The Van Allen radiation belts are the most dangerous regions in near-Earth space, home to electrons with energies as high as millions of electron volts, which can damage or destroy satellites. One of the most enduring mysteries of space physics is just how these “killer electrons” get accelerated to such high energies in the first place—energies more typically seen in the realms of exploding stars and black holes. Scientists have debated this ever since the belts were discovered by the first U.S. satellite, Explorer 1, in 1958.
One longstanding suspicion has been that plasma waves play a key role. The plasma surrounding Earth is like an ocean, awash with waves and vibrations propagating through it, and these could be accelerating particles to higher speeds and energies. However, waves don’t necessarily transfer momentum to the particles themselves; after all, in the ocean, waves usually pass right through, whereas the actual water molecules bob in circles.
To better understand how wave action could accelerate electrons to deathly speeds, Shklyar made detailed calculations of a slightly different scenario, one in which killer electrons receive their energy at the expense of other particles. In this scheme, waves act as the middleman, transferring energy from one group of particles to electrons, gradually pushing them faster and faster.
There’s one issue with this scenario: To make it work, there must be a way for the receiving electrons to continue to draw energy from particles slower than themselves. Thermodynamics generally frowns on this. When a faster particle collides with a slower particle, the faster one usually slows down and the slower one speeds up—not the other way around.
However, the author’s calculations show that this transfer is possible under certain conditions, namely, that the less energetic particles must have an unstable distribution. This allows some of their energy to go into waves, which can, in turn, push more stable electrons to higher and higher speeds.
Crucially, the author’s work also suggests a ubiquitous source for these processes: lightning bolts. Lightning is the prime source of a category of plasma waves called whistlers, named for their characteristic sharp fall in pitch, which makes them sound like whistles on radio receivers.
The author’s calculations show that lightning-induced whistler waves open a thermodynamic pathway: As charged particles spiral through the electric and magnetic fields, they can strengthen whistler waves, adding energy at each turn in a resonance. These whistler waves can then interact with higher-energy electrons through another mechanism called the Cerenkov resonance, pushing them to even greater energies—and killer status.
In this situation, the waves themselves are not the source of energy—they simply mediate the transaction from slower particles to faster ones. The work shows that this mechanism is a viable one and suggests that it plays an important role in creating the Van Allen belts. (Journal of Geophysical Research: Space Physics, https://doi.org/10.1002/2016JA023263, 2017)
—Mark Zastrow, Freelance Writer