by modulated heating of the auroral electrojet, useful because generating VLF waves ordinarily requires gigantic antennas Generating artificial Airglow, which is typically subvisual but routinely detectable. Under certain geophysical conditions and transmitter configurations, it can be bright enough to observe with the unaided eye. Generating extremely low frequency (ELF) waves in the 0.1 Hz range. These are next to impossible to produce any other way, because the length of an antenna is dictated by the wavelength of the signal it emits or receives. Generating whistler-mode VLF signals that enter the magnetosphere and propagate to the other hemisphere, interacting with Van Allen radiation belt particles along the way VLF remote sensing of the heated ionosphere Research at the HAARP has included: Plasma line observations Stimulated electron emission observations Gyro frequency heating research Spread F observations (blurring of ionospheric echoes of radio waves due to irregularities in electron density in the F layer) High-velocity trace runs Airglow observations Heating induced scintillation observations VLF and ELF generation observations Radio observations of meteors Polar mesospheric summer echoes (PMSE) have been studied, probing the mesosphere using the IRI as a powerful radar, and with a 28 MHz radar and two VHF radars at 49 MHz and 139 MHz. The presence of multiple radars spanning both HF and VHF bands allows scientists to make comparative measurements that may someday lead to an understanding of the processes that form these elusive phenomena. Research into extraterrestrial HF radar echos: the Lunar Echo experiment (2008). Testing of spread spectrum Transmitters (2009) Meteor shower impacts on the ionosphere Response and recovery of the ionosphere from solar flares and geomagnetic storms The effect of ionospheric disturbances on GPS satellite signal quality Producing high density plasma clouds in Earth's upper atmosphere Research conducted at the HAARP facility has allowed the US military to perfect communications with its fleet of submarines by sending radio signals over long distances. Instrumentation and operation The main instrument at HAARP is the Ionospheric Research Instrument (IRI). This is a high-power, high-frequency phased array radio transmitter with a set of 180 antennas, disposed in an array of 12×15 units that occupy a rectangle of about . The IRI is used to temporarily energize a small portion of the ionosphere. The study of these disturbed volumes yields important information for understanding natural ionospheric processes. During active ionospheric research, the signal generated by the transmitter system is delivered to the