The temporal structure of intensity scintillations near the magnetic equator

Abstract

The temporal structure of weak and strong intensity scintillations observed with a geostationary satellite at 249 MHz near the magnetic equator is studied. It is shown that for weak intensity scintillations, the power spectra exhibit an asymptote immediately beyond the Fresnel frequency, which signifies a power law variation of power spectral density with frequency yielding spectral indices ranging between −3 and −3.5. By the use of simultaneous irregularity drift measurements, it is found that the power law portion of the spectrum for weak scintillations is caused by F region irregularities in the scale length range of about 700 m to 100 m. On the other hand, for strong intensity scintillations, the asymptote signifies a somewhat higher spectral index ranging between −3 and −4.5. Considering both weak and strong scintillations, an average spectral index of −3.5 is obtained which is found to be consistent with the average one‐dimensional in situ spectral index for irregularities with scale lengths less than 1 km. In addition, it is shown that for strong intensity scintillations, a substantial decrease of the correlation time occurs which is dictated not only by the increased strength of scattering but by the increased irregularity drift as well. At 249 MHz the shortest correlation length transverse to the propagation path is found to be about 70 m near the magnetic equator. This is a factor of 5 larger than the corresponding value obtained away from the equator near the crest of the equatorial anomaly at Ascension Island.