Super-fast Energetic Electron Rain from Earth’s Radiation Belts

The near-Earth space is highly dynamic and filled with orbiting charged particles that make up the Van Allen radiation belts. Here we reveal a new source of super-fast, energetic electrons raining down on our planet, which can have implications for space infrastructure and atmospheric modeling.

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Space physics is the study of plasmas within the solar system, and our work is mostly on the energy flow from the Sun to the Earth. An important carrier of this energy is plasma waves, analogous to sound waves in the air, except made up of charged particles flowing through space. My interest in waves starts visually: among all the phenomena in space measurements, waves in particular can be seen intuitively as the most regular (often sinusoidal) electromagnetic vibrations. Interestingly, these waves can be generated naturally in the Van Allen radiation belts and in turn energize the electrons within, which stretches out their travel paths along magnetic field lines between the Earth’s north and south magnetic poles. Under certain conditions, this energization can occur so effectively that electrons will be accelerated and lost into the atmosphere, creating the electron rain, as illustrated in Fig. 1. This can excite molecules in the upper atmosphere to release light, commonly known as aurora in polar regions.

Fig. 1: Electrons typically spiral back and forth between Earth’s magnetic poles in the radiation belts, show as yellow and red cross-sections. However, waves in the radiation belts can boost electrons out of their typical orbits, making them shower down at the north and south poles, where they can spark the auroras. Credit: Emmanuel Masongsong (UCLA).

The majority of satellites focus on characteristics of these waves near the equator (such as NASA’s THEMIS spacecraft), but the electron rain itself can only be captured near the poles (at high latitudes). Such observations only became available recently from UCLA’s ELFIN (Electron Losses and Fields Investigation) CubeSats in a low-Earth orbit. By combining distant THEMIS satellite observations of the whistler waves with low-altitude ELFIN electron data and sophisticated computer modeling, we were able to show and explain the unexpectedly rapid downpour of electrons colliding with the atmosphere (as illustrated in Fig. 2). This study not only verifies the most recent theories in plasma waves, but also addresses our confusion from these apparently “abnormal” data.

Fig. 2: The THEMIS and ELFIN satellites (orbits shown in cyan and green, respectively) worked together to help understand the mystery of electron rain. When whistler waves (purple) interact with the electrons (moving along the blue spiral), they can give them extra energy (red spiral), which causes them to fall into the atmosphere. Credit: Zhang et al., 2022.

As in other experimental physics fields, the biggest hurdle we encountered in this study is data calibration. Since the start of the ELFIN mission there have been over 300 undergraduate students involved; a large amount of time and effort have been devoted to streamlining the data downlink and data processing, thanks to our dedicated student operations team. It took them about one year to calibrate the electron data from ELFIN and release it to the community for scientific analysis. An important lesson learned from this process is to standardize the data format and analyzing software (such as SPEDAS, the Space Environment Data Analysis Software, free at spedas.org), to make it widely accessible and easy to analyze. Nowadays, space physics is no longer data-deprived, as opposed to its beginning in the 1950s with limited spacecraft observations. Instead, we are more and more overwhelmed by the growing satellite datasets available to us, so improving standardization is very important. In my opinion, this should be the next step to prioritize in space physics or space industry. Unified data formatting (after cleanup) and clearly categorized datasets within mission teams or across the entire community will greatly increase our efficiency in exploring space measurements.

Owing to recent advances in technology, space physics has welcomed a rapid escalation of small satellites and CubeSats in targeted investigations. The success of our study can be largely attributed to the ELFIN CubeSats, which provide high-resolution angular and energy measurements that fill the long-standing measurement gaps of energetic electrons at low altitudes. To continue our heritage and success from ELFIN, we are ready to embark on the next adventure by proposing a new CubeSat mission, DUCHESS (DUcted CHorus Electron Scattering Satellite). This mission aims to solve an important, yet challenging mystery that remains in space physics: where exactly is the electron rain generated along its travel path? This new mission will again fill the gap of high-latitude/low-altitude wave measurements that have been predicted to cause highly energetic electron rain towards Earth, which directly affects atmospheric chemistry. I believe that CubeSats and other small satellites are the future of comprehensive study of the near-Earth space environment, because of their low cost and quick development turnaround. With the enhanced understanding of plasmas in our solar system and lunar-planetary environments gained from these CubeSats, human space travel and colonization may no longer be limited to our imagination.

On another front, recently wave data can not only can be seen, they can be directly heard, thanks to recent efforts in sonification, the one-to-one conversion of spacecraft data into sound (learn more at listen.spacescience.org). This breakthrough enables space physicists and the public alike to use sound as a perceptual gateway for experiencing space plasmas. It has been shown that  data discovery is more efficient and fruitful when scientists use sound and visual analysis employed together. Please hear and feel the waves in our event, as they stream along the magnetic field lines while communicating with the background electrons.

Let us end this journey with a quote from Albert Einstein: “Imagination is more important than knowledge. For knowledge is limited, whereas imagination embraces the entire world, stimulating progress, giving birth to evolution.” Our experience perfectly exemplifies how imagination contributes to the success of this investigation and the advancement of our knowledge of the space environment. We are fortunate that, because space physics is a relatively young field, there is enough freedom granted for our imagination, and thus it is incredibly fulfilling when your ideas are realized.

The paper is accessible via this link: https://www.nature.com/articles/s41467-022-29291-8

Xiao-Jia Zhang

Associate Researcher, University of California Los Angeles