WHE IS A EARTH that rocked Nepal in 2015, geophysicists were surprised to see its reflection in the ionosphere, a layer of atmosphere beyond about 75 km above the 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 epicenter of the earthquake.
Similar effects have been observed with other phenomena, both natural (hurricanes, volcanoes and the passage of meteors) and artificial (explosions associated with mining). All of these things cause electrical changes in the sky that can be detected hundreds of miles away.
Such observations have led David Lewis, a program director at America̵
Reason and feeling
The reaction of the ionosphere to events on the ground is basically simple but complex in practice. Events such as earthquakes cause disturbances in the lower atmosphere. These can take the form of sound waves, shock waves or slower movements called atmospheric gravity waves. Such oscillations are usually spread in all directions, including upward toward the ionosphere.
Before they get there, these waves travel through electrically neutral gas. This makes them difficult to detect remotely because there are no electrical effects, such as radio emissions, for detectors to exploit. However, it changes when they arrive. The movements of high altitude charged particles (electrons and the electrically charged atoms, called ions, which give the ionosphere its name) in response to passing waves can be picked up in several ways. One is from their effect on GPS satellite reception (so the Nepalese earthquake was discovered). Another is the use of ionic probes, a special type of radar that bounces radio waves from this atmospheric layer. The third is to use radio waves to stimulate the ionosphere so that it glows. This is akin to creating an artificial aurora and making visible ionospheric movements. Observing ionospheric waves in these ways can answer questions about what sets these waves in motion. But that means collecting lots of data to understand what’s going on.
One of AtmoSense’s first goals is to locate and study phenomena at or near the Earth’s surface – storms, earthquakes, volcanic eruptions, mining and “rock waves”, which are winds associated with mountain ranges. The goal is to see if atmospheric sensing can surpass existing methods: seismographs for earthquakes, Doppler weather for storms and so on.
Thereafter, the project will continue to try to collect data beyond the capacity of current sensors. Radar can, for example, track the positions of the storms, but AtmoSense may also be able to indicate how much energy a storm releases and thus determine whether it is weakened or amplified. Seismology can also be improved. According to Colonel Lewis, “there may be precursors to earthquakes that are not picked up on the ground. There can be all kinds of information. “It holds out the alluring prospect of being able to give earthquake warnings.
Ripples in the air
The armed forces, which pay for the project, are particularly interested in gaining insight into activities such as missile committees and underground nuclear power experiments. These should really be within the grasp of the system when fully developed. But it may also be possible to record smaller events as well.
If a meteor’s shock waves can be tracked across the ionosphere, for example, what about a hypersonic missile? Or an airplane? Previous work suggests that events as small as a ton of explosives that go off can be detected. With the right sensors and filters it may be possible to detect even minor interference. Colonel Lewis will explore new ways to do this, as well as try to upgrade the old ones. Among the new methods considered are instruments carried by high altitude drones and balloons, both of which can linger in the upper atmosphere for several days at a time.
The first 27 months of the program will be devoted to understanding the science behind ripples in both the ionosphere and the electrically neutral atmosphere beneath it and then building computer simulations of what is happening. These are tested to see if they can replicate exact effects seen before.
Once the team has a better understanding of basic science, they will proceed to the second stage: field trials. This will involve three tests at three-month intervals in which researchers try to locate relevant events, such as storm cells, mining and earthquakes. If it works, the project will then move on to issues of military interest by spying on rockets, tracking aircraft and even looking at underground bunkers that are being dug. The result, if all goes well, may be the world’s first real panopticon. ■
This article was published in the Science & technology section of the print edition under the heading “Something’s in the air”