Van Allen belt
The Van Allen Radiation Belt (named after James Van Allen ) is the Earth's radiation belt . It is a ring ( torus ) of high-energy charged particles in space that are captured by the earth's magnetic field . The magnetosphere acts as a protective shield for the earth, because it prevents such deadly particles from reaching the earth's inhabitants. Other planets are also surrounded by similar belts, but the Van Allen Belt only denotes the radiation belt around the earth.
The belt essentially consists of two radiation zones:
- the interior extends in low latitudes , i.e. H. near the equator, in an area of about 700 to 6,000 kilometers above the earth's surface and consists mainly of high-energy protons .
- the outer one is at an altitude of about 16,000 to 58,000 kilometers and contains mainly electrons .
So far it has been assumed that the particles of the Van Allen Belt originate mainly from the solar wind and cosmic rays . The latest investigations of the results of the “Van Allen A” and “Van Allen B” probes show, however, that the majority of the particles are created in the belt itself, as atoms are quasi torn apart by electromagnetic fields and electrons are released.
The charged cosmic particles in the Van Allen Belt are deflected by the Earth's magnetic field as a result of the Lorentz force , enclosed in a magnetic bottle and thus oscillate back and forth between the earth's poles with an oscillation period of about one second.
The presence of a radiation belt was suspected even before the space age. The theory was confirmed on January 31, 1958 by the mission of Explorer 1 and by the follow-up mission Explorer 3 , which were led by James Van Allen . Further explorer missions were able to map the particles.
The graphic shows the distribution of the particle density around the earth. High-energy protons (top picture) are concentrated in the inner radiation belt above 3,000 and 6,000 km above the earth's surface. High-energy electrons (below) strengthen the inner and form the outer radiation belt at an altitude of 25,000 km. The particle density of protons with an energy of more than 10 MeV and of electrons with more than 0.5 MeV is of the order of 10 6 particles / (cm² · s). The ionization radiation exposure from electrons to electrical components is 0.1 to 1 krad / h (1 to 10 Gy / h), and from protons (behind 1 cm of aluminum shielding) it is two orders of magnitude lower.
As part of the Pamela experiment in 2011, it was demonstrated that there is an accumulation of antimatter in the inner radiation belt of the magnetosphere . The detected antiprotons are probably formed when high-energy cosmic rays collide with the earth's atmosphere.
In September 2012, the Van Allen probes were able to detect, in addition to the two known radiation belts of the earth, a third one, clearly separated from the outer Van Allen belt by a gap. After the temporary radiation belt was measurable for about a month with constant intensity, it was dissolved by a strong solar flare . NASA researchers suspect that such temporary radiation belts are more common.
The equivalent dose of radiation in both main zones is behind 3 mm thick aluminum under extreme circumstances up to 200 mSv / h ( millisievert per hour) in the core area of the inner belt and up to 50 mSv / h in the core area of the outer belt. The standard values in the entire Van Allen Belt are 0.7–1.5 mSv per day ( effective dose ). This discrepancy can be explained on the one hand by the various measurement methods, but on the other hand by the dependence of the radiation on the strong fluctuations solar activity. This means that values that are 1000 times higher can sometimes be measured. On Earth, radiation from the inner Van Allen Belt in the area of the South Atlantic Anomaly can be clearly observed.
For comparison: in Europe the mean radiation dose at sea level is around 2 mSv / a ≈ 0.2 µSv / h.
Significance for space travel
Manned space travel
The intensity of the radiation within the Van Allen Belt can reach health-endangering values for a limited space and time . For this reason, the aspect of radiation protection must not be neglected in manned space missions in earth orbit. How great the load on the human organism is depends on solar activity , the nature of the spacecraft hull, the trajectory and the orbit speed or the duration of the mission.
The Russian space station MIR , the Skylab and the international space station ISS orbited the earth at an altitude of around 400 km (see also satellite orbit # types ). The Russian space program , the Mercury space program, and the US Gemini space program had their greatest distance from the earth below. The numerous space shuttle missions (see the list of space shuttle missions ) also circled the earth mostly at this smaller distance from the earth, only the Hubble space telescope was deployed at an altitude of around 550 km. The space missions of the manned space flight took place in a near-earth orbit , the Van Allen radiation belt was not reached (see also Radiation Exposure # Overview: Cosmic Radiation and Radiation Measurement on the ISS ).
Only the Apollo missions to the moon passed through the Van Allen radiation belt and beyond. The space travelers were also exposed to direct radiation from the sun and possible radioactivity from the lunar surface. Such radiation exposure on a manned Mars flight would be considerable.
- Richard B. Horne, (et al.): Wave acceleration of electrons in the Van Allen radiation belts. In: Nature. Volume 437, 2005, doi: 10.1038 / nature03939 , pp. 227-230.
- DN Baker, (et al.): An extreme distortion of the Van Allen belt arising from the 'Hallowe'en' solar storm in 2003. In: Nature. Volume 432, 2004, doi: 10.1038 / nature03116 , pp. 878-881.
- Van Allen belt graphic
- The Van Allen Belt
- Earth's Radiation Belts with Safe Zone Orbit NASA / Goddard Space Flight Center Scientific Visualization Studio
- Science - Electron Acceleration in the Heart of the Van Allen Radiation Belts by GD Reeves et. all. Science, July 25, 2013, accessed July 26, 2013 .
- Van Allen Belt: Researchers solve the mystery of earthly radiation rings spiegel.de, accessed on July 27, 2013
- Oscar Adriani, (et al.): The Discovery of Geomagnetically Trapped Cosmic-Ray Antiprotons. In: The Astrophysical Journal Letters. Volume 737, No. 2, 2011, doi: 10.1088 / 2041-8205 / 737/2 / L29 , pp. 1–5 ( preprint article at arXiv.org; 126 kB ).
- Detection in the particle flow: Antimatter in orbit - Article in Spiegel Online , August 6, 2011
- Particle Physics: Antiproton Ring Envelops the Earth - Article in Spektrum der Wissenschaft , August 8, 2011
- Third radiation belt discovered around the earth , Wissenschaft aktuell from March 1, 2013.
- Third radiation belt discovered on earth. - Article at astronews.de, March 4, 2013
- Van Allen Probes Mission: NASA probes find Earth's radiation belt. Spiegel Online , March 1, 2013, accessed March 10, 2013 .