electronics parts, and plan for worst case radiation environment scenarios. These methods work well for these ambitious missions that can afford the costs associated with these meticulous efforts. However, there have been more small spacecraft and CubeSats with smaller duration missions entering the space arena, which can take some additional risks, but cannot afford to implement all of these risk-reducing methods. Therefore, one way to quantify the radiation exposure risk for these smaller spacecraft would be to investigate the radiation environment pertinent to the mission to better understand these radiation exposures, rather than always designing to the infrequent, worst-case environment. In this study, we have investigated 34 historical solar particle events (1974-2010) that occurred during a time period when the sun spot number (SSN) was less than 30. These events contain Ground Level Events (GLE), sub-GLEs, and sub-sub-GLEs(sup 1-3). GLEs are extremely energetic solar particle events (SPEs) having proton energies often extending into the several GeV range and producing secondary particles in the atmosphere, mostly neutrons, observed with ground station neutron monitors. Sub-GLE events are less energetic, extending into the several hundred MeV range, but without producing detectable levels of secondary atmospheric particles. Sub-sub GLEs are even less energetic with an observable increase in protons at energies greater than 30 MeV, but no observable proton flux above 300 MeV. The spectra for these events were fitted using a double power law fit in particle rigidity, called the Band fit method. The differential spectra were then input into the NASA Langley Research Center HZETRN 2005, which is a high-energy particle transport/dose code, to determine the dose in various thicknesses of aluminum, representing the spacecraft. This paper will detail the absorbed dose results of each of these environments, as well as analyze the data to better understand the doses over small thicknesses that are more relevant to small spacecraft and satellites, such as CubeSats. In addition, we will discuss the implications of these data and provide some recommendations that may be useful to spacecraft designers of these smaller spacecraft/CubeSat-type missions.

Atwell, William↗

NASA NTRS2013

Source Regions of the Interplanetary Magnetic Field and Variability in Heavy-Ion Elemental Composition in Gradual Solar Energetic Particle Events

Gradual solar energetic particle (SEP) events are those in which ions are accelerated to their observed energies by interactions with a shock driven by a fast coronal mass-ejection (CME). Previous studies have shown that much of the observed event-to-event variability can be understood in terms of shock speed and evolution in the shock-normal angle. But an equally important factor, particularly for the elemental composition, is the origin of the suprathermal seed particles upon which the shock acts. To tackle this issue, we (1) use observed solar-wind speed, magnetograms, and the PFSS model to map the Sun-L1 interplanetary magnetic field (IMF) line back to its source region on the Sun at the time of the SEP observations; and (2) then look for correlation between SEP composition (as measured by Wind and ACE at approx. 2-30 MeV/nucleon) and characteristics of the identified IMF-source regions. The study is based on 24 SEP events, identified as a statistically-significant increase in approx. 20 MeV protons and occurring in 1998 and 2003-2006, when the rate of newly-emergent solar magnetic flux and CMEs was lower than in solar-maximum years and the field-line tracing is therefore more likely to be successful. We find that the gradual SEP Fe/O is correlated with the field strength at the IMF-source, with the largest enhancements occurring when the footpoint field is strong, due to the nearby presence of an active region. In these cases, other elemental ratios show a strong charge-to-mass (q/M) ordering, at least on average, similar to that found in impulsive events. These results lead us to suggest that magnetic reconnection in footpoint regions near active regions bias the heavy-ion composition of suprathermal seed ions by processes qualitatively similar to those that produce larger heavy-ion enhancements in impulsive SEP events. To address potential technical concerns about our analysis, we also discuss efforts to exclude impulsive SEP events from our event sample.

magnetic field↗

NASA NTRS2010

Radiation Dose Assessments of Solar Particle Events with Spectral Representation at High Energies for the Improvement of Radiation Protection

For radiation dose assessments of major solar particle events (SPEs), spectral functional forms of SPEs have been made by fitting available satellite measurements up to approx.100 MeV. However, very high-energy protons (above ~500 MeV) have been observed with neutron monitors (NMs) in ground level enhancements (GLEs), which generally present the most severe radiation hazards to astronauts. Due to technical difficulties in converting NM data into absolutely normalized fluence measurements, those functional forms were made with little or no use of NM data. A new analysis of NM data has found that a double power law in rigidity (the so-called Band function) generally provides a satisfactory representation of the combined satellite and NM data from approx.10 MeV to approx.10 GeV in major SPEs (Tylka & Dietrich 2009). We use the Band function fits to re-assess human exposures from large SPEs. Using different spectral representations of large SPEs, variations of exposure levels were compared. The results can be applied to the development of approaches of improved radiation protection for astronauts, as well as the optimization of mission planning and shielding for future space missions.

