Editing Ionosphere (section)

===Overview===
Scientists explore the structure of the ionosphere by a wide variety of methods. They include:
* passive observations of optical and radio emissions generated in the ionosphere
* bouncing radio waves of different frequencies from it
* [[incoherent scatter]] radars such as the [[EISCAT]], Sondre Stromfjord, [[Millstone Hill Observatory|Millstone Hill]], [[Arecibo Telescope|Arecibo]], Advanced Modular Incoherent Scatter Radar (AMISR) and [[Jicamarca Radio Observatory|Jicamarca]] radars
* coherent scatter radars such as the [[Super Dual Auroral Radar Network|Super Dual Auroral Radar Network (SuperDARN)]] radars
* special receivers to detect how the reflected waves have changed from the transmitted waves.

A variety of experiments, such as HAARP ([[High Frequency Active Auroral Research Program]]), involve high power radio transmitters to modify the properties of the ionosphere. These investigations focus on studying the properties and behavior of ionospheric plasma, with particular emphasis on being able to understand and use it to enhance communications and surveillance systems for both civilian and military purposes. HAARP was started in 1993 as a proposed twenty-year experiment, and is currently active near Gakona, Alaska.

The SuperDARN radar project researches the high- and mid-latitudes using coherent backscatter of radio waves in the 8 to 20&nbsp;MHz range. Coherent backscatter is similar to Bragg scattering in crystals and involves the constructive interference of scattering from ionospheric density irregularities. The project involves more than 11 countries and multiple radars in both hemispheres.

Scientists are also examining the ionosphere by the changes to radio waves, from satellites and stars, passing through it. The [[Arecibo Telescope]] located in [[Puerto Rico]], was originally intended to study Earth's ionosphere.