Overall hazards from the space environment can be seen from the graphic below based on increasing charged particle energy.
To understand the effects of radiation on spacecraft and electronics, one must first examine the environment in space. I suggest the online information available from NASA. I would start with the following general review on the space environment from the Marshall Space Flight Center: The Natural Space Environment: Effects on Spacecraft (NASA RP-1350), which can be downloaded in pdf format. If you are in a hurry, I suggest looking at just the following four items in either the online or pdf version: Plasma, Geomagnetic Field, Solar Environment, and Ionizing Radiation. A more basic (and detailed) look at these topics can be found in The Exploration of the Earth's Magnetosphere from NASA's Goddard Space Flight Center [alternate site at http://www.phy6.org/Education/Intro.html].
For our purposes the sources of radiation in Earth space are separated into four categories:
Generally, plasma effects are not categorized with radiation effects since plasma energies are low (eV to keV) compared to the energy of ionizing radiation (keV to GeV). The plasma causes spacecraft surface charging, whereas the energetic particles affect spacecraft internal charging, displacement damage, ionizing dose, and single event effects. Another environmental factor is the geomagnetic field which strongly influences plasma/particle motions and locations.
The sun also has an impact on plasma density, ionizing radiation levels, and magnetic field characteristics. The sun undergoes an 11 year cycle with four years of solar minimum followed by seven years of solar maximum. Sun flares are a major contributor to the overall ionizing radiation level. A major solar flare can emit energetic protons that reach Earth within 30 minutes of the flare's peak. One to four days after a flare, a slower cloud of solar material and magnetic fields reach Earth, buffeting the magnetosphere and resulting in a geomagnetic storm. A geomagnetic storm is the result of solar storm particles being injected into the Earth's magnetic field. Magnetic substorms are a common occurrence in geosynchronous orbit (GEO) and result in the injection of high energy electrons (50 to 150 keV) into the outer Van Allen belt. Geomagnetic storms have been directly correlated to the incidence of spacecraft charging.
The geomagnetic field influences particle motions in the Earth's orbit, and it also deflects some particles from interplanetary space. The geomagnetic field would have a dipole shape except for the solar wind that distorts the geomagnetic field into a parabolic shape. The magnetic field traps charged particles, mostly electrons and protons, into the Van Allen radiation belts. The magnetic field is strongest at low altitudes. The dipole field geometry results in a region, known as the South Atlantic Anomaly (SAA), where the radiation belts reach their lowest altitude. The South Atlantic Anomaly (SAA) is a region of particularly intense proton flux. The figure below shows an overview of the Earth's magnetosphere.
Plasma is basically neutral, having equal numbers of electrons and ions at equal energies (low energies as compared to ionizing radiation). The plasma environment is divided into several regions, which are greatly controlled by the geomagnetic field. The magnetic field strength causes plasma densities to vary with latitude: particle densities are larger near the magnetic equator as compared to the magnetic poles for a given altitude. Major plasma regions are shown in the figure above. Whipple summarizes equilibrium potentials at increasing altitude:
Equilibrium Potentials at Increasing Altitude
[Source: E.C. Whipple, “Potentials of surfaces in space,” Reports on Progress in Physics, vol. 44, pp. 1197-1250, 1981]
|Ionosphere:||a few tenths of a volt negative|
|Magnetosphere:||normally, a few volts positive; in eclipse, may become highly negative|
|Solar Wind:||a few volts positive|
|Interstellar Space:||a few volts positive or negative|
Grard et al. tabulate the plasma density, kinetic energies and current densities for the plasmasphere, plasma sheet and solar wind as shown in the table below. The plasma density, n, along with electron/ion velocities, v, directly result in the ambient flux (i.e., =nv) from which the current density may be computed (J=q). The kinetic energy yields not only the velocity (i.e., EK=˝mv2) but also the ambient plasma temperature (kT) from which shadowed surface potentials can be estimated. The Debye length is the distance over which a satellite significantly perturbs the ambient medium.
Typical Space Plasma Environments
[Source: R. Grard, K. Knott, Pedersen, "Spacecraft charging effects,"
Space Science Reviews, vol. 34, pp. 289-304, 1983]
|Parameter||Plasmasphere||Plasma sheet||Solar wind|
|Plasma density, cm-3||10 - 1000||1||6|
|Electron mean kinetic energy, eV||1||1000||15|
|Ion mean kinetic energy, eV||1||6000||10|
|Electron random current density, µA/m˛||0.25 - 25||0.85||0.62|
|Ion random current density, µA/m˛||0.006 - 0.6||0.05||0.012|
|Electron Debye length, m||2.5 - 0.25||240||12|
Like the Van Allen radiation belts, the ionosphere is located within the cavity of the magnetosphere. The ionosphere is a layer of Earth's upper atmosphere where photoionization by solar X-rays and extreme ultraviolet rays creates free electrons.
