Note: Much of this material is attributed to Ken LaBel and Janet Barth, NASA Goddard Space Flight Center.
Note: Much of this material is centered around SEE. I'll add some more material on total dose when I either get a good pointer, some time, or some contributions.
Radiation damage to on-board electronics may be separated into two categories: total ionizing dose and single event effects. Total ionizing dose (TID) is a cumulative long-term degradation of the device when exposed to ionizing radiation. Single event effects (SEEs) are individual events which occur when a single incident ionizing particle deposits enough energy to cause an effect in a device.
The main sources of energetic particles that are of concern to spacecraft designers are:
- protons and electrons trapped in the Van Allen belts
- heavy ions trapped in the magnetosphere
- cosmic ray protons and heavy ions, and
- protons and heavy ions from solar flares.
The levels of all of these sources are affected by the activity of the sun. The solar cycle is divided into two activity phases: the solar minimum and the solar maximum. An average cycle lasts about eleven years with the length varying from nine to thirteen years. Generally, the models of the radiation environment reflect the particle level changes with respect to the changes in solar activity.
From the information provided by the mapping of the trapped heavy ions by the SAMPEX satellite, we know that these ions do not have sufficient energy to penetrate the satellite and to generate the ionization in electronic parts necessary to cause SEEs. Also, electrons are not known to induce SEEs. Therefore, trapped heavy ions and trapped electrons are not included in a radiation environment definition for SEEs.
In the past, analyses of SEEs focused on energetic heavy ion induced phenomena. However, SEE data from recent spacecraft have shown that newer, high density electronic parts can have higher upset rates from protons than from heavy ions because of their low threshold LET value. In addition, it is difficult to shield against the high energy protons that cause SEE problems within the weight budget of a spacecraft. As a result, any successful and cost effective SEE mitigation plan must include a careful definition of the trapped proton environment and its variations.
Protons are the most important component of the "inner" Van Allen belt. In the equatorial plane, the high energy protons (E>30 MeV) extend only to about 2.4 earth radii. The energies range from keV to hundreds of MeV. The intensities range from 1 proton/cm2/sec to 1 x 105 protons/cm2/sec. The location of the peak flux intensities varies with particle energy. This is a fairly stable population but three known variations are important when defining requirements for SEE analyses. The most well known variatcion in the population is due to the cylic activity of the sun. During solar maximum, the trapped proton populations near the atmospheric cut-off at the inner edge of the belt are at the lowest levels and, during solar minimum, they are at their highest. Second, the trapped protons are subject to perturbations at the outer edge of the inner belt and in the region between two and three earth radii due to geomagnetic storms and/or solar flare events. Last, the particle population is affected by the gradual change (secular variation) of the earth's magnetic field.
Analyses of data gathered in flight before, during, and after geomagnetic storms and solar flare events have shown that the trapped proton population is affected by these phenomena at the outer edges of their trapping domain. It was observed on the CRRES satellite that flew during solar maximum that the so called "slot" region of the magnetosphere (2 < L < 3) can become filled with very energetic trapped protons as a result of solar flare events. The decay time of the second belt is estimated to be on the order of 6-8 months. Phillips Laboratory has modeled this second proton belt as detected by the CRRES satellite. The Air Force DMSP satellite flew during solar minimum. Particle flux monitors on board the DMSP showed that, after a major magnetic storm, the inner proton belt was reconfigured and eroded such that a second belt was formed.
The uncertainty factor defined for the trapped proton AP8 model is two. This is based on the statistical error inherent in merging the several spacecraft data sets that make up the model and does not include the substantial variations that occur over time. The largest variability in the trapped proton predictions is a function of the trajectory of the spacecraft. Therefore, applying proton predictions for a satellite in one orbit trajectory to another trajectory can result in errors up to several orders of magnitude.
To reduce the uncertainty of trapped proton calculations for SEE application, the definition of the trapped proton environment must specifically take into account:
- how long the mission will be in each phase of the solar cycle and the effect of changes in the launch date, the orbit trajectory
- analysis of the effects of the secular variation of the geomagnetic field (especially for orbits under 1000 km)
- analysis of the variation in the outer edges of the proton trapping regions due to solar flare events/magnetic storms, and
- the amount of spacecraft shielding surrounding the SEE sensitive part(s).
