High Frequency Active Auroral Research Program imaging riometer diagnostic

This paper describes the prototype 16-beam, 38.6-MHz riometer system develo ped by Advanced Power Technologies, Inc., and the University of Maryland fo r the High Frequency Active Amoral Research Program (HAARP). The prototype system is the forerunner for a full-scale imaging riometer diagnostic instr ument for characterizing the ionospheric volume perturbed by controlled rad io frequency (RF) heating experiments. The prototype system, installed at t he HAARP site near Gakona, Alaska, consists of a 1 x 16 antenna array phase d in one dimension (beam width of similar to 6.7 degrees) and oriented appr oximately along the magnetic meridian. The system responds sensitively to n atural variations of amoral absorption, such as those caused by magnetosphe ric substorms, and provides clear evidence of its capability to discern spa tial structure and motion. A newly observed feature seen near dusk are inte nse, short-duration absorption spikes accompanied by only weak magnetic sig natures. Because the HAARP heating facility is still a work in progress, th ere have been only limited opportunities to conduct science investigations with it. Consistent with estimates, heater-induced effects in the ionospher e have not been observed with the riometer at the power levels currently av ailable. It has been noted that the proximity of the prototype system to th e HAARP RF heater can result in interference to the riometer signal for som e frequencies and operating modes of the heater. The full-scale imaging cap ability of the proposed instrument, and a remote location, may be required to detect small-scale modifications of the ionosphere caused by the heater.