The use of an imaging riometer, as illustrated in Figure 1, provides a diagnostic technique that has the potential to measure critical ionospheric heating parameters. As shown here, an imaging riometer consists of an array of independent receiving antennas, whose signals can be phased and combined to measure the background cosmic noise radiation at a specific frequency. The pointing capability and beam resolution of the phased array allow the riometer system to collect radiation incident from a conical region of the atmosphere. The level of conductivity modulation due to heating within this region can be deduced from the modulation in the received energy. Spatial resolution of the heated region can be achieved by forming multiple simultaneous beams, thus forming an image over the region of interest.
The HAARP Imaging Riometer Diagnostic, as originally proposed, would consist of a 256-element antenna array 16x16 and a Butler matrix phasing system (e.g., see Detrick and Rosenberg ). Of the 256 beams possible with this design, it was proposed to use the central-most 164, covering an angular field of view extending approximately 60 from the zenith. Figure 2 shows the projection of the antenna pattern onto a flat ionosphere at a height of 90 km above the surface, the approximate altitude where HF radiation from the HAARP transmitter is efficiently absorbed. The angular field of view (to the -3 dB locus) of an individual beam is approximately 6.7, somewhat larger than the approximately 5 angular radiation pattern of the RF heater.
It is important to sample the heated volume rapidly and with high time resolution in
order to follow the ionospheric response to the rapid rise and fall times of the heater pulses.
To accomplish this, each of the 164 beam-forming outputs of the Butler matrix would be
sampled by a dedicated fast-response receiver (operating at 38.6 MHz), specifically designed
for the purpose. Several operating modes would be provided to obtain rapid continuous (1
ms) or synoptic (0.1 ms) sampling of single beams or small clusters of beams during heating
experiments; at other times, the full array would be
sampled continuously at lower resolution (1 s).
A prototype imaging riometer system, described
in the next section, was designed, constructed and installed at Gakona, Alaska.