Self-scattering of the HF heater emissions observed at geographically dispersed receiving sites

Results of investigating the scattering and propagation effects for the HF radiation from the HAARP heating facility (Alaska, USA) are presented. The latest data were collected during the HAARP measurement campaign that was conducted from February 21 through March 3, 2008. The transmissions from the HAARP HF heater were monitored at geographically dispersed sites in the USA, Europe, and Arctic, using digital Doppler receivers of the Institute of Radio Astronomy (Kharkov, Ukraine) and Digisondes of the University of Massachusetts Lowell (Lowell, USA). A number of characteristics features of the detected scattered signals suggest that the received radiowaves were scattered by ionospheric irregularities produced by the same HF emission. This effect is known as "self-scattering." Average relaxation times for the self-scattered signals were shown to be several tens of seconds (40-60 s), while the observed risetime was much longer, up to a few minutes. These numbers are characteristics of the types of the irregularities responsible for the scattering of the heater signal. A time constant of 50-60 s is usually associated with the relaxation time of small-scale irregularities with spatial scale sizes from a few meters to tens of meters. Several possibilities for explaining the observed longer risetime constant of two to three minutes are discussed. The power and spectral width of the scattered signals depend on the HAARP beam orientation and local ionospheric conditions. The possibility of extracting parameters of the stimulated plasma irregularities is discussed.