Universe Today
Remember the big solar storm back in May 2024? The one that caused auroras as far south as Florida and Mexico? Scientists are still parsing out the impacts that storm had on the local environment, and a new paper published in Geophysical Research Letters by Yoshizumi Miyoshi of Nagoya University and his co-authors shows it might have allowed some nearby electrons to accelerate to relativistic speeds using something called a chorus wave.
To understand why that’s important, let’s first take a look at Earth’s local radiation environment. It’s dominated by what scientists call the Van Allen belts. These were discovered early on in the Space Age by Explorer 1, and they are Earth’s primary defense against high energy electrons and other ions spewed out by the Sun. Scientists have long split them into two categories - the “inner” belt is made mainly of protons, and the “outer” belt is covered in high-energy electrons.
But in 2014, NASA’s Van Allen Probe mission discovered a noticeable boundary - the ultra-relativistic “killer electrons” that move near the speed of light seem to hit some sort of glass wall at around 11,500 kilometers above the Earth’s surface. To scientists, it seemed that this “shield” protected the inner area directly around the planet from the most dangerous form of radiation.
Fraser talks about what the Van Allen radiation belts are.Which sounded great - until Nature decided not to do it anymore. Luckily we know when exactly that happened, because Japan’s Arase satellite (also known as the Exploration of energization and Radiation in Geospace - ERG mission), which has been flying through Earth’s magnetosphere since 2017, was watching. For its entire lifetime up to that point, it noted that high energy electrons were being held at bay. Until May 2024.
During that storm, solar wind gusts topping out at 1,200 kilometers per second compressed Earth’s magnetic field, and triggered the most intense geomagnetic disturbance since the arguably even more famous Halloween storm of 2003. As the storm peaked, Arase counted ultra-relativistic electrons at levels 5,000 times higher than the typical background. This wasn't the first time we had seen spikes in radiation during solar storms, but older spacecraft weren’t able to tell what was actually happening.
Arase provided the key data to help guide the researchers, who used a mathematical method called electron phase space density. This tracks the particle distribution of those electrons by energy and motion. If, say, the electrons were coming in from outside where the protective barrier originally was, this metric should very clearly show a downward trend from outer space towards Earth. However, Arase saw a very distinct peak right where the barrier used to be. To space physics, that represents a smoking gun for “local” acceleration - i.e. electrons being sped up exactly where they were detected.
NASA Goddard video about the radiation environment surrounding Earth. Credit - NASA Goddard YouTube ChannelWhich then begs the question, what was accelerating them? According to the paper, it was something called a “chorus wave”. Typically, the inner radiation belt is covered in a shroud of charged gas known as the plasmasphere. Charged gases do a great job of dampening high frequency electromagnetic waves, as they simply absorb their energy. But when the May 2024 storm hit, the plasmasphere was crushed back toward Earth, creating a void where it would normally be.
Combine that with a “negative ionospheric storm”, which massively depletes local plasma densities, and you create a very empty electromagnetic environment. Which is the perfect situation for chorus waves. These are low frequency electromagnetic waves, which sound like whistling when translated into audio, that are generated by lower-energy particles during magnetic storms. In a depleted plasma environment, these waves are able to transfer their energy very efficiently to the surrounding low energy electrons, boosting them from sub-million electron volt energies to multi-MeV ranges - essentially creating high energy electron radiation within what is normally an exclusion zone.
Over time this new high energy radiation spread out both towards and away from Earth - and the effect lasted for months. Its effects were visible to Arase until another solar storm hit five months later in October 2024, and changed the magnetic field situation up again.
But ultimately, this paper acts as a warning of a new type of space weather hazard - localized radiation caused not by a massive solar explosion but by a retreat of our own protective plasma sheath. As we start to expand out into the area most affected by this, it’s certainly better to be aware of that phenomenon now, but unfortunately it might require even more shielding to protect our sensitive electronic and biological cargo in the future.
Learn More:
Nagoya University - 2024 solar storm triggered ultra-relativistic electrons inside Earth’s “impenetrable barrier”
Y. Miyoshi et al - Local Acceleration of Multi-MeV Electrons Inside Earth's Impenetrable Barrier During the May 2024 Superstorm
UT - The Final Journey of Van Allen Probe A
UT - NASA CubeSat Discovers New Radiation Belts After Intense Solar Storm
