A 2.5-km long slewable broadside receiving-antenna array was used to study ionospherically propagated backscatter from the Pacific Ocean. Swept-frequency continuous-wave recordings revealed a new type of amplitude variation, periodic in both time delay and radio frequency, which caused recordings of time delay versus frequency to resemble large thumbprints. This new phenomenon was seen frequently during the fall and winter and also during spring and summer evenings. It is shown that magnetoionic splitting in the ionosphere (polarization rotation) was responsible for a modulation on the received backscatter signals. This concept was verified experimentally by switching the polarization of the transmitting antenna between vertical and horizontal. Computer ray tracing was used to contruct a synthetic backscatter record displaying lines of signal enhancement due to polarization rotation.
When the receiving antenna beamwidth is broadened from 1/2 to 4° the amplitude variation due to polarization rotation is undetectable, which probably explains why the effect was not positively identified in backscatter using F-layer propagation taken prior to the availability of the 2.5-km array. This result is also explained by the use of computer ray tracing. The modulation lines have not been seen to date by the author in recordings of backscatter from the land, probably because of the irregularities of backscattering from land surfaces.