Space sometimes looks as twilight zone to us, but we do not have to go that far to find one. The radiation belts are regions of high-energy particles, mainly protons and electrons, held captive by the magnetic influence of the Earth. They have two main sources. A small but very intense "inner belt" (some call it "The Van Allen Belt" because it was discovered in 1958 by James Van Allen) is trapped within 6500 km or so of the Earth's surface. It consists mainly a high-energy protons (10-50 MeV) and is a by-product of the cosmic radiation, a thin drizzle of very fast protons and nuclei which fill all our galaxy. In addition there exist electrons and protons (and also oxygen particles from the upper atmosphere) given moderate energies (say 1-100 keV) by processes inside the domain of the Earth's magnetic field. Some of these electrons produce the polar aurora ("northern lights") when they hit the upper atmosphere, but many get trapped, and among those, protons and positive particles have most of the energy .
Another point of particular interest to high-energy astrophysics is the South Atlantic Anomaly (SAA).We all know that the Earth's magnetic axis is not the same as its rotational axis. As the Earth's molten, ferromagnetic liquid core churns, it generates a magnetic field, and the north-south axis of this field is tilted about 16° from the rotational axis. The north magnetic pole is in the north Canadian islands, but it moves around a lot, and it's currently headed northwest at about 64 km per year. Here's the part that many people don't know. While the north magnetic pole is about 7° from the north rotational pole, the south magnetic pole is about 25° from the south rotational pole. A line drawn from the north magnetic pole to the south does not pass through the center of the Earth. Our magnetic field is torus shaped, like a giant donut around the earth. But it's not only tilted, it's also pulled to one side, such that one inner surface of the donut is more squished up against the side of the earth than the other. It's this offset that causes the South Atlantic Anomaly to be at just one spot on the Earth. This is a region of very high particle flux about 250 km above the Atlantic Ocean off the coast of Brazil and is a result of the fact that the Earth's rotational and magnetic axes are not aligned. The particle flux is so high in this region that often the detectors on our satellites must be shut off (or at least placed in a "safe" mode) to protect them from the radiation. Below is a map of the SAA at an altitude of around 560 km. The map was produced ROSAT by monitoring the presence of charged particles. The dark red area shows the extent of the SAA. The green to yellow to orange areas show Earth's particle belts.
The South Atlantic Anomaly comes about because the Earth's field is not completely symmetric. If we were to represent it by a compact magnet (which reproduces the main effect, not the local wiggles), that magnet would not be at the center of the Earth but a few hundred km away, in the direction away from the "anomaly". Thus the anomaly is the region most distant from the "source magnet" and its magnetic field (at any given height) is thus relatively weak. The reason trapped particles don't reach the atmosphere is that they are repelled (sort of) by strong magnetic fields, and the weak field in the anomaly allows them to reach further down than elsewhere.
The shape of this anomaly changes over time. Since its initial discovery in 1958] the southern limits of the SAA have remained roughly constant while a long-term expansion has been measured to the northwest, the north, the northeast, and the east. Additionally, the shape and particle density of the anomaly varies on a diurnal basis, with greatest particle density corresponding roughly to local noon. At an altitude of approximately 500 km, it spans from -50° to 0° geographic latitude and from -90° to +40° longitude. The highest intensity portion of the SAA drifts to the west at a speed of about 0.3 degrees per year. The drift rate is very close to the rotation differential between the Earth's core and its surface, estimated to be between 0.3 and 0.5 degrees per year. Current literature suggests that a slow weakening of the geomagnetic field is one of several causes for the changes in the borders since its discovery. As the geomagnetic field continues to weaken, the inner Van Allen belt gets closer to the Earth, with a commensurate enlargement of the anomaly at given altitudes.
Now, what are the effects of this monster? The South Atlantic Anomaly is of great significance to astronomical satellites and other spacecraft that orbit the Earth at several hundred kilometers altitude; these orbits take satellites through the anomaly periodically, exposing them to several minutes of strong radiation, caused by the trapped protons in the inner Van Allen belt. The ISS, orbiting with an inclination of 51.6°, requires extra shielding to deal with this problem. The Hubble Space Telescope does not take observations while passing through this anomaly. Astronauts are also affected by this region which is said to be the cause of peculiar 'shooting stars' (phosphenes) seen in the visual field of astronauts. One of the current ISS astrounaouts, Don Pettit, describes this effects in his blog. The eye retina is an amazing structure - it’s more impressive than film or a CCD camera chip, and it reacts to more than just light. It also reacts to cosmic rays, which are plentiful in space. When a cosmic ray happens to pass through the retina it causes the rods and cones to fire, and you perceive a flash of light that is really not there. The triggered cells are localized around the spot where the cosmic ray passes, so the flash has some structure. A perpendicular ray appears as a fuzzy dot. A ray at an angle appears as a segmented line. Sometimes the tracks have side branches, giving the impression of an electric spark. The rate or frequency at which these flashes are seen varies with orbital position, Don continues to say. When passing through anomaly, where the flux of cosmic rays is 10 to 100 times greater than the rest of the orbital path, eye flashes will increase from one or two every 10 minutes to several per minute.
Passing through the South Atlantic Anomaly is thought to be the reason for the early failures of the Globalstar network's satellites. The PAMELA experiment, while passing through this anomaly, detected antiproton levels that were orders of magnitude higher than those expected from normal particle decay. This suggests the Van Allen belt confines antiparticles produced by the interaction of the Earth's upper atmosphere with cosmic rays. NASA has reported that modern laptops have crashed when the space shuttle flights passed through the anomaly and Don has confirmed that since in his blog adding cameras suffer too. During the Apollo missions, astronauts saw these flashes after their eyes had become dark-adapted. When it was dark, they reported a flash every 2.9 minutes on average. Only one Apollo crew member involved in the experiments did not report seeing the phenomenon, Apollo 16′s Command Module Pilot Ken Mattingly, who stated that he had poor night vision.
There are experiments on board the ISS to monitor how much radiation the crew is receiving. One experiment is the Phantom Torso, a mummy-looking mock-up of the human body which determines the distribution of radiation doses inside the human body at various tissues and organs.
There’s also the Alpha Magnetic Spectrometer experiment, a particle physics experiment module that is mounted on the ISS. It is designed to search for various types of unusual matter by measuring cosmic rays, and hopefully will also tell us more about the origins of both those crazy flashes seen in space, and also the origins of the Universe.
We know that the South Atlantic Anomaly is hazardous to electronic equipment and to humans who spend time inside it. We know that it dips down close to the Earth. Although the Anomaly is a dangerous place, its edges are pretty well defined. The closest it ever gets to the Earth's surface is about 200 km, and at that height it's very small. Your commercial airplane won't reach that altitude for sure. And with everything we know today about it, it is hardly a twilight zone either.
Credits: NASA, Wikpedia, Don Pettit, Astrobiology Magazine, Brian Dunning