Unlocking the mysteries of the Earth’s radiation belts

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. 

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.

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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Man Hua is a fifth year PhD student in Wuhan University. She will graduate in June 2021. Her academic interests focus on space physics, mainly about the wave-particle interactions in the Earth's radiation belt. She can be contacted via e-mail here. 

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Come and join in with Magnetic Earth!

 

Magnetic Earth is a web resource to share the science of geomagnetism. Aimed at students and new researchers (or anyone with an interest), I created the website to provide a friendly introduction to the subject with links to other locations on the internet for people to dig deeper. Moving forward, I want to encourage contributions from others in order to grow this concept as an open community hub to help people navigate research activities, tools, and organisations relevant to studying Earth’s magnetic field.

At magneticearth.org you will find a handful of pages with very short introductions to topics related to geomagnetism - mainly about global field modelling as that is what I am familiar with. The content is intentionally kept succinct so that it can quickly give a very broad overview of many areas and be understandable by a wide audience. It is not a tutorial, but a starting point and reference to interlink other resources on the web.

At swarm.magneticearth.org (“Swarm Notebooks”) you will find a collection of Python-based recipes that can help scientists get started with accessing and analysing data from ESA’s Swarm spacecraft mission which is mapping and monitoring Earth’s magnetic field from space. I am developing this as part of my work in supporting the Swarm Virtual Research Environment - a project which provides a cloud-based computing environment enabling people to more easily write and run code directly through a web browser. The wider goal here is to foster more collaboration and development of sustainable software to interact with Swarm mission products. The Swarm Notebooks website is an example of a more in-depth guide that can be linked to from Magnetic Earth.

How can you help?

An informational resource is only as useful as its content. With more voices we can cover more stuff and multiply its usefulness, so I am trying to make it as easy as possible for anyone to contribute!

1. Take a quick look at the website - is it useful to you? what is missing?

2. Check the instructions there for the multiple ways you can give feedback or get involved

How does it work?

The website is generated from code which sits in a repository on GitHub, under the Magnetic Earth GitHub Organisation. GitHub provides the collaborative tooling that lets us robustly and openly develop the website, and directly publish it using GitHub Pages - all for free. I own the magneticearth.org domain so that we have a nicer URL, and it also leaves the option open to change the web hosting without changing the location.

The code-driven approach to the website makes it more difficult for people with fewer software skills to edit it but is a longer term maintainable solution - the content source files are written in Markdown so can be transferred to different website generation systems in the future. To alleviate this problem, I provide several no-code ways that people can submit suggestions for content changes/additions. The spirit with which I have set up the website is following in the footsteps of several other great resources out there, such as The Turing Way - a guide to open source community-driven data science.

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Ashley Smith is a postdoctoral researcher at the University of Edinburgh, working as part of the Swarm DISC (Data, Innovation, and Science Cluster). He is passionate about computing technologies and open source software and his research interests include geomagnetic field modelling and space weather. He can be contacted by email at ashley.smith@ed.ac.uk.

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Geomagnetic Observatories, Surveys and Analyses : IAGA Division V

Dr. Masahito Nosé is the Division Chair for Division V : Geomagnetic Observatories, Surveys and Analyses. Here, he answers some of our questions about himself and his division. 

1) Could you please tell us something about yourself?

I am Dr. Masahito Nosé, working for Institute for Space-Earth Environmental Research, Nagoya University, Japan. I earned Ph.D. in 1998 and had jobs at Applied Physics Laboratory, Johns Hopkins University, USA for 1998–2001 and at World Data Center for Geomagnetism, Kyoto/Graduate School of Science, Kyoto University, Japan for 2001–2018. My research interests include geomagnetic field variations, geomagnetic indices, energetic particle acceleration, plasma ion composition, and ring current dynamics. Recent topic is development of low-cost magnetometer with magneto-impedance sensor, which make it possible to deploy a dense ground observation network.

2) What are the basic research questions of the IAGA division you head? 

IAGA Division V, Geomagnetic Observatories, Surveys and Analyses, has objectives to promote high quality standards in geomagnetic data acquisition, observatory and survey procedures, geomagnetic indices, and data dissemination; to analyse magnetic observation data for the purposes of understanding the various sources of the magnetic field. In this respect, the following three working groups are active: Working Group V-OBS: Geomagnetic Observation; Working Group V-MOD: Geomagnetic Field Modeling; and Working Group V-DAT: Geomagnetic Data and Indices.

 

Location of geomagnetic observatories in operation. Image from BGS.

3) What are the past important results of this division? 

Working Group (WG) V-OBS maintains a list of observatories operating worldwide now and in the past, and allocates IAGA codes. WG is also responsible for the organisation of a workshop on geomagnetic observatory instruments, data acquisition and processing every two years. These efforts keep quality for measurements and for data processing. Two IAGA Guides related to geomagnetic observations have been published: “IAGA Guide for Magnetic Measurements and Observatory Practice” by J. Jankowski and C. Sucksdorff in 1996 and “IAGA Guide for Magnetic Repeat Station Surveys” by L.R. Newitt, C.E. Barton, and J. Bitterly in 1997.

Working Group V-MOD released the 13th Generation International Geomagnetic Reference Field (IGRF-13) in December 2019. The IGRF model is widely used not only in research community but also in ordinary society. This is the outcome of enduring efforts of this working group and continuous observations of geomagnetic field on ground and by satellites.

Earth's magnetic structure. Image from NOAA NCEI.

Working Group V-DAT has been supporting derivation of geomagnetic indices and event lists such as aa index, am index, Kp index, Disturbance storm-time (Dst) index, Auroral Electrojet (AE) index, Polar Cap (PC) index, SSC (storm sudden commencement) and SFE (solar flare effect), and Q-Days and D-Days. These indices and event list are inevitable for research in space physics and space weather. This working group also designed the geomagnetic field data dissemination format called IAGA2002 format, which is now used as standard in data exchange.

4) What do you think would be the future applications or impacts through this research?

Continuous activity of Division V is essential to keep precise geomagnetic observations and products depending on them, for example, IAGA-endorsed geomagnetic indices, event list, and IGRF model. High-quality geomagnetic field data and geomagnetic indices can be applied to space weather forecast and geomagnetically induced current research.