Written by: Megan Wetegrove
In this post, planetary radiation belts are introduced, with special emphasis on Earth’s planetary radiation belts – the Van Allen Belts.
Requirements for Planetary Radiation Belts
In order for a radiation belt to form around a planet, the planet must have a magnetic field. This magnetic field is able to deflect charged particles that are traveling in the planet’s direction. The area around the planet in which the magnetic field is able to control particles is called the magnetosphere. When a magnetosphere is present around a planet, most space radiation directed at the planet is deflected. Below is an image of Earth’s magnetic field deflecting charged particles from the Sun.
Image Source: http://en.wikipedia.org/wiki/File:Magnetosphere_rendition.jpg
A planet’s magnetic field is able to offer the planet a great amount of protection from charged particles. At the same time, the presence of a magnetic field gives rise to planetary radiation belts on account of the field trapping a number of charged particles along magnetic field lines. Consequently, these particles form donut-shaped ‘belts’ around the planet. Currently, Mercury, Earth, Jupiter, Saturn, Uranus, and Neptune are surrounded by trapped radiation. Venus and Mars do not have magnetic fields; thus, these planets are not able to trap charged particles.
The Van Allen Belts
The Van Allen belts are two planetary radiation belts surrounding Earth. The belts are donut-shaped crescents that do not extend as far as Earth’s poles. The inner belt, extending from approximately 400 km to 18,400 km (measured from the equator) consists mainly of electrons with maximum energy of 10 MeV. Electron fluxes with energies greater than 2 MeV peak in the inner belt at approximately 2500 km above Earth’s surface (also measured from the equator). The outer region consists of trapped protons and heavy ions with maximum energy of 100 MeV and of electrons with maximum energy of 10 MeV. Proton fluxes with energies greater than 30 MeV peak at approximately 2300 km above Earth’s surface (also measured from the equator), and electron fluxes in this outer belt with energies greater than 2 MeV peak at approximately 20,000 km above Earth’s surface. In the image below, the Van Allen belts are depicted along with the relative positions of the International Space Station, GPS satellites, the Van Allen probes, and SDO. (Clicking on the picture will enlarge it.)
Also shown in the image is a gap between the Van Allen belts. This gap is sometimes referred to as the slot region. Despite the misleading term, the slot region does contain a trapped particle population, although the population is not as dense and highly populated as the inner and outer belts. Due to changes in the magnetosphere resulting from solar and magnetic storms, the slot region population can increase by several orders of magnitude. It is important to note that a third radiation belt surrounded Earth at one time. News of the third belt was released in February 2013 after being discovered by NASA’s Van Allen probes. NASA was able to study the belt for four weeks before a shock wave from the Sun wiped the belt out.
Earth’s magnetic axis differs from Earth’s spin axis by approximately 11 degrees, which leads to a displacement of the magnetic center from the center of Earth. Consequently, a dip in the inner belt down to approximately 200 km exists over the South Atlantic Ocean. The South Atlantic Anomaly (SAA), as this dip is called, is responsible for a large amount of radiation exposure during spaceflight and for the International Space Station (ISS). However, the flux levels in the SAA are much lower than those at higher altitudes. Therefore, for space missions outside of the Van Allen belts, radiation exposure to astronauts and electronic equipment due to increased flux levels must be taken into account. For those wondering, with the exception of the Apollo missions, no manned space mission has traveled beyond the Van Allen belts.
In this image, the SAA can be seen. Notice how the cross-sectional view of the inner belt on the left side of Earth – the part of Earth where South America is located – is closer to Earth than the right side cross-sectional view.
Image Source: http://history.nasa.gov/EP-177/i3-7.jpg
Jupiter’s Planetary Radiation Belts
Jupiter’s magnetic field is the strongest magnetic field in our solar system (dipole moment of 1.55 x 1020 T·m3, approximately 20,000 times larger than Earth’s dipole moment). Jupiter also has the largest magnetosphere of the planets in our solar system, with a distance of 3 million km for the side of the magnetosphere facing the Sun and a distance of 650 million km for the tail end of the magnetosphere. In fact, its magnetosphere is the largest object in the solar system. Interestingly, the Sun has a diameter of approximately 1.3 million km. This means that the Sun could comfortably fit inside Jupiter’s magnetosphere! Also interesting is the fact that the side of the magnetosphere not facing the Sun extends past the orbit of Saturn.
As a result of the strong magnetic field, Jupiter’s radiation belts are much more dangerous than Earth’s belts. NASA has estimated that if astronauts were to ever venture as close to Jupiter as did the Voyager 1 spacecraft, the astronauts would receive a dose 1,000 times greater than the lethal dose. It is unlikely that manned spaceflight to Jupiter will take place in the near future; nevertheless, when planning for a manned space mission to or past Jupiter, mission planners will need to take Jupiter’s intense radiation field into consideration. Unmanned space missions to Jupiter and beyond have already had great success. Undoubtedly, space radiation could have negatively impacted these missions, but these spacecraft were designed to withstand harsh radiation environments.
Other Planetary Radiation Belts in the Solar System
Saturn, Uranus, and Neptune each have magnetic fields with strengths comparable to Earth’s magnetic field strength (dipole moment of 7.91 x 1015 T·m3). The data in the table below shows that the magnetic field strengths for Saturn, Uranus, and Neptune are indeed comparable to Earth, but also that these fields are somewhat stronger than Earth’s magnetic field. Consequently, the planetary radiation belts surrounding these planets are not expected to be much larger than Earth’s radiation belts. Thus, these outer planet radiation belts do not pose as great of a threat to spacecraft and astronauts as does Jupiter’s radiation belts.
Table 1. Planetary Magnetic Field Strengths
|Magnetic Field Strength Relative to Earth
Data Source: The Cambridge Guide to the Solar System by Kenneth R. Lang
Mercury, on the other hand, has a very weak magnetic field. Accordingly, the trapped particle population is estimated to be very low in comparison to Earth’s trapped particle population.
 J.L. Barth, C.S. Dyer, and E.G. Stassinopoulos, “Space, Atmospheric, and Terrestrial Radiation Environments,” IEEE Trans. Nucl. Sci., vol. 50, no. 3, pp. 466-482, June 2003.
 C.T. Russell and J.G. Luhmann, “Jupiter: Magnetic Field and Magnetosphere,” Encyclopedia of Planetary Sciences, pp. 372-373, Chapman and Hall, New York, 1997.
 Kenneth R. Lang, “The Cambridge Guide to the Solar System,” Cambridge University Press, 2nd ed., March 2011.