Solar flux dependence of coherence scales in scintillation patterns produced by ESF irregularities

The coherence scale length, defined as the 50% decorrelation scale length along the magnetic east-west di- rection, in the ground scintillation pattern obtained at a dip equatorial location, due to scattering of VHF radio waves by equatorial spread F (ESF) irregularities, is calculated, us- ing amplitude scintillation data recorded by two spaced re- ceivers. The average east-west drift of the ground scintilla- tion pattern, during the pre- and post-midnight periods, also calculated from the same observations, shows an almost lin- ear increase with 10.7-cm solar flux. In the present paper the variability of the drift is automatically taken into account in the calculation of the coherence scale length of the ground scintillation pattern. For weak scintillations, the coherence scale depends on the Fresnel scale, which varies with the height of the irregularity layer, and also on the spectral index of the irregularity power spectrum. It is found that for weak scintillations, the coherence scales are much better organized according to the 10.7-cm solar flux, during the pre-midnight period, than during the post-midnight period, with a general trend of coherence scale length increasing with 10.7-cm so- lar flux except for cases with F 10.7-cm solar flux <100. This indicates that, during the initial phase of ESF irregu- larity development, the irregularity spectrum does not have much variability while further evolution of the spatial struc- ture in ESF irