A new study reveals that low-frequency electromagnetic waves accompany intense heating events at low altitudes.
On 17 December 1971, scientists observed a bizarre new phenomenon in the outermost region of Earth’s atmosphere, where the planet’s magnetic field orchestrates the flow of charged particles and produces such phenomena as auroras. In the midst of a massive geomagnetic storm induced by a surge of solar radiation, satellites detected large flows of heavy oxygen (O+) ions streaming away from Earth, seemingly in defiance of gravity. Ever since, scientists have been trying to figure out what propelled the ions, in part because disturbances in this zone—known as the ionosphere—can disrupt communication systems. A new study by Shen et al. reveals for the first time how low-altitude electromagnetic waves help launch these ions toward outer space.
Normally, the upward diffusion of O+ ions in Earth’s ionosphere is balanced by gravity, resulting in a state of equilibrium. Surges of energy from the Sun can disrupt this balance, however, causing flows of plasma that can hurtle outward like a fountain, then fall back toward Earth. To investigate how O+ ions become energized enough to escape Earth’s gravity, the researchers used measurements from the Canadian-based CASSIOPE satellite. One of CASSIOPE’s missions is to become the world’s first commercial space-based digital courier service, picking up and dropping off massive packages of digital data all over the globe. Another is to collect data on solar storms in the upper atmosphere using the Enhanced Polar Outflow Probe (e‑POP) payload, a collection of eight instruments that can, among other capabilities, measure the energy distribution of ions in the ionosphere and image auroral emissions.
Over the course of 1 year, the authors observed e-POP measurements at relatively low altitudes, between 325 and 730 kilometers. They looked for hot spots in the ionosphere, which occur when O+ ions become energized enough to escape Earth’s gravitational field. The team analyzed 24 such hot spots, examining their relation to the bulk flow of ions along the magnetic field lines around Earth, as well as low-frequency electromagnetic waves and currents.
In the first statistical study of its kind, the team observed that extremely low frequency electromagnetic waves known as BBELF waves energize O+ ions, even at relatively low altitudes, heating them and accelerating them outward. This phenomenon, called transverse ion heating, has long been thought to occur predominantly at higher altitudes—not as low as 350 kilometers, as the authors observed. The researchers discovered patches of ionosphere where this heating was particularly intense, reaching up to 4.5 eV (50,000 K) in areas about 2 kilometers across, they report.
Researchers traditionally have attributed heating in the lower reaches of the ionosphere to frictional interaction with Earth’s atmosphere, but the new study suggests that electromagnetic waves can also cause heating events. The study also reported downward flowing jets of ionospheric ions, which the authors attribute to downward pointing electric fields associated with auroral currents. (Journal of Geophysical Research: Space Physics, https://doi.org/10.1002/2017JA024955, 2018)
—Emily Underwood, Freelance Writer