at a 50% duty cycle. A 2 msec pulse length and a 4 msec IPP was chosen for the observations. Azimuth scans using the 430 MHz incoherent scatter radar yielded no detectable cases of HF-induced irregularities in a heated E region. An upper limit for heater-induced irregularities was experimentally set at 5% in amplitude for irregularities having spacial scale sizes greater than 5 km. In an effort to detect field-aligned irregularities having dimensions of 3 m across the earth's magnetic field lines, a portable 50 MHz radar was set up on the island of St. Croix and operated in a backscatter mode. The 50 MHz radar was pointed in a direction perpendicular to the earth's magnetic field at an altitude of 105 km above the HF heating facility. The operation of the St. Croix radar was coordinated with measurements made at the Arecibo Observatory. During the St. Croix observations, the HF transmitter was restricted to pulsed operation at a maximum duty cycle of 50%. No heater-induced signal returns were apparent on an A-scope monitor, which was viewed while the 50 MHz radar was operated. On the basis of A-scope observations, an upper limit of 1 m2 has been set for the total scattering cross section for irregularities produced by HF waves penetrating the E region. The sensitivity of the experiment to scattering will be increased 2-3 orders of magnitude following detailed processing of the data.
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