A supposedly stable belt of radiation 7000 km above the Earth’s surface may in fact be producing damaging bursts of high-energy electrons. According to scientists at the University of Colorado Boulder, US, the bursts appear to be triggered by lightning, and understanding them could help determine the safest “windows” for launching spacecraft – especially those with a human cargo.
The Earth is surrounded by two doughnut-shaped radiation belts that lie within our planet’s magnetosphere. While both belts contain high concentrations of energetic electrons, the electrons in the outer belt (which starts from about 4 Earth radii above the Earth’s surface and extends to about 9–10 Earth radii) typically have energies in the MeV range. In contrast, electrons in the inner belt, which is located between about 1.1 and 2 Earth radii, have energies between 10 and a few hundred kilo-electronvolts (KeV).
At the higher end of this energy scale, these electrons easily penetrate the walls of spacecraft and can damage sensitive electronics inside. They also pose risks to astronauts who leave the protective environment of their spacecraft to perform extravehicular activities.
The size of the radiation belts, as well as the energy and number of electrons they contain, varies considerably over time. One cause of these variations is sub-second bursts of energetic electrons that enter the atmosphere from the magnetosphere that surrounds it. These rapid microbursts are most commonly seen in the outer radiation belt, where they are the result of interactions with phenomena called whistler mode chorus radio waves. However, they can also be observed in the inner belt, where they are generated by whistlers produced by lightning storms. Such lightening-induced precipitation, as it is known, typically occurs at low energies of 10s to 100 KeV.
Outer-belt energies in inner-belt electrons
In the new study, researchers led by CU Boulder aerospace engineering student Max Feinland observed clumps of electrons with MeV energies in the inner belt for the first time. This serendipitous discovery came while Feinland was analysing data from a now-decommissioned NASA satellite called the Solar, Anomalous, and Magnetospheric Particle Explorer (SAMPEX). He originally intended to focus on outer-belt electrons, but “after stumbling across these events in the inner belt, we thought they were interesting and decided to investigate further,” he tells Physics World.
After careful analysis, Feinland, who was working as an undergraduate research assistant in Lauren Blum’s team at CU Boulder’s Laboratory for Atmospheric and Space Physics at the time, identified 45 bursts of high-energy electrons in the inner belt in data from 1996 to 2006. At first, he and his colleagues weren’t sure what could be causing them, since the chorus waves known to produce such high-energy bursts are generally an outer-belt phenomenon. “We actually hypothesized a number of processes that could explain our observations,” he says. “We even thought that they might be due to Very Low Frequency (VLF) transmitters used for naval communications.”
The lightbulb moment, however, came when Feinland compared the bursts to records of lightning strikes in North America. Intriguingly, he found that several of the peaks in the electron bursts seemed to happen less than a second after the lighting strikes.
A lightning trigger
The researchers’ explanation for this is that radio waves produced after a lightning strike interact with electrons in the inner belt. These electrons then begin to oscillate between the Earth’s northern and southern hemispheres with a period of just 0.2 seconds. With each oscillation, some electrons drop out of the inner belt and into the atmosphere. This last finding was unexpected: while researchers knew that high-energy electrons can fall into the atmosphere from the outer radiation belt, this is the first time that they have observed them coming from the inner belt.
Feinland says the team’s discovery could help space-launch firms and national agencies decide when to launch their most sensitive payloads. With further studies, he adds, it might even be possible to determine how long these high-energy electrons remain in the inner belt after geomagnetic storms. “If we can quantify these lifetimes, we could determine when it is safest to launch spacecraft,” he says.
The researchers are now seeking to calculate the exact energies of the electrons. “Some of them may be even more energetic than 1 MeV,” Feinland says.
The present work is detailed in Nature Communications.
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