Understanding space weather, the changing behavior of plasma near Earth, is key to the safe and reliable performance of electronics on board satellites, spacecraft, and space stations. Of particular interest in this plasma are highly relativistic electrons, those with energies reaching millions of electron volts (megaelectron volts, or MeV), in the planet’s radiation belts. During geomagnetic storms, in which surges in the solar wind can greatly perturb Earth’s magnetic environment, the population of these high-energy electrons decreases substantially.
As a storm subsides, the population of MeV electrons rebounds on the timescale of an hour, and the particles are broadly distributed in space. However, NASA’s Van Allen Probes mission has observed a sharp and nearly impenetrable barrier at a distance of about 2.8 Earth radii from the planet, inward of which MeV electrons are not observed. Foster et al. propose a new formation mechanism for this barrier, in which the generation of MeV electrons is suppressed by interaction with terrestrial radio signals.
Prior work has shown that the excitement of MeV electrons after a storm occurs in two phases. First, the motion of electrons with energies on the order of 10 kiloelectron volts generates chorus waves, radio waves in the VLF part of the electromagnetic spectrum. Interaction with these VLF waves then boosts electrons with energies on the order of 100 kiloelectron volts into the MeV range. Chorus wave production grows nonlinearly, which accounts for the rapid replenishment of MeV electrons.
Naval forces use VLF radio signals with frequencies of 20–25 kilohertz to communicate with underwater submarines. These omnidirectional broadcasts propagate from Earth to a distance that corresponds closely with the observed barrier at 2.8 Earth radii. On the basis of data from a 2017 solar storm, the researchers postulate that interference from these naval VLF signals precipitates local electrons in resonance with the transmitters, thus depleting populations of the approximately 10-kiloelectron-volt electrons that would otherwise drive the growth of chorus waves.
Without strong chorus wave growth, there is nothing to accelerate approximately 100-kiloelectron-volt electrons to MeV energies within 2.8 Earth radii of the planet, the researchers say. This effect can persist for days or weeks after the population of MeV electrons beyond this distance has rebounded, giving the perception of an “impenetrable” barrier to these relativistic particles. (Journal of Geophysical Research: Space Physics, https://doi.org/10.1029/2020JA027913, 2020)
—Morgan Rehnberg, Science Writer