How HAARP works

Nature has a good article about the High Frequency Active Auroral Research Program (HAARP), an ionospheric heater that became fully operational last July.
The High Frequency Active Auroral Research Program (HAARP) has been entwined with controversy since its birth. Originally envisioned as a way to facilitate communications with nuclear-armed submarines, HAARP took almost two decades to build and has incurred around US$250 million in construction and operating costs. It consists of 360 radio transmitters and 180 antennas, and covers some 14 hectares near the town of Gakona about 250 kilometres northeast of Anchorage.

With 3.6 megawatts of power at its command, HAARP is the most powerful ionospheric heater in the world. At its heart is a phased-array radar that emits radio waves that are partially absorbed between 100 kilometres and 350 kilometres in altitude, accelerating electrons there and 'heating' the ionosphere (see graphic, right). In effect, HAARP allows scientists to turn the ionosphere, the uppermost and one of the least understood regions of the atmosphere, into a natural laboratory.

Link (via the day they tried to kill me)