Kim, Myung-Hee↗

NASA NTRS2004

SEP Time-to-Maximum (TTM) Studies and the Ionic Charge States of Solar Heavy Ions

1. We published a report on the TTM method and presented the results at the 28 International Cosmic Ray Conference. Included in this paper were our results on the energy-dependent charge state of Fe in the 200 1 April 15 ground- level event, the largest of Cycle 23. We showed that our results agreed well with direct measurements from SAMPEX in this event. We also presented results for the 2001 April 14 event, one of the largest impulsive events of Cycle 23. 2. We extracted TTM values for protons and ions of various energies in approx. 50 SEP events in Cycle 23, using time profiles measured by ACE, Wind, GOES, IMPS, SOHO, and SAMPEX. This event sample included many large gradual events, as well as several impulsive events. The TTM analyses typically yielded approx. 300 datapoints per event. 3. We used these data to make preliminary event classifications according to: (a) shapes of the rigidity and/or speed dependence of TTMs in each event; (b) behavior of proton TTMs compared to that of heavier ions; and (c) the behavior of Fe TTMs compared to that of lighter Z greater than I ions. 4. We cross-checked proton fluence and TTM values from 8 instruments on 6 satellites, so as to understand the origin of systematic instrument-to-instrument discrepancies. By also comparing the observed proton spectra, it was possible to extract correction factors to the nominal proton energy bins that removed or greatly reduced the instrument-to-instrument spread among proton TTMs. 5. We began comparisons of proton and He-4 TTM values in some particularly well measured, large SEP events measured by IMP8 in Cycles 21 and 22 when the instruments onboard were functioning better. It is hoped that these data will provide a better understanding of event-to-event variation in the functional dependence of TTMs on velocity and rigidity.

Tylka, Allan J.↗

NASA NTRS2001

Solar Energetic Particles and Space Weather

The solar energetic particles (SEPs) of consequence to space weather are accelerated at shock waves driven out from the Sun by fast coronal mass ejections (CMEs). In the large events, these great shocks fill half of the heliosphere. SEP intensity profiles change appearance with longitude. Events with significant intensities of greater than ten MeV protons occur at an average rate of approx. 13 per year near solar maximum and several events with high intensities of > 100 McV protons occur each decade. As particles stream out along magnetic field lines from a shock near the Sun, they generate waves that scatter subsequent particles. At high intensities, wave growth throttles the flow below the 'streaming limit.' However, if the shock maintains its strength, particle intensities can rise above this limit to a peak when the shock itself passes over the observer creating a 'delayed' radiation hazard, even for protons with energies up to approx. one GeV. The streaming limit makes us blind to the intensities at the oncoming shock, however, heavier elements such as He, O, and Fe probe the shape of the wave spectrum, and variation in abundances of these elements allow us to evade the limit and probe conditions at the shock, with the aid of detailed modeling. At high energies, spectra steepen to form a spectral 'knee'. The location of the proton spectral knee can vary from approx. ten MeV to approx. one GeV, depending on shock conditions, greatly affecting the radiation hazard. Hard spectra are a serious threat to astronauts, placing challenging requirements for shielding, especially on long-duration missions to the moon or Mars.

Reames, Donald V.↗

NASA NTRS1995

Characteristics of low energy ions in the Heavy Ions In Space (HIIS) experiment

We present preliminary data on heavy ions (Z greater than or equal to 10) detected in the topmost Lexan sheets of the track detector stacks of the Heavy Ions in space (HIIS) experiment (M0001) on LDEF. The energy interval covered by these observations varies with the element, with (for example) Ne observable at 18-100 MeV nuc and Fe at 45-200 MeV/nuc. All of the observed ions are at energies far below the geomagnetic cutoff for fully-ionized particles at the LDEF orbit. Above 50 MeV/nuc (where most of our observed particles are Fe), the ions arrive primarily from the direction of lowest geomagnetic cutoff. This suggests that these particles originate outside the magnetosphere from a source with a steeply-falling spectrum and may therefore be associated with solar energetic particle (SEP) events. Below 50 MeV/nuc, the distribution of arrival directions suggests that most of the observed heavy ions are trapped in the Earth's magnetic field. Preliminary analysis, however, shows that these trapped heavy ions have a very surprising composition: they include not only Ne and Ar, which are expected from the trapping of anomalous cosmic rays (ACR's), but also Mg and Si, which are not part of the anomalous component. Our preliminary analysis shows that trapped heavy ions at 12 less than or equal to Zeta less than or equal to 14 have a steeply-falling spectrum, similar to that reported by the Kiel experiment (exp 1,2,3) on LDEF (M0002) for trapped Ar and Fe at E less than 50 MeV/nuc. The trapped Mg, Si, and Fe may also be associated with SEP events, but the mechanism by which they have appeared to deep in the inner magnetosphere requires further theoretical investigation.

Kleis, Thomas↗

NASA NTRS1995

Results from the Heavy Ions In Space (HIIS) experiment on the ionic charge state of solar energetic particles

It has long been known that low-energy solar energetic particles (SEP's) are partially-ionized. For example, in large, so-called 'gradual' solar energetic particle events, at approximately 1 MeV/nucleon the measured mean ionic charge state, Q, of Fe ions is 14.1 +/- 0.2, corresponding to a plasma temperature of approximately 2 MK in the coronal or solar-wind source material. Recent studies, which have greatly clarified the origin of solar energetic particles and their relation to solar flares, suggest that ions in these SEP events are accelerated not at a flare site, but by shocks propagating through relatively low-density regions in the interplanetary medium. As a result, the partially-ionized states observed at low energies are expected to continue to higher energies. However, up to now there have been no high-energy measurements of ionic charge states to confirm this notion. We report here HIIS observations of Fe-group ions at 50-600 MeV/nucleon, at energies and fluences which cannot be explained by fully-ionized galactic cosmic rays, even in the presence of severe geomagnetic cutoff suppression. Above approximately 200 MeV/nucleon, all features of our data -- fluence, energy spectrum, elemental composition, and arrival directions -- can be explained by the large SEP events of October 1989, provided that the mean ionic charge state at these high energies is comparable to the measured value at approximately 1 MeV/nucleon. By comparing the HIIS observations with measurements in interplanetary space in October 1989, we determine the mean ionic charge state of SEP Fe ions at approximately 200-600 MeV/nucleon to be Q = 13.4 plus or minus 1.0, in good agreement with the observed value at approximately 1 MeV/nucleon. The source of the ions below approximately 200 MeV/nucleon is not yet clear. Partially-ionized ions are less effectively deflected by the Earth's magnetic field than fully-ionized cosmic rays and therefore have greatly enhanced access to low-Earth orbit. Moreover, at the high energies observed in HIIS, these ions can penetrate typical amounts of shielding. We discuss the significance of the HIIS results for estimates of the radiation hazard posed by large SEP events to satellites in low-Earth orbit, including the proposed Space Station orbit. Finally, we comment on previous reports of low-energy below-cutoff Fe-group ions, which some authors have interpreted as evidence for partially-ionized galactic cosmic rays. The LDEF flux levels are much smaller than the corresponding fluxes in these previous reports, implying that the source of these ions has an unusual solar-cycle variation and/or strongly increases with decreasing altitude.

Tylka, Allan J.↗

NASA NTRS1993

Progress report on the Heavy Ions in Space (HIIS) experiment

One of the objectives of the Heavy Ions In Space (HIIS) experiment is to investigate heavy ions which appear at Long Duration Exposure Facility (LDEF) below the geomagnetic cutoff for fully-ionized galactic cosmic rays. Possible sources of such 'below-cutoff' particles are partially-ionized solar energetic particles, the anomalous component of cosmic rays, and magnetospherically-trapped particles. In recent years, there have also been reports of below-cutoff ions which do not appear to be from any known source. Although most of these observations are based on only a handful of ions, they have led to speculation about 'partially-ionized galactic cosmic rays' and 'near-by cosmic ray sources'. The collecting power of HIIS is order of magnitude larger than that of the instruments which reported these results, so HIIS should be able to confirm these observations and perhaps discover the source of these particles. Preliminary results on below-cutoff heavy-ions are reported. Observations to possible known sources of such ions are compared. A second objective of the HIIS experiment is to measure the elemental composition of ultraheavy galactic cosmic rays, beginning in the tin-barium region of the periodic table. A report on the status of this analysis is presented.

Adams, James H., Jr.↗

NASA NTRS1992

Heavy Ions In Space (HIIS) experiment

The Heavy Ions In Space (HIIS) experiment has two primary objectives: (1) to measure the elemental composition of ultraheavy Galactic cosmic rays, beginning in the tin-barium region of the periodic table; and (2) to study heavy ions which arrive at LDEF below the geomagnetic cutoff, either because they are not fully stripped of electrons or because their source is within the magnetosphere. Both have practical as well as astrophysical consequences. The HIIS experiment used eight thick stacks of plastic track detectors mounted in two trays on the space facing end of LDEF. Since the last LDEF symposium, the statistics were increased of the observations and have extended the analysis to a second stack and to detector sheets near the top of a stack. New results are reported on the detector resolution and on the observations of both stopping and relativistic particles.

Adams, James H., Jr.↗

NASA NTRS1992

Preliminary results from the heavy ions in space experiment

The Heavy Ions In Space (HIIS) experiment has two primary objectives: (1) to measure the elemental composition of ultraheavy galactic cosmic rays, beginning in the tin-barium region of the periodic table; and (2) to study heavy ions which arrive at LDEF below the geomagnetic cutoff, either because they are not fully stripped of electrons or because their source is within the magnetosphere. Both of these objectives have practical as well as astrophysical consequences. In particular, the high atomic number of the ultraheavy galactic cosmic rays puts them among the most intensely ionizing particles in Nature. They are therefore capable of upsetting electronic components normally considered immune to such effects. The below cutoff heavy ions are intensely ionizing because of their low velocity. They can be a significant source of microelectronic anomalies in low inclination orbits, where Earth's magnetic field protects satellites from most particles from interplanetary space. The HIIS results will lead to significantly improved estimates of the intensely ionizing radiation environment.

Adams, James H., Jr.↗