Dielectica traverses through the literature on this topic– and summarizes as they appear.
Correspondence prepared by: Suman Chakraborty, Physical Research Laboratory, Ahmadabad, Gujrat, India, email: suman.chakrabarty37@gmail.com (05:10:2020, 18:30 IST)
Key Words: Magnetosphere, Radiation belts, Space environment, Radiation Belt Storm Probes
Ahmedabad, India: Any planet that has a magnetic field is capable of deflecting charged particles travelling towards it and forms a protective cavity around itself called the magnetosphere. Besides shielding the planet from charged particles, the magnetosphere is also capable of trapping charged particles along magnetic field lines, forming a region called the radiation belts. The Earth’s radiation belts were discovered in 1958 by James Van Allen, a physicist at the University of Iowa using a simple cosmic ray experiment consisting of a Geiger counter and a tape recorder on the first US satellite, Explorer 1. During the flight, radiation levels seemed to increase and then suddenly drop to zero and then again to increase followed by a further sudden drop to zero. What the team soon realized was that regions appearing as zero were really off the scale! These high-radiation regions were mapped and identified as the radiation belts which are now often dubbed as the “Van Allen Radiation Belts” after the name of James Van Allen [1].
So, what are the radiation belts?
The Earth’s radiation belts are two giant donut-shaped regions of magnetically trapped, highly energetic charged particles with the Earth sitting at the center of the “donut hole”. It comprises of an inner belt and an outer belt with a “slot region” separating the two belts [1,2].
The figure shows the Van Allen Radiation Belts with the Earth at its center. We can clearly see a two-belt structure with a region of depleted charged particles in between the two belts. Although numerous studies have been conducted for the past several decades, the most prominent outcomes came since the launch of the Van Allen Probes in 2012. The Van Allen Probes (VAP; 2012 – 2019), initially known as the Radiation Belt Storm Probes (RBSP), were two identical spacecrafts that were deployed to study the Van Allen Radiation Belts. NASA conducted the VAP mission as part of the “Living with a Star” program. It was initially planned for 2 years, but eventually operated for 7 years. The probes showed that the radiation belts are even more complex than previously imagined and the shape of the belts depends on what particles are being studied. Such observations are more important today as our society now relies on more than 800 satellites operating in the radiation belts for communication and navigation. Dr. David Sibeck, VAP mission scientist at NASA’s Goddard Space Flight Center in Maryland, in a 2018 statement said, “Our current technology is ever more susceptible to these accelerated particles because even a single hit from a particle can upset our even smaller instruments and electronics. As technology advances, it’s actually becoming even more pressing to understand and predict our space environment“[1].
Part of interest to study the radiation belts came from its location. It is known that the radiation belts can expand and contract depending on solar activity. During intense solar activity, the belt can expand and even extend over the orbit of the International Space Station (ISS; Orbit height: 408 km). The ISS has been permanently inhabited since 2000, with typical astronauts staying there for six months at a time. In 2015 – 2016, NASA astronaut Scott Kelly and Russian cosmonaut Mikhail Kornienko remained there for almost a year. As astronauts stay in orbit for longer duration, their radiation exposure may also increase, leading to concerns about long-term habitation for astronauts in space. The knowledge of space environment is also necessary to design future spacecrafts in order to prevent them from short out due to an electrical overload that may lead to disruption of communication. All these aspects make the radiation belts a hot topic of research for the space science community. In an August 2016 NASA statement, Dr. Sibeck said, “We study radiation belts because they pose a hazard to spacecraft and astronauts. If you knew how bad the radiation could get, you would build a better spacecraft to accommodate that” [1].
Sources:
[1] https://www.space.com/33948-van-allen-radiation-belts.html
[2] https://www.nasa.gov/mission_pages/rbsp/mission/fun-facts.html