Galactic cosmic ray particles originate outside the solar system. They include ions of all elements from atomic number 1 through 92. The flux levels of these particles are low but, because they include highly energetic particles (10s of MeV/n ~ E ~ 100s of GeV/n) of heavy elements such as iron, they produce intense ionization as they pass through matter. As with the high energy trapped protons, they are difficult to shield against. Therefore, in spite of their low levels, they constitute a significant hazard to electronics in terms of SEEs.
As with the trapped proton population, the galactic cosmic ray particle population varies with the solar cycle. It is at its peak level during solar minimum and at its lowest level during solar maximum. The earth's magnetic field provides spacecraft with varying degrees of protection from the cosmic rays depending primarily on the inclination and secondarily on the altitude of the trajectory. However, cosmic rays have free access over the polar regions where field lines are open to interplanetary space. The exposure of a given orbit is determined by rigidity functions calculated with geomagnetic field models. The coefficients in the models include a time variation so that the rigidity functions can be calculated for the epoch of a mission.
The basic uncertainty factor defined for the CREME model is two. The CHIME model will provide more updated abundances when it is available.
To reduce the uncertainty in the predictions of the galactic cosmic ray heavy ion levels, the definition must consider:
- how long the mission will be in each phase of the solar cycle and the effect of changes in the launch date
- the effect of the ionization state of the anomalous component
- the amount of geomagnetic shielding for the orbit, and
- estimate of the amount of shielding surrounding the SEE sensitive part(s).
The average eleven year solar cycle can be divided into four inactive years with a small number of flare events (solar minimum) and seven active years with a large number of events (solar maximum). During the solar minimum phase, few significant solar flare events occur; therefore, only the seven active years of the solar cycle are usually considered for spacecraft mission evaluations. Large solar flare events may occur several times during each solar maximum phase. Events last from several hours to a few days. The proton energies may reach a few hundred MeV and the heavy ion component ranges in energy from 10s of MeV/n to 100s of GeV/n. As with the galactic cosmic ray particles, the solar flare particles are attenuated by the earth's magnetosphere. The rigidity functions that are used to attenuate those particles can also be used to attenuate the solar flare protons and heavy ions. When setting part requirements, it is important to keep in mind that solar flare conditions exist for only about two percent of the total mission time during solar maximum.
The component of the environment that presents the largest uncertainty in predictions is the solar flare protons. Some solar cycles (Cycle 21) contain no extremely large flares at all. Other cycles contain as many as eight extremely large events (Cycle 22). The problem of providing solar flare predictions to those concerned with SEE criticality analysis is compounded by the limitations of the models. They are designed for determining mission integrated total dose or solar cell degradation levels and do not adequately address the SEE problem. That is, they provide event integrated fluences (SOLPRO) or mission integrated and daily fluences. These values are not adequate for determining worst case SEE vulnerability during the peak flux levels of the flares. The new CHIME model promises to provide these options for users when it becomes available. In the meantime, the best option is to use the peak flux spectrum from the August 1972 event [14]. The fluence levels provided by the SOLPRO and JPL92 models are a function of confidence level and mission duration.
To reduce the uncertainty in solar flare proton predictions, the definition must take into account:
- how long the mission will be in each phase of the solar cycle and the effect of changes in the launch date
- the level of confidence selected by the project
- fluence levels for an extremely large event
- flux levels for the peak of an event
- the amount of geomagnetic shielding for the orbit
- estimate of how many times such an event will occur, and
- the amount of shielding surrounding the SEE sensitive part(s).
As with the solar flare proton portion of the model, the heavy ion model gives fluences as a function of confidence level and mission duration. Again, for SEE analysis, a peak spectrum must be analyzed for worst case conditions.
The solar flare heavy ion predictions must take into account:
- how long the mission will be in each phase of the solar cycle and the effect of changes in the launch date
- the level of confidence selected by the project
- fluence levels for an extremely large event
- flux levels for the peak of an event
- the amount of geomagnetic shielding for the orbit
- estimate of how many times such an event will occur, and
- the amount of shielding surrounding the SEE sensitive part(s).
