In situ measurements of the spectral characteristics ofFregion ionospheric irregularities

The retarding potential analyzer aboard Ogo 6 has provided high-resolution observations of the ion concentration along the satellite path. Changes in ion concentration as small as 0.03% and at times as small as 0.01% could be measured. Spatial resolution varied from 35 to 380 m. Samples of data have been analyzed to determine the spectral properties of the F region irregularities observed. The most common frequency spectrum observed follows to a very good approximation a power law of the form Aαf−nαSn, where A is the irregularity amplitude, f is the observed frequency, and S is the irregularity scale size. The spectrum was measured over the scale size range 70 m to 7 km, and the values of n obtained were close to 1, the average value being 0.95. The spectral index n is found to be insensitive to irregularity amplitude. This type of spectrum suggests that the irregularities in this scale size range result from the turbulent dissipation of larger irregularities. At the equator the larger irregularities are probably produced by convective electric fields. At high latitudes, electric fields may also be involved, but other factors such as precipitating particles may contribute to, or be primarily responsible for, the production of large irregularities. Examples of other types of spectra associated with wavelike irregularities and with ‘ground glass’ (high-frequency noise) irregularities are also shown.