antenna array and transmitted in an upward direction. At an altitude between (depending on operating frequency), the signal is partially absorbed in a small volume several tens of kilometers in diameter and a few meters thick over the IRI. The intensity of the HF signal in the ionosphere is less than 3 µW/cm2, tens of thousands of times less than the Sun's natural electromagnetic radiation reaching the earth and hundreds of times less than even the normal random variations in intensity of the Sun's natural ultraviolet (UV) energy which creates the ionosphere. The small effects that are produced can be observed with the sensitive scientific instruments installed at the HAARP facility. These observations can provide information about the dynamics of plasmas and insight into the processes of solar-terrestrial interactions. Each antenna element consists of a crossed dipole that can be polarized for linear, ordinary mode (O-mode), or extraordinary mode (X-mode) transmission and reception. Each part of the two section crossed dipoles is individually fed from a specially designed, custom-built transmitter that operates at very low distortion levels. The effective radiated power (ERP) of the IRI is limited by more than a factor of 10 at its lower operating frequencies. Much of this is due to higher antenna losses and a less efficient antenna pattern. The IRI can transmit between 2.7 and 10 MHz, a frequency range that lies above the AM radio broadcast band and well below Citizens' Band frequency allocations. HAARP is licensed to transmit only in certain segments of this frequency range. When the IRI is transmitting, the bandwidth of the transmitted signal is 100 kHz or less. The IRI can transmit in continuous waves (CW) or in pulses as short as 10 microseconds (µs). CW transmission is generally used for ionospheric modification, while transmission in short pulses frequently repeated is used as a radar system. Researchers can run experiments that use both modes of transmission, first modifying the ionosphere for a predetermined amount of time, then measuring the decay of modification effects with pulsed transmissions. There are other geophysical instruments for research located at the HAARP facility. Some of them are: A fluxgate magnetometer built by the University of Alaska Fairbanks Geophysical Institute, available to chart variations in the Earth's magnetic field. Rapid and sharp changes of the magnetic field may indicate a geomagnetic storm. A digisonde that can provide ionospheric profiles, allowing scientists to choose appropriate