Radiation Belts and Trapped Particles

The Van Allen Radiation Belts were discovered in 1958 during the early days of the Space Age. There are two of these donut-shaped clouds that gird the equatorial regions of the Earth and are roughly symmetric with its equatorial plane. The inner belts extend from approximately 1,000 to 5,000 km and contain very high-energy protons trapped in the Earth's magnetic field. The outer belt extends from approximately 16,000 to 24,000 km and consists mostly of high-energy electrons. Geosynchronous satellites orbit the Earth just outside the limits of the outer belt, and human space activity is confined to the zone within the inner edge of the inner belt.

Within Earth's magnetic field, electrically-charged particles tend to bounce from pole to pole and drift east or west. The Van Allen belt particles do likewise. The particles that make up the Van Allen Belts bounce along the north and south-directed magnetic lines of force to which they are trapped like water flowing in a pipe. At the same time, there is a slow drift of these particles to the west if they are positively charged or eastwards if they are negatively charged. An electron makes one complete orbit towards the east in about 50 minutes. This is a much faster orbit rate than for satellites because electromagnetic forces are much stronger than gravitational ones.

representation of the trajectory of trapped particles

Schematic represenation of the trajectory of trapped particles

Careful satellite studies over the last 50 years show that there are actually two kinds of already familiar particles that make up the Belts: electrons and protons. The individual particles carry a lot of energy, and it is convenient to talk in terms of their energies when describing the Belt particles. This is where the story gets a bit interesting. There are two electron belts and one proton belt.

The proton belt is located from about 500 kilometers above Earth's surface and extends to 13,000 km. This Inner Belt contains protons with energies greater than 10 million volts. Scientists currently think that these protons are trapped cosmic ray particles from outside the solar system, or from the Sun itself possibly during severe solar flares.

The low-energy electron belt actually overlaps the volumes of space where the proton belt is located in the Inner Belt. The electrons carry between 1 - 5 million volts of energy, on average.

The high-energy electron belt is located further out than the two overlapping inner belts. Electrons in this Outer Belt carry between 10 to 100 million volts of energy, on average.

Inner and outer Van Allen radiation belts, drawing (Credit: Michael Palomino)

Although we have learned a lot about the Belt particles in the last 50 years, there are still some very big questions about them that, as yet, have no answers. Space physicists don't completely understand where they come from, or how their energies can be so "astronomical" compared to either the plasmasphere particles or Ring Current particles. Typical "Belt" particles carry energies between 1 and 100 million volts. The rest of the particles that we can encounter near the earth barely have energies higher than 200,000 volts.

There is also another aspect to these Belts of particular interest to manned space flight and satellites. Because the magnetic field of Earth doesn't exactly line up with the Earth's rotation axis, the Belts are actually tilted a bit. Their influence is stronger in equatorial regions over South America. This also means that, because the Belts follow the Earth's magnetic field not its geographic shape, they are closer to the ground over South America and the South Atlantic. This means that if you were in a Space Shuttle, Space Station or operating a satellite as it passes over the South Atlantic, you will be closer to the Belts and receive a larger than average dose of radiation from them as their particles penetrate your spacecraft or satellite skin. This region is called the South Atlantic Anomaly. It affects astronaut radiation dosages as well as data and signal transmission quality from all spacecraft passing through this continent-sized region.

Space Radiation and Astronauts Exposure

Should astronomers and astronauts be concerned about radiation belts? Can the very energetic particles penetrate the shielding of a spacecraft or the spacesuit of an astronaut?

Much like extreme weather phenomena in Earth's atmosphere can seriously affect humans and human structures on the ground, space weather is extremely unpredictable and can affect or even be fatal in space. Solar storms and accompanying geomagnetic storms involve fast moving particles and high energy radiation that affect and can significantly alter the flow of energy and matter through interplanetary space. Protons and electrons speed up almost to the speed of light and these together with the produced X-ray radiation can disrupt short-wave communication on Earth.

A scientific satellite contains very sensitive circuits that control its movement or the pointing of various science instruments, for instance. Should these be affected by severe space weather conditions, the satellite will become uncontrollable and most probably unusable, resulting to a serious loss of investment (in terms of money and scientific effort). Obviously science operators controlling the satellite on Earth can try to recover the damage but this is not always possible. Unfortunately this is a rather common situation resulting to a loss of billions of dollars/euros in recent years.

Space weather effects (Image Credit: NASA)

Besides metallic structures, the human body in space is subject to the penetration of cosmic radiation, in particular high-energy rays. If the astronauts happen to be outside their spacecraft (e.g. for a spacewalk), the blast of radiation from a severe solar storm could be deadly. Even on a normal working day at the International Space Station astronauts may receive in a few days twice as much radiation compared to the yearly dosage of radiation that humans receive on Earth.

How can we protect our satellites and astronauts from the harmful effects related to Earth's radiation belts and how predictable are they? Extreme space weather phenomena follow the 11-year sunspot cycle, so we expect more solar flares and explosions during solar maxima. Extreme space weather phenomena start with intense auroras in the arctic regions and, as the highly energetic particles of the solar wind interact with the Earth's magnetic field, satellites will start experiencing problems. Solar «coronal mass ejections» also release fast moving plasma which usually reaches the Earth within a few days.