The inner magnetosphere is the region where the Earth's magnetic field lines are approximately dipolar. The base of this region is the ionosphere; at higher altitudes the inner magnetosphere includes the plasmasphere, the geomagnetically-trapped Van Allen particles, and the ring current. The plasmasphere is a region of ionized, rarefied gas. Within the plasmasphere the plasma temperature is low, but the equatorial density is high (103 to 105 cm-3). In contrast, the plasma sheet in the outer magnetosphere on the midnight side of the Earth has a small ( 2 cm-3) plasma density, and plasma motion is controlled by both magnetic and electric fields.
The magnetospheric plasma is important to spacecraft charging. At altitudes > 3 RE, ambient plasma density is generally less than 106 m-3, so photoelectron current dominates; at altitudes < 2 RE, the plasma density is > 109 m-3 and temperature is < 1 eV, such that plasma electron flux dominates. During times of geomagnetic activity, energetic electrons are injected into the region of synchronous orbit near the midnight meridian. A magnetic substorm consists of a dense plasma cloud of energetic particles (1-10 per cm3) with electron temperatures (kT) of 5 to 20 kV. The electrons move along the magnetic field lines from the midnight meridian toward the dawn meridian, whereas the protons drift toward the dusk meridian.
The magnetosheath is the interface that separates interplanetary space and the magnetosphere. This interface is very dynamic, normally the magnetopause lies at a distance equivalent to about 10 Earth radii (RE) in the direction of the Sun. However, during elevated solar wind conditions, the magnetopause can be pushed inward to within 6.6 RE (GEO altitudes).
External to the magnetosphere is the solar wind, cosmic rays, and some energetic electrons and ions. The solar wind is basically a high-speed (~ 400 to 700 km/sec) low-temperature, low-density plasma, primarily composed of protons (H+) with some alpha particles (He2+). The solar wind has the solar magnetic field embedded within it, and together they cause magnetic substorms and storms.
The ionizing particles are distinguished from plasma based on their lower densities (<1 cm-3) and higher energies (~MeV). The main sources of energetic particles (ionizing radiation) in space are
The trapped heavy ions do not have sufficient energy to generate the ionization required to cause single-event effects (SEEs) nor do they make a significant contribution to total ionizing dose (TID). In addition, electrons are not known to cause SEEs. The energetic particles causing single-event upset include galactic cosmic rays, cosmic solar particles (which are heavily influenced by solar flares), and trapped protons in the radiation belts.
The trapped particles in the Van Allen radiation belts, as shown in the figure above, include trapped electrons of E<7 MeV, which are easier to shield than the trapped protons with E<500 MeV. The trapped proton energies vary approximately inversely with altitude (energies of 400 MeV close to Earth). The dose is affected by altitude and geomagnetic latitude. The radiation belts include an inner zone and an outer zone as compared in the table below.
|Inner Zone (< 2.5RE)||Outer Zone (> 2.5RE)|
|Proton flux dominates||About 10 times higher electron flux in outer zone than inner zone|
|Electron energies < 5 MeV||Electron energies around 7 MeV|
|Electron and proton fluxes peak at 1.5RE to 2.0RE||Electron flux peaks at about 5RE|
The Earth's geomagnetic field traps charged particles (protons, electrons and some heavier ions) in the Van Allen radiation belts. These trapped particles spiral back and forth along the magnetic field lines as shown in the figure below. The regions inside the magnetopause are generally described in terms of the L field line value, which is essentially the radial distance from the center of the Earth (in units of Earth radii) to the minimum B on that line (the magnetic equator). Hence, L=RE is at the Earth's surface---the radius of the Earth is RE=6371 km. L is the dipolar shell parameter defined by McIlwain as L=RE/cos2(M) where RE is the number of Earth radii and M is the geomagnetic (dipolar) latitude.
The region up to L~2.5 is the inner zone, and beyond L~3 to the magnetopause is the outer zone. The inner zone and outer zone terminology really applies to electron populations. Between these two zones is the slot, which has during magnetically quiet periods relatively few electrons, but during magnetic storms, the slot is filled by electrons from the outer zone. The slot can become filled with extremely energetic protons as a result of solar flare events. These L boundary values on the zone are not hard definitions. Some references indicate that the radiation belts consist of one belt of protons, peaking at RE~2.5, and two electron belts, peaking at RE~1.5 and RE~5.5.
