Driving the glow of the northern lights is an energetic region of the atmosphere known as the ionosphere. Situated at the edge of space, the ionosphere is constantly changing, shaped by solar radiation from above and atmospheric winds from below.
A mirror at the edge of space
The ionosphere acts like a mirror for low-frequency radio waves, reflecting ground-based signals "over the horizon." Engineers use this reflection to channel navigation, communication, and radar signals thousands of kilometers around the globe.
To enable these over-the-horizon technologies, countries have spent millions of dollars measuring the upper regions of the ionosphere.
Too high for balloons, too low for satellites
All the while, a lower region, the D-region, holds the key to billions of dollars in novel space weather and over-the-horizon radio technologies. But the D-region is extremely difficult to measure. It is too high for weather balloons and too low for satellites. Without these measurements, it remains nearly useless for next-generation innovation.
Scanning the sky with lightning
Today, SkyCT is building the first real-time, high-resolution map of the global D-region. To do this, SkyCT uses lightning. Each lightning stroke emits a radio signal whose waveform is shaped by the D-region it reflects off of. A receiver thousands of kilometers away can analyze that waveform, effectively "scanning" the D-region along the path from stroke to receiver.
With six million lightning strikes occurring daily and a growing global network of over 15 receivers, these scans are pieced together into a 3D map of the D-region, in much the same way a medical CT scan builds a complete image of the body from many individual snapshots.
"CT" stands for computed tomography: a method of reconstructing a 3D density map from many cross-sectional measurements, used both in medical CT scans and by SkyCT.
During the Great American Solar Eclipse of August 2017, SkyCT's algorithm captured the eclipse shadow as a localized drop in D-region electron density sweeping across North America.
What a real-time map unlocks
With a real-time map of the D-region in hand, SkyCT unlocks critical capabilities: bolstering over-the-horizon communications during solar storms, providing early warning of atmospheric expansion events that drag satellites out of orbit, and pioneering navigation systems that operate independently of vulnerable satellite constellations.
The D-region also responds rapidly to space weather events such as solar flares, geomagnetic storms, and particle precipitation, as well as to disturbances from below, including geomagnetic waves and potential seismic activity. That makes it a powerful channel for studying both space and the Earth itself.