Van Allen radiation belt

The Van Allen radiation belts (named after James Van Allen) is a ring ( torus ) of energetic charged particles that are trapped by the magnetic field of the earth. Although other planets are surrounded by similar belts, the term Van Allen belts refers specifically to the radiation belts around the Earth.

The belt consists substantially of two radiation zones:

• The inner extending into low latitudes, ie in the vicinity of the equator is within a range of about 700 to 6,000 miles above the earth's surface and is composed mainly of high energy protons.
• The exterior is in about 15,000 to 25,000 kilometers altitude and contains mainly electrons.

Previously it was assumed that the particles of the Van Allen belt mainly come from the solar wind and cosmic radiation. Recent studies of the results of the probes " Van Allen A" and "Van Allen B", however, show that a majority of the particles occurs even in the belt, there by less torn atoms of electromagnetic fields and electrons are removed.

The charged cosmic particles are deflected by the Earth's magnetic field due to the Lorentz force in the Van Allen belts, trapped in a magnetic bottle and swing as between the Earth's poles with an oscillation period of about one second back and forth.

If the belt is overloaded, the particles strike the Earth's upper atmosphere and encourage them to fluorescence, causing the aurora is created.

Proof

The presence of a radiation belt was suspected before the space age. The theory was confirmed on 31 January 1958 under the mission of Explorer 1 and the follow-on mission Explorer 3, which were directed by James Van Allen. More Explorer missions could map the particles.

The graph illustrates the distribution of the particle density around the Earth. High-energy protons ( upper panel) are concentrated in the inner radiation belt above 3000 and 6000 km of the Earth's surface. High-energy electrons ( below) reinforce the inner and outer radiation belts form the 25,000 km altitude. The particle density of the protons to an energy greater than 10 MeV and greater than 0.5 MeV electrons is of the order of 106 particles / (cm ² · sec). The ionization radiation by electrons at electrical components is 0.1 to 1 krad / hr ( 1 to 10 Gy / h), by protons ( 1 cm behind aluminum shielding) two orders of magnitude lower.

As part of the Pamela experiment in 2011 demonstrated that in the inner radiation belt of the magnetosphere an accumulation of antimatter exists. The detected antiprotons are probably formed in the collision of high-energy cosmic rays with the Earth's atmosphere.

In March 2013, a third, previously unknown radiation belt was discovered by the two Van Allen radiation probes.

In September 2012, the twin probes Van Allen Probes could also prove a temporary radiation belt in addition to the two known radiation belts of the earth. This splits off to the latest findings from the outer belt. After the temporary radiation belt was about a month measurable with constant intensity, it was dissolved by a strong solar flare. NASA researchers suggest that such temporary radiation belt more common.

Radiation exposure

The equivalent dose of radiation of both main zones is behind 3mm thick aluminum under extreme conditions up to 200 mSv / h ( millisieverts per hour ) in the core area of ​​the inner belt and up to 50 mSv / h in the core area of the outer belt. As standard values ​​are valid throughout the Van Allen belts 0.7-1.5 mSv per day ( effective dose), this discrepancy can be explained both by the different measurement methods, on the other, by the dependence of the radiation from the strong fluctuations of solar activity. This 1000-fold higher values ​​can sometimes be measured. On Earth, the radiation from the inner Van Allen Belt is clearly observed in the South Atlantic anomaly.

For comparison: In Europe, the mean radiation dose at sea level is about 2 mSv / a ≈ 0.2 mSv / h

Importance for space

Human Spaceflight

The intensity of the radiation within the Van Allen belts can achieve spatially and temporally restricted hazardous values ​​. Therefore, the aspect of radiation protection in manned space missions in Earth orbit must not be neglected. How big is the burden on the human organism depends on the solar activity, the nature of the spacecraft hull, the trajectory and the line speed or the mission duration.

Meters

Astronomical instruments such as the X-ray telescope Chandra can provide meaningful data only outside the radiation belt and must be brought to correspondingly high orbits.