Protons are the most important component of the inner zone. In the equatorial plane, the protons extend out to about RE~2.8. For protons there is a systematic increase in the L value at which the proton flux peaks for lower energy protons (e.g., Ep = 10 MeV peaks at L~2.5 and Ep = 1 MeV peaks at L~3). The protons in the SAA provide the most intense radiation source in low earth orbit (LEO).
The peak electron fluxes in the outer zone exceed those in the inner zone by around one order of magnitude. Within the inner zone the electron flux is soft, i.e., it has a steep energy spectrum with few electrons above 1 MeV. The outer zone, which includes the synchronous orbits, shows much more variability. Several orders of magnitude increase occurs after large magnetic storms. The energetic electrons that are injected have a residence time of tens of days. At the higher altitudes, synchronous and above, a quasi-equilibrium condition exists for electrons in the 0.1 to 1 MeV range because of the frequent magnetic disturbances which occur there. At altitudes of L 5, magnetic substorms and storms inject quantities of hot plasma from the tail of the magnetosphere.
Particles originating from cosmic and solar sources are attenuated by the Earth's magnetosphere. The Earth's magnetic field provides natural shielding from both cosmic and solar particles depending primarily on the inclination and secondarily on the altitude. As inclination reaches auroral to polar regions, a satellite is outside the protection of the geomagnetic field lines. At polar orbits intense fluxes of energetic electrons, known as precipitating electrons, propagate down along magnetic field lines (and create the aurora). As altitude increases, the exposure to these particles gradually increases. During large solar events or magnetic storms, magnetic field lines are compressed allowing cosmic and solar particles to penetrate lower altitudes and inclinations. The galactic particle population peaks at solar minimum; whereas the solar particle levels peak at solar maximum.
The composition of galactic cosmic rays includes: 85% protons, 14% alpha particles, and 1% nuclides with Z>4, but heavy ions of Z>26 (iron) are rare. The cosmic rays include particles with energies from 0 to over 10 GeV. The bulk of these heavy ions are Hydrogen (proton), He (alpha), Carbon, and Oxygen with peak energies around 1 GeV. The cosmic rays have a very low flux compared to trapped particles, but have much higher energy; hence, which is difficult to shield against.
The solar cycle has an approximately eleven-year periodicity. Solar activity affects the level of each of the above space radiation sources.
|Solar Min||Solar Max|
|Cosmic Ray Population||peak level||low level|
Solar flare intensity varies over a wide range such that many systems never experience a large flare event. The heaviest doses occur at solar maximum every 10-12 yrs. The Earth's magnetic field provides natural shielding. Solar flares produce heavy ions and protons. The solar flares are about 90% protons with the remainder being alpha, heavy ions and electrons. Heavy ion fluxes from solar flares are generally less than galactic background but can be 4 times greater. The solar heavy ion spectrum is less energetic than galactic cosmic ray spectrum. The solar protons are energetic (10 MeV to 1 GeV). Protons from solar flares are important for earth orbiting and deep space programs. Protons from a single flare produce fluences up to ~2x1010 p/cm2. Shielding can be effective for protons of lower energies. The shielding density necessary to reduce the radiation dose from a large solar proton event is shown in the figure below.
Here we classify the effects from the natural space environment into four types:
The ionizing radiation of the space environment causes both total dose and single-event effects (SEEs). The physical mechanisms of total ionizing dose (TID) and SEEs are very different. Charged particle effects in the space environment are summarized below according to the particle source.
|Spacecraft Charging||Total Ionizing Dose||Displacement Damage||Single Event Effects|
|· Surface Charging from Plasma|
· Deep Dielectric from High Energy Electrons
|· Trapped Protons and Electrons|
· Solar Protons
|· Protons: both Trapped and Solar|
· Heavy Ions: both Galactic Cosmic Rays and Solar Events
The table below summaries the three main components of the space radiation environment along with their effects on CMOS devices.
|Summary of Space Radiation Environments
and their Effects on CMOS Devices
(from The NASA ASIC Guide: Assuring ASICS for Space, by Wall & Macdonald)
|Radiation Source||Particle Types||Primary Effects in Devices|
|Trapped radiation belts||Electrons||Ionization damage|
|Protons||Ionization damage; SEE in sensitive devices|
|Galactic cosmic rays||High-energy charged particles||Single-event effects (SEEs)|
|Solar flares||Electrons||Ionization damage|
|Protons||Ionization damage; SEE in sensitive devices|
|Lower energy/heavy-charged particles||SEE|