Focus: An Unnatural Aurora

Published June 30, 2000  |  Phys. Rev. Focus 5, 30 (2000)  |  DOI: 10.1103/PhysRevFocus.5.30

The Structure of Electromagnetic Wave-Induced 557.7-nm Emission Associated with a Sporadic- E Event over Arecibo

L. M. Kagan, M. C. Kelley, F. Garcia, P. A. Bernhardt, F. T. Djuth, M. P. Sulzer, and C. A. Tepley

Published July 3, 2000
Figure 1
Jan Curtis/Univ. of Alaska

Nature’s fireworks. Natural auroras are common in Alaska, but researchers can also create their own auroras to learn more about the ionosphere.

Auroras are colorful nighttime light shows that occur in the upper atmosphere when conditions are right, but researchers can also create artificial auroras to learn about the atmosphere. In the 3 July PRL a team describes a new way of using such an aurora to illuminate the lower ionosphere–a layer that is difficult to visualize by other means. They transmitted powerful radio waves upward at night and took pictures as low-lying clouds of ionized atoms glowed green in response. The technique will allow more detailed observations of a turbulent and poorly understood atmospheric layer that affects satellite and radio communications.

If our eyes were more sensitive, we would see a dim red and green glow in the night sky from excited nitrogen and oxygen hundreds of kilometers up. In the main part of the ionosphere–the so-called F layer between 200 and 500 km in altitude–ions are plentiful and long-lived enough that the glow can be seen by sensitive detectors. But to measure the E layer below–the one best known for reflecting radio broadcasts from far-away cities–researchers usually send up rockets or use narrowly focused radar beams, which reveal only small regions of the sky. Because of the lower ion density, “the E region doesn’t glow, so you need a flashlight,” says Paul Bernhardt of the Naval Research Laboratory in Washington, DC.

The flashlight for Bernhardt and his colleagues was an array of radio transmitter antennas near Arecibo, Puerto Rico. Their main task was to excite ions in the F region with radio waves, but one night, during a period of about a half hour, a series of clouds of ionized metal atoms at a height of 85-90 km rolled in and provided an unusually high density of ions at that low altitude. Such “sporadic E layers” are regularly created when meteors burn up in the atmosphere. The electromagnetic waves converted to electric field waves in the ionosphere and excited free electrons to energies high enough that their collisions with oxygen atoms resulted in green fluorescence. The process is similar to that of a natural aurora, except that the energy originates from a radio transmitter, rather than the Earth’s magnetosphere or the solar wind.

The green fluorescence allowed the researchers to take the first images of structures in the lower ionosphere, with some clouds stretching 25 by 50 km across the sky. Team leader Michael Kelley of Cornell University suggests repeating the procedure during seasons when the ion clouds are more common, which will allow researchers to probe the constituents and interactions in the E layer with a controlled source of auroral energy. “It’s kind of a mini-auroral laboratory,” he says.

“It’s a superb paper,” says William Gordon of Rice University in Houston. He says it’s important that the team has found a method for mapping ion clouds because they can have important effects on radio communications (AM, FM, and military) and satellite-based navigation with the Global Positioning System (GPS). For example, GPS signals received through a sporadic E layer can slow down a bit, reducing the accuracy of the positioning data. As Bernhardt summarizes the work, “We now have a new technique to study these layers just by taking pictures of them.”