What are the Earth’s radiation belts?
The discovery of the Earth’s radiation belts by Explorer 1 in 1958 was a major milestone in geophysics and astronomy, which marked the birth of magnetospheric physics. The Earth’s radiation belts are two donut-shaped regions encircling the earth, filled with high-energy particles, mostly electrons and ions trapped by Earth’s magnetic field. The inner belt mainly consists of protons, while electrons primarily dominate the outer belt.
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| Radiation belts with 2 probe satellites flying through them. Image : NASA |
Why do we care about the radiation belts?
These high-energy electrons, also known as “killer electrons”, can cause problems for satellite and astronauts as they have strong radiation. The radiation belts exhibit highly dynamic variations, where multiple physical mechanisms can contribute to alter the topology of the radiation belts and the charged particle fluxes in it. Better understanding of what drives these variations leads to better protection of our satellite and astronauts in space.
How do we study the dynamics of the radiation belts?
Years of efforts have been made in unlocking the mysteries of radiation belts. There are two basic ways to study the dynamics of the radiation belts: observations and simulations. Observations from both satellites in space and ground-based facilities can help us understand the wave activity and charged particle evolution, such as Van Allen Probes, Arase, ground-based magnetometer, etc. Furthermore, the conjunction analysis based on LEO and MEO satellite can link the observed waves with the simultaneously occurring particle precipitations. There are many satellites that help us understand what is going on near Earth, including Cluster, THEMIS, Van Allen Probes, MMS, Arase, etc., which provide a giant database of observation. However, there is a limitation of the observation from satellites as they can only provide limited spatial measurements during each time stamp; this is where simulations come in to help us analyze the physical mechanism that dominantly leads to the observed results. One of the most important advantage of the simulation method is that we can choose to turn on or off a specific factor to see whether this physical process contribute to the simulated results, which can help to interpret the observed variations in the radiation belts. Therefore, we can quantify the role of each mechanism in altering the radiation belt dynamics.
What is the current understanding of the radiation belts?
Owing to continuous accumulations of the high-resolution wave and particle measurements from multiple satellite missions in geospace and the development of the state-of-art modeling during the Van Allen Probe era, significant advances have been made in understanding the dynamic size and structure and their underlying physical mechanisms in the radiation belts. It has been well acknowledged that the size and shape of the radiation belt significantly depend on the solar activity as well as wave-particle interactions in the radiation belts. When the magnetopause is compressed by strong solar wind, the particles can be lost outside the magnetopause, which creates a negative radial gradient in electron phase space density and further leads to outward radial diffusion. Electron flux enhancements can occur by electron injections during substorm or enhanced convection.
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| Magnetospheric waves. Image : Thorne et al. 2021 |
There are plenty of plasma waves inside the Earth’s magnetosphere, including those naturally occurring waves: ultra-low-frequency (ULF) waves, plasmaspheric hiss, lightening-generated whistlers (LGW), magnetosonic waves, whistler-mode chorus, electromagnetic ion cyclotron (EMIC) waves, and electrostatic electron cyclotron harmonic (ECH) waves. Besides, the ground-based very-low-frequency (VLF) transmitters radiate emissions, which propagate mostly within the Earth-ionosphere waveguide, penetrate through the imperfectly reflecting ionosphere and leak a portion of their power into the magnetosphere. All these plasma wave modes play an important role in changing the radiation belt dynamics by wave-particle interactions. For example, ULF waves dominantly drive inward radial diffusion, causing energization of particles as they are transported inwards from the source region at high L-shell. Plasmaspheric hiss and EMIC waves can efficiently lead to electron precipitation. Chorus waves provide an efficient mechanism for energy transfer through the wave generation by injected lower-energy electrons and the relativistic electron acceleration by the generated waves. In particular, recent study has proved that VLF transmitter waves can efficiently scatter energetic electrons in the near-Earth space, leading to the bifurcation of the inner energetic electron belts.
Although our understanding of the dynamics of the Earth’s radiation belts has been unprecedently advanced, there are still lots of challenging questions waiting for us to solve. For instance, how to quantify the relative contribution of magnetopause shadowing effects and local wave-particle interactions by EMIC waves to electron dropout? How to incorporate the non-diffusive terms in the current diffusive radiation belt model? When are the nonlinear effects of wave-particle interactions important compared to the quasi-linear results? Does the similar wave-particle interaction also occur on other magnetized planets?
So, let’s get started and prepared to be fascinated by the beauty of the Earth’s radiation belt!
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