Development of the South Atlantic Anomaly // based on data collected by the Swarm Satellite Constellation from 2014-2020. Credit ESA:
South Atlantic Anomaly //SAA
The anomaly is a a weakening of the Earth’s magnetic field in the area of the southern Atlantic and extending west over South America. This weakening allows the lower van Allen belt to dip lower than anywhere else between 60°N and 60°S. In the polar regions, the lower van Allen belt reaches the surface of the Earth, following the lines of the magnetic flux.
The South Atlantic Anomaly (SSA) is created by a rising of the Earth’s iron/nickel core beneath the continent of Africa. This creates a crest in the flux, and behind it, in the wake of the Earth’s rotation, is the trough of the SSA. The trough is conical in shape; wider east to west than it is north to south, with the weakest point being about 300 miles/480 kilometers south of Rio de Janeiro (30°S 40°W).
Because of the weakness in the flux, the lower Van Allen belt dips lower into the cone. Near the center of the anomaly, the Van Allen belt is both lower and has a softer gradient edge. While technically the Van Allen belt touches the surface of the Earth in the core region, its ionic charge there is negligible, with the flux measured in the nanotesla range. It has no notable effect at the surface.
The eastern edge of the anomaly starts at an altitude of 280 miles/450 kilometers in southern Africa (35°S 40°E) and extends to the Pacific coast of South America (25°S 80°W). The widest from north to south lies at 40°W, from the equator to 55°S. Because the uprising of the Earth’s core is a dynamic process in geological time, the anomaly drifts westward at a creeping pace.
Low Earth Orbit
Low Earth Orbits (LEOs) are calculated to stay under the lower van Allen belt. Early space exploration satellites had a high failure rate when they were placed within the belt, caused mainly by proton collisions with electronic equipment. As understanding of the failure process developed, electronics were better shielded from the radiation that was generated by the proton collisions.
As man began to spend more time in orbit, the issues of radiation exposure became more critical, and the art of lightweight and effective shielding advanced to better protect those venturing into orbit. One of the changes was limiting manned orbits to altitudes below the lower Van Allen to reduce levels of radiation generated by proton collisions. Hence manned orbits remain within an envelope of 54°N to 54°S at 260 miles/420 kilometers.
This envelope has one area where proton collision is unavoidable; the area of the SAA. To avoid it fully, the orbit would be too low to maintain properly. Anything in LEO will undergo two or three orbits daily that pass through the SAA. Peak orbit is the orbit that passes closest to the low point of the anomaly where the proton density and collision radiation is highest (core).
Protons are among the heavier atomic particles, and at non-relativistic speeds, a collision with one is non-elastic. This means that a proton striking an object is deposited on the surface of the object. At relativistic velocity (such as a proton beam), the proton is elastic, and it would penetrate the object, slowing the collision more slowly, and extending the energy release at lower levels over a longer period of time. But orbital velocity is not sufficient for penetration.
Under the principle of conservation of mass and energy, a change in velocity involves a release of energy, especially for an atomic particle with an ionic charge. Protons in the lower van Allen are positively charged and are measured in electron volts (eV); the range usually runs from 250 keV to 60 MeV. Most of this comes from the solar wind reacting with the Earth’s magnetic field, and the protons act as collectors for this excess electrical energy. It is this charge reacting with the Earth’s magnetic field that keeps the van Allen belt in place, and the solar winds propel the currents within the belt’s layers. The van Allen in the lowest part of the SAA is almost a whirlwind, similar to a tornado’s vectors.
The collision is defined by four items. First is the relative velocity of the collision. Second is the electrical charge of the proton. Third is the molecular structure of the item being collided with. Fourth is temperature of the item being collided with.
In orbit, velocity doesn’t vary much, 28,000 km/h – 17,000 mph is pretty much standard. It is sufficient for a collision to result in a release of energy. What it hits depends on where on the object it hits. Head on perpendicular is a harder collision than one that glances off the side. What molecule is the other side of the elastic collision made of? If it’s proton to metal, no elasticity. And how charged is the proton? The less energy transfer per unit time will create a lower level of energy release. The more energy transfer, the higher the level of release.
The vast majority of the radiation release is electromagnetic force (emf). This is the energy that includes static, batteries, wall sockets, radios, light, and x-rays. The softest transfer is static electricity. The station is equipped to keep it grounded, preventing a plasma buildup. As the energy transfer of the collision increases, the frequency increases, from radio, through microwave and x-ray, and to gamma at the top end. Gamma (soft) is usually produced from a hard collision of 25 MeV or more.
Cameras and Visual FX
Cameras are shielded to survive orbit for as long as possible. Nothing gets through the shielding except that which can’t be stopped. Gamma radiation produces the ultimate collision elasticity.
The camera’s shielding absorbs all the lower forms of emf as static electricity. The vast majority of collisions become plasma residing on the surface. Only the highest-power collisions can reach the camera plates (sensors) by a route other than through the lens.
When gamma radiation passes through the shielding and the sensor, it is weakened by the collisions, and it leaves a residual static electrical charge on the sensor in its wake. The sensor responds to this latent charge as it would to visible light. A flash in the image sequence.
Two types of cameras are used by NASA, those which use a Bayer sensor, and those that use a multi-sensor/prism. With a Bayer, because the camera uses a single sensor with micro color filtration, the visual flashes are white.
With the CCD cameras, a prism breaks the light into three spectrum images, red, green, and blue, each targeted to separate sensors. When the gamma radiation strikes the red sensor, the flash is red; the green sensor produces a green flash; the blue sensor, a blue flash.
Notes of interest:
Gamma radiation generated in proton collisions is “soft” gamma. It’s waveform is uniform. This is not the same product as hard gamma, which is generated in nuclear decay. Hard gamma has a signature spike in the waveform that identifies the isotope decaying. That spike is considerably more harmful to living tissue than the base waveform. This is not to say that gamma radiation of any sort is considered healthy. Soft gamma is just not quite as nasty.