Molniya orbits), these are the primary orbits of interest in the majority of cases. The trapped protons distribution. The trapped particles in the Van Allen radiation ...
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NRIAG Journal of Astronomy and Astrophysics, Special Issue, PP.139 –150, (2006)
COMPARATIVE STUDY OF THE ENERGETIC PARTICLE FLUENCES FOR DIFFERENT ORBITAL TRAJECTORIES Samwel, S.W. *, Garrett, H. B. **, Hanna, Y.S. * Ibrahim, Makram *, Mikhail, J.S. *, Hady, A.A. ***
ABSTRACT: The radiation environment is a significant source of spacecraft failures. There is, in addition, a large variation in the level and type of hazard depending on the orbit of a given spacecraft. In the present work, we have carried out a comparative study of the energetic particle fluences along different orbital trajectories. The energetic particles under study are trapped protons and electrons, the solar protons, and the galactic cosmic rays. The Hubble Space Telescope (HST) is selected as representative of the Low Earth Orbit (LEO), the Defense Meteorological Satellite Program (DMSP F-10) to represent polar orbits, and the Global positioning System (GPS BII-09) to show the medium earth orbit (MEO) environment. The NILESAT 101 has been picked as a geostationary mission. Finally, the Combined Release and Radiation Effect Satellite (CRRES) is used for geostationary transfer orbit. Based on these comparisons, we find that there are extremely large variations in the radiation fluence levels for the different components of the radiation environment. These variations are shown to be very sensitive to the details of the trajectory that a given spacecraft follows through the radiation sources.
INTRODUCTION The space environment provides an assortment of hazards whose bad effects can range from degraded performance up to catastrophic loss of a spacecraft. The radiation environment is believed to be the most significant in terms of spacecraft failures. It consists mainly of protons, electrons and heavy ions, with energies which are able to cause ionization and displacement damage in spacecraft materials and electronic circuits [cf. Reedy, 2006]. Electrons and protons trapped in the magnetic field of the earth are the major contributors to the total particle fluence experienced. Also, solar flares and CME may be encountered during a mission, contributing extensively to the received dose and single event ____________________________________________________________________ * National Research Institute Of Astronomy and Geophysics, Helwan, Cairo, Egypt ** La Crescenta, CA , USA. ***Faculty of Science, Cairo University, Giza, Cairo, Egypt.
COMPARATIVE STUDY OF THE ENERGETIC PARTICLE …..
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effects. These radiation fluences are transient, however, compared to the trapped particles. In addition, the galactic cosmic radiation (GCR), originating from far outside our solar system, contribute but at a very much lower fluence level than the trapped and solar particles (they do, however, contribute to the single event effect rates). Thus, radiation encountered in space can roughly be classified into three groups [cf. Garrett, 1993 and Barth, 1997]: 1Trapped radiation 2Solar protons 3Galactic Cosmic Rays In order to assure reliability of the spacecraft systems, it is necessary to investigate the radiation environment encountered in space first, and determine the particle fluence (i.e. the number of particles encountered during mission time). The particle fluence experienced by the spacecraft is highly dependent on the orbit [cf. Barth, 1997] since the magnetic field of the Earth partially shields a spacecraft from the transient solar particles and the trapped protons and electrons are functions of the magnetic field location. Therefore, a comparative study of the energetic particle fluences along different orbital trajectories is required if we are to be able to predict the effects of the space environment adequately. Section 2 outlines the numerical methods which are used to estimate the fluence of the different energetic particles, while section 3 employs these models to assess the radiation environment for different missions at different orbital trajectories. Finally, a summary of the results obtained for the different classes of orbits will be presented as our conclusion in section 4. MODELING THE RADIATION ENVIRONMENT Radiation belt models The most widely used models for evaluating the trapped particle environments are the AE-8 and AP-8 for electrons and protons respectively. These models were developed by James Vette in a joint program sponsored by NASA, the US Air Force (USAF), in co-operation with various university teams and with data on the trapped particle radiation environment from a variety of spacecraft circa the 1960s [Vette, 1966, and Sawyer and Vette, 1976]. The current versions of AE8 and AP8 (released in 1983 and 1976) incorporate data from 43 satellites and are applicable to both the maximum and minimum states of solar activity. _______________________________________________________________________ 140 NRIAG Journal of Astronomy and Astrophysics, Special Issue, (2006)
Samwel, S.W. et al. _____________________________________________________________________
The models are empirical and represent static conditions based on long term average observations throughout the inner magnetosphere. The AP-8 and AE-8 models [Vette, 1991b] consist of maps that contain omnidirectional, integral electron (AE maps) and proton (AP maps) fluxes in the energy range from 0.04 MeV to 7 MeV for electrons and 0.1 MeV to 400 MeV for protons in the Earth's radiation belt. The NASA Ap-8 and AE-8 radiation belt models are still the standard models for engineering applications. This is mainly due to the fact that, up till now, they have been the only models that completely cover the region of the radiation belts, and have a wide energy range for both protons and electrons (see also the CRRES family of radiation models). Solar proton model Several models of the solar proton event environment exist. Here however, we have used the popular “JPL Solar Proton Model” developed by Joan Feynman and colleagues at the Jet Propulsion Laboratory, California Institute of Technology [Feynman et al., 1993, 2002]. The JPL model provides a statistical estimate of the fluences that a spacecraft in the interplanetary medium can be expected to experience during missions of varying length [Garrett, 1993 and Feynman et al., 1993, 2002]. The current JPL-91 model attempts to statically predict the long term proton fluences for a given mission, for a given confidence level (CL, or level of confidence that the solar proton flux will not exceed the model values [Feynman et al., 1993]) as represented in table 1. It is based on a set of data collected by the OGO 1 and IMP 1, 2, 3, 5, 6, 7, and 8 series of spacecrafts between 1963 and 1991 [Feynman et al., 2002]. As the interplanetary proton fluence below 100 Mev is dominated by the solar protons, the model is concerned with solar proton fluences in the energy ranges >1, >4, >10, >30, and >60 MeV. Using the dates of the solar maximum as the zero reference year for each cycle, Feynman et al. (1990a and b) divided the sunspot cycle into two periods: a high-fluence, active sun of 7 years period and a low-fluence quite sun of 4 years period. The active period begins 2.5 years before the solar maximum and ends 4.5 years later after the solar maximum [cf. Feynman et al., 1990a&b, and Meusel et al., 2005]. The model considers the statistical properties of the 7 active years and ignores those of the 4 quite years. It was assumed that no significant proton fluence occurs during quite periods. Therefore, only data collected during the 7 active years of the cycle were used. _______________________________________________________________________ NRIAG Journal o f Astronomy and Astrophysics, Special Issue, (2006) 141
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_______________________________________________________________________ Table 1. Recommended confidence levels for the JPL models
Mission duration (years) Confidence level (%) 1 97 2 95 3 95 4 90 5 90 6 90 7 90
Galactic Cosmic Ray Models Although the fluences of the GCR are very low, it is now recognized that there is a need for an accurate model that includes the galactic cosmic radiation environment. This is becoming increasingly important in light of manned space exploration missions and increasing sensitivity of microelectronics to this background environment [Badhwar and O'Neil, 1992]. James Adams and his colleagues developed the Cosmic Ray Effects in Micro-Electronic (CREME86) code which was first released in 1984 [Tylka et al., 1996] to calculate the galactic cosmic rays population energy spectra in addition to the anomalous component energy and the solar energetic particle events spectra. Recently, Tylka et al. (1997), with the sponsorship of the NASA Space Environments and Effects program, released an update to the CREME package, CREME96. The GCR environment calculated by CREME is based on data from several researchers collected by Adams et al. (1981). Galactic Cosmic Ray environment is comprised of particles ranging from protons to iron nuclei of low flux but high energy. It consists mainly of protons (85%), alpha particles (14%) [cf. Nieminen, 2001]. Because of the dissimilar shape of their energy spectra, the hydrogen, helium, and iron ion distributions are treated as separate cases, and the other elements scaled to one of the three spectra, as appropriate, using the relative abundances of the elements. _______________________________________________________________________ 142 NRIAG Journal of Astronomy and Astrophysics, Special Issue, (2006)
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ANALYSIS OF THE RADIATION ENVIRONMENT FOR DIFFERENT ORBITAL TRAJECTORIES In the present section, the energetic particle fluences for different orbital trajectories are investigated for a one year mission length (during the solar maximum) starting at the orbital epoch 1998, July, 01. The energetic particles under study are the solar protons, the trapped protons and electrons, and GCRs. Table (2) outlines the models which are used in this study in order to estimate the fluences of the trapped protons & electrons, the solar protons and the galactic cosmic rays. Table 2. Summary of radiation environment and models
Particle Origin Trapped Transient
Particle Type Protons Electrons Solar Protons Galactic cosmic rays
Model AP-8 AE-8 JPL CREME
The Hubble Space Telescope (HST) is selected as representative to a spacecraft at (LEO) orbit. The DMSP F-10 to represent polar orbits, and the GPS BII-09 to show the MEO environment. The NILESAT 101 has been picked as a geostationary mission. Finally, CRRES is used to represent a geo-transfer orbit (GTO). Although this does not cover all classes of spacecraft orbits (e.g., high altitude, eccentric orbits (HEO) and Molniya orbits), these are the primary orbits of interest in the majority of cases. The trapped protons distribution The trapped particles in the Van Allen radiation belts include trapped electrons of E
