Sweepers, Communications and Solar Activity

Sweepers, wide-band noise pulses found in the HF range, have been observed at many locations on the globe. They display a changing frequency vs time characteristic. Two types, described as alpha and beta, have been noted. The duration of the beta type is so short that as it sweeps through 500-kc bandwidth its time rate of change of frequency exceeds the time resolution of the equipment. It appears singly or as multiples but most commonly as a double or triple event. The gross features of beta sweepers appear similar to those of atmospherics. Alpha sweepers display a measurable rate of change of frequency with time. They appear in trains; no multiple has yet been found. Alpha sweepers may result from man made or industrial processes, or may arise from solar or cosmic plasma interactions. In appearance on the f-t plane they resemble some solar radio noise bursts. Both sweeper types constitute interference to radio wave communications, especially to wideband systems. Measured sweeper intensities have varied from microvolts to millivolts at a bandwidth of 500 kc. HF sweeper intensities are modified by propagation conditions, distance from the source, solar activity, etc. For example, during the peak of solar activity both alpha and beta sweepers at 26 Mc would propagate more efficiently because of the higher existing MUF.