A plan to turn the atmosphere into one, enormous sensor

WHEN AN EARTHQUAKE rocked Nepal in 2015, geophysicists were surprised to find that they could see its reflection in the ionosphere, a layer of the atmosphere beyond about 75km above Earth’s surface. Ground-based instruments saw changes in the density of free electrons in this electrically charged zone. These rippled outward from the point in the ionosphere directly above the earthquake’s epicentre.Similar effects have been observed with other phenomena, both natural (hurricanes, volcanoes and the passage of meteors) and artificial (blasting associated with mining operations). All of these things cause electrical changes in the sky that can be detected hundreds of kilometres away.