Where Did that Electron Come From?
Tracking Charged Particles into Earth’s Atmosphere with ELFIN
By Mara Johnson-Groh
NASA’s Goddard Space Flight Center
On September 2, 2019 — after a year of quiet conditions in space since its September 2018 launch — a NASA CubeSat the size of a large toaster flew straight through a solar storm, when a burst of material ejected by the Sun dramatically increased the number of highly charged particles coursing through Earth’s magnetic environment. These observations from the CubeSat — called ELFIN, short for Electron Losses and Fields Investigation — allowed the scientists to see events that are usually too weak to see under normal conditions.
ELFIN’s job, as it circles through Earth’s polar regions, is to measure super-speedy charged particles falling into Earth’s atmosphere, and for the first time, uncover what pushed them there. The highly energetic electrons and ions measured by ELFIN originate in the Van Allen radiation belts, the concentric rings of charged particles trapped around Earth by the planet’s magnetic field. These charged particles can spark aurora, and if strong enough, disrupt telecommunications, so understanding what sends them hurtling towards Earth is important to protecting our assets in space and on the ground.
Here’s what that ELFIN data looked like in the solar storm.
The graphs show data over a period of just a few minutes on September 2 with each color (right axis) showing how many particles are present at a given energy (left axis). Red represents higher numbers — and the spike in the middle shows that the particle count was in the millions across a wide range of energies. Because ELFIN can also determine the direction in which the particles are traveling relative to the Earth’s magnetic field — a measurement known as pitch angle — they can figure out which of these particles are circling around Earth, trapped by the magnetic fields, versus those that are raining down out of the belts toward our planet. ELFIN is the first satellite to quickly survey the whole latitudinal range of the radiation belts with this capability — taking measurements of pitch angle while simultaneously measuring the particles’ energies at high resolution.
In this case, the particles were falling into Earth’s atmosphere as it flew over Norway and the North Sea. Having seen a precipitation event, the scientists looked to see if they could identify what caused it. Particles typically get dislodged by electromagnetic waves pushing them out of orbit. Different waves dislodge particles with different energies or different travel directions. By looking at the distribution of particles that fell into the atmosphere, the scientists hoped to find out which type of wave was responsible. In particular, ELFIN scientists are looking to see if a type of wave known as an electromagnetic ion cyclotron wave, or EMIC wave, can scatter these particles into Earth’s atmosphere. This type of wave typically knocks down only high-energy particles — those with energies above 900,000 electronvolts.
The measurements, shown in the bottom panel of the graph above, show a spike of precipitating particles at these high energies, suggesting EMIC waves might be involved. But since it did not also measure EMIC waves, which often occur farther out from where the particles precipitate, the case is not yet closed. The mission expects to answer this question as it continues to collect data over the next one and a half years.
Other NASA missions — like the Magnetospheric Multiscale mission and the Time History of Events and Macroscale Interactions during Substorms mission, which orbit farther out — may be able to collaborate with ELFIN by directly measuring the EMIC waves near the equator that launch the particles, which follow along magnetic field lines all the way down to ELFIN. These types of conjunction measurements from different instruments and vantage points will allow scientists to learn more about EMIC waves scattering phenomena than any single-point observation could.
ELFIN was developed at the University of California, Los Angeles, where over 200 students have contributed to the mission. The mission is funded by NASA and the National Science Foundation.