HAARP, the most powerful ionosphere heater on Earth

Todd Pedersen is a scientist at the Air Force Research Laboratory’s Space Vehicles Directorate at Kirtland Air Force Base in Albuquerque, New Mexico.

https://physicstoday.scitation.org/doi/10.1063/PT.3.3032

Our modern world of Wi-Fi, smartphones, and location apps relies on radio waves to link up all our gadgets. Most of us, though, are unaware that the ionosphere high above Earth affects the location services in our phones and the directions relayed by the navigation units in our cars. The complex dynamics of the ionospheric plasma, a gas of electrons and ions enveloping our planet, can be studied by research facilities such as the High Frequency Active Auroral Research Program (HAARP), located in Alaska. During the past 15 years, HAARP has produced many interesting and unexpected results, perhaps most spectacularly the production of an artificial ionospheric plasma generated by radio waves.

The ionosphere is the region of the upper atmosphere characterized by a large population of free electrons and ions—the atmospheric shrapnel that arises when UV photons from the Sun knock electrons from atmospheric gas. (For a tour of the upper atmosphere, see the Quick Study by John Emmert, Physics Today, December 2008, page 70.) Its density is controlled by the relative rates of ion production and the recombination of ions with electrons to re-create neutral molecules.

The ionospheric plasma can distort and delay satellite communications and navigation signals passing through it; indeed, the primary practical motivation for studying the ionosphere is to get a handle on those effects. At the low power of day-to-day devices, the ionospheric plasma can alter radio waves, but the plasma itself is unaffected. At high enough power densities, however, radio waves can affect the plasma and generate feedback between the waves and plasma, a phenomenon that offers a unique means—so-called ionospheric heating—of studying the ionosphere.
The HAARP facility began operating in 1999 with a 6 × 8 array of transmitting antennas that, in total, produced 960 kW of RF power—about the same as generated by 10 AM radio stations. (The figure shows today’s 12 × 15 array.) The HAARP beam is broad like a flashlight’s, not narrow like a laser’s, but it can be electronically steered anywhere within 30° of zenith—that is, local vertical—and it can operate at 3–10 MHz. Its powerful radio waves drive ionospheric electrons back and forth in what are called plasma waves. As those driven electrons collide with each other and with background species, their temperature goes up, which is why HAARP is called a heater.

Heating and observing the ionosphere. Generators at the High Frequency Active Auroral Research Program (HAARP) operations center in Alaska (buildings to the upper left) feed power to the large antenna array to the right. That array, in turn, transmits RF waves that interact with the ionosphere. Shelters down the road from the array house optical instruments for observing the resulting excitations; one of the instruments is visible through the clear dome in the lower inset. (Backdrop photo by A. Lee Snyder; inset photo by Robert Esposito.) The red and green regions in the upper inset (courtesy of Jeffrey Holmes) represent regions of the ionosphere in which oxygen atoms excited by ionospheric heating relax to lower-energy states. Behind the HAARP site rises Mount Drum.

Mount Drum is a stratovolcano in the Wrangell Mountains of east-central Alaska in the United States.It is located at the extreme western end of the Wrangells, 18 miles (29 km) west-southwest of Mount Sanford and the same distance west-northwest of Mount Wrangell.