A primary effect of RF absorption in the D and lower E regions of the ionosphere is the modification of the conductivity of the irradiated layer. Nonlinear processes accompanying conductivity modulation can give rise to new wave energy at frequencies different from the modulation frequency. Much of the physics of these processes can be studied through controlled heating experiments using high power, high frequency (HF) transmitters. The conductivity modification when an RF heater is on, followed by fast relaxation to its ambient value when the heater is off, is the basis of controlled ULF/ELF/VLF wave generation [Stubbe et al., 1982; Papadopoulos et al., 1990]. In the presence of the horizontal ionospheric electric field, which is perpendicular to the approximately vertical geomagnetic field at polar latitudes, the conductivity modulation induces a modulation of the Pedersen and Hall currents flowing through the perturbed region, thereby creating a virtual radiating antenna at the HF modulation frequency.

One of the goals of HAARP is to determine the efficiency of the scaling of HF power to ELF power, and to try to effect ways to increase it. The scaling depends on several key factors, e.g., the height-integrated Pedersen and Hall modulated conductivity, the modified area, and the vertical extent of the modified region. In order to obtain independent measurements of these factors, HAARP intends to support and operate a complementary diagnostic facility employing optical, magnetic, and radio wave instrumentation. The extensive set of scientific research instruments envisioned for this facility will also be valuable for observing naturally-occurring ionospheric and auroral processes when artificial heating experiments are not being conducted.

One of the instruments being considered for the HAARP diagnostic facility is an imaging riometer, based in part on the design described by Detrick and Rosenberg [1990]. This paper describes the prototype 16-beam riometer system, forerunner to a full-scale imaging riometer diagnostic array, developed by APTI and the University of Maryland and deployed at the HAARP site near Gakona, Alaska (N 62.41, E 214.88, $ L\sim 4.9$). Some examples of naturally-occurring ionospheric disturbances obtained with this prototype system will be presented.

Allan T. Weatherwax