C H A P T E R
N ° 4
The Van Allen Belts
In 1958, astrophysicist James Van Allen and colleagues used data from NASA’s Explorer 1 Satellite – USA’s first successfully launched satellite and the first to carry science instruments - and discovered that the Earth’s magnetosphere (i.e., magnetic field) traps incoming space radiation as a part of its shielding mechanism and, thus, shields the planet. These trapped particles revealed to create two zones of energetic charged (ionizing) particles, consequently forming radiation belts around the planet. It is these belts that are known as the ‘Van Allen Belts’ and consist of an inner and outer radiation belt.
The inner radiation belt is the closest to Earth and has an altitude of ~6000-12.000 km above the planet. It contains ionizing radiation in the form of very energetic protons that are by-products of collisions between Galactic Cosmic Rays (GCRs) and atoms from Earth’s atmosphere. The outer radiation belt and, thus, the furthest from Earth is located at an altitude of ~25.000-45.000 km and consists of plasma. This plasma is a quasi-neutral (i.e., has an equal number of electrons and ions) ionized gas made up of high-energy particles - mostly protons and electrons - that moves under the influence of its interplanetary magnetic field, and originates from the Sun. As the particles are electrically charged and consists of an interplanetary magnetic field, they can be affected by electrical and magnetic fields. The interplanetary magnetic field originates from the Sun where the magnetic field is '“open“. When a space weather event occurs, the explosion of plasma from a solar flare and Coronal Mass Ejection (CME) drags magnetic field out from the Sun and into the space environment. This also occurs during the continuous leak of solar wind into the space environment. This interplanetary magnetic field is what carries the Solar Energetic Particles (SEPs).
A significant difference between the belts is the origin of the particles found within them. Where the inner belt is dominated by Galactic Cosmic Rays (GCRs) originating from supernova (i.e., when a star has reached the end of its life and explodes in a burst of light), the outer belt is dominated by Solar Energetic Particles (SEPs) originating from the Sun. As indicated, the Van Allen Belts are, thus, enormous belts consisting of energetic electrically charged particles.
Between Earth and the inner belt, and between the inner and outer belts, empty ‘slot’ regions can be found. These are caused by the Very Low Frequency (VLF) radio waves (~3-30 kHz) from Earth’s magnetosphere that can scatter the particles into certain L-shells. The scattered particles then get lost into the planet’s atmosphere, consequently leading to the slow loss of particles from the radiation belts. Earth’s magnetosphere is, thus, a key element in controlling dynamics of the Van Allen Belts.
Image Credit: Unknown: Illustration showing the Van Allen Belts.
The Van Allen Belts and Space Weather
During space weather events such as Geomagnetic Storms, there is an equal possibility for a rapid loss and a rapid increase of particles within the belts. This is because; when incoming particles originating from the Sun travels through the outer part of Earth’s magnetosphere to the inner radiation belt during solar storms, the electrons can gain a little bit of energy. This increase in the energy level of the particles enables them to enhance electromagnetic waves in the inner radiation belt as they travel through it. This consequently causes other particles within the inner belt to accelerate to very high energy levels. However, in addition to this process, this interaction can likewise scatter the particles within the inner belt, causing them to be lost from the belt. The outcome of whether there will be a rapid increase or decrease of electrons within the inner radiation belt, thus, depends on the balance between these processes.
Radiation belts like the Van Allen Belts are, however, not unique to Earth. Research have shown that similar radiation belts can be found on other plants with a global dipole magnetic field (i.e., a magnetic field surrounding a whole planet), such as Jupiter, Neptune, Uranus, and Saturn. In contrast, planets like Mars, Venus, and Mercury do not have these belts due to the lack of a similar type of magnetic field. Yet, depending on the planet, the radiation belts can look different and, therefore, cause different effects on its surrounding space environment. For example: Earth’s global dipole magnetic field is not truly dipolar in space or at the Earth’s surface. This causes the internal magnetic field to have a depression centered over the South Atlantic region, which is known as the ‘South Atlantic Anomaly’. This anomaly brings the radiation belts closer to Earth at this region, which for example exposes spacecrafts flying at only a few hundred kilometers to experience a substantial radiation dose. The radiation belts surrounding Saturn do, however, look completely different, as its rings blocks energetic particles.
Image Credit: NASA Goddard Space Flight Center/Scientific Visualization Studio: Artist’s concept of the Van Allen Belts with a cutaway section of the two zones of radiation surrounding Earth.
Why are the Van Allen Belts important?
The discovery of the radiation belts was a historical moment in space history and a ‘point of no return’ for the space sector, as it enabled a better understanding and awareness of the space environment surrounding Earth. This helped uncover the key to enable exploration of outer space. James Van Allen estimated that it would be possible to fly through the weaker regions of radiation to reach outer space and, thus, discovered a way to enable human exploration missions of our Solar System. Furthermore, the enablement of a better understanding of the radiation environment surrounding Earth, additionally led to a change in scientists and engineers understanding of the relation between satellites, their orbit, and the surrounding outer space environment.
Image Credit: Roen Kelly: Cartoon illustrating the Van Allen Belts and the location of the International Space Station (ISS).
Sources
James A. Van allen (2021): “Van Allen Belts”. The Gale Encyclopedia of Science. Edited by Katherine H. Nemeh and Jacqueline L. Longe. 6’th Edit, Vol, 8. pp. 4637-4640. Gale Academic OneFile. Link:link.gale.com/apps/doc/CX8124402562/AONE?u=anon~4c76c819&sid=googleScholar&xid=25c69d64
W. Li; M.K. Hudson (2019): ”Earth’s Van Allen Radiation Belts: From Discovery to the Van Allen Probes Era”.JGR Space Physics. Vol. 124, Issue 11. DOI: https://doi.org/10.1029/2018JA025940
NASA (n.d.): ”What are the Van Allen Bels and why do they matter?”. https://science.nasa.gov/biological-physical/stories/van-allen-belts/
ESA (2013): ”Earth’s Plasmasphere and the Van Allen Belts”. https://sci.esa.int/web/cluster/-/52831-earth-plasmasphere-and-the-van-allen-belts
T.P. Dachev et al. (2015): “Overview of the Liulin type instruments for space radiation measurement and their scientific results”. Life Sciences in Space Research. Vol 4. PP. 92-114. DOI: https://doi.org/10.1016/j.lssr.2015.01.005.
ESA (2020): “Types of orbits”. https://www.esa.int/Enabling_Support/Space_Transportation/Types_of_orbits#MEO