There are extremely large variations in the SEE inducing flux levels that a given spacecraft encounters depending on its trajectory through the radiation sources. Some of the typical orbit configurations are discussed below with emphasis given to considerations that are important when calculating SEE rate predictions.
Low Earth Orbits (LEOs)
The most important characteristic of the environment encountered by satellites in LEOs is that several times each day they pass through the proton and electron particles trapped in the Van Allen belts. The level of fluxes seen during these passes varies greatly with orbit inclination and altitude. The greatest inclination dependencies occur in the range of 0°< i < 30°. For inclinations over 30°, the fluxes rise more gradually until about 60°. Over 60° the inclination has little effect on the flux levels. The largest altitude variations occur between 200 to 600 km where large increases in flux levels are seen as the altitude rises. For altitudes over 600 km, the flux increase with increasing altitude is more gradual. The location of the peak fluxes depends on the energy of the particle. For trapped protons with E > 10 MeV, the peak is at about 4000 km. For normal geomagnetic and solar activity conditions, these proton flux levels drop gradually at altitudes above 4000 km. However, as discussed above, inflated proton levels for energies E > 10 MeV have been detected at these higher altitudes after large geomagnetic storms and solar flare events.
The amount of protection that the geomagnetic field provides a satellite from the cosmic ray and solar flare particles is also dependent on the inclination and to a smaller degree the altitude of the orbit. As altitude increases, the exposure to cosmic ray and solar flare particles gradually increases. However, the effect that the inclination has on the exposure to these particles is much more important. As the inclination increases, the satellite spends more and more of its time in regions accessible to these particles. As the inclination reaches polar regions, it is outside the closed geomagnetic field lines and is fully exposed to cosmic ray and solar flare particles for a significant portion of the orbit.
Under normal magnetic conditions, satellites with inclinations below 45° will be completely shielded from solar flare protons. During large solar events, the pressure on the magnetosphere will cause the magnetic field lines to be compressed resulting in solar flare and cosmic ray particles reaching previously unattainable altitudes and inclinations. The same can be true for cosmic ray particles during large magnetic storms.
Highly Elliptical Orbits (HEOs)
Highly elliptical orbits are similar to LEO orbits in that they pass through the Van Allen belts each day. However, because of their high apogee altitude (greater than about 30,000 km), they also have long exposures to the cosmic ray and solar flare environments regardless of their inclination. The levels of trapped proton fluxes that HEOs encounter depend on the perigee position of the orbit including altitude, latitude, and longitude. If this position drifts during the course of the mission, the degree of drift must be taken into account when predicting proton flux levels.
Geostationary Orbits (GEOs)
At geostationary altitudes, the only trapped protons that are present are below energy levels necessary to initiate the nuclear events in materials surrounding the sensitive region of the device that cause SEEs. However, GEOs are almost fully exposed to the galactic cosmic ray and solar flare particles. Protons below about 40-50 MeV are normally geomagnetically attenuated, however, this attenuation breaks down during solar flare events and geomagnetic storms. Field lines that cross the equator at about 7 earth radii during normal conditions can be compressed down to about 4 earth radii during these events. As a result, particles that were previously deflected have access to much lower latitudes and altitudes.
Planetary and Interplanetary
The evaluation of the radiation environment for these missions can be extremely complex depending on the number of times the trajectory passes through the earth's radiation belts, how close the spacecraft passes to the sun, and how well known the radiation environment of the planet is. Each of these factors must be taken very carefully into account for the exact mission trajectory.
Careful analysis is especially important for missions that fly during solar maximum and that have trajectories that place the spacecraft close to the sun. Guidelines for scaling the intensities of particles of solar origin for spacecraft outside of 1 AU have been determined by a panel of experts. They recommend that a factor of 1 AU x 1/r2 be used for distances less than 1 AU and that values of 1 AU x 1/r3 be used for distances greater than 1 AU.
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Last Revised: January 09, 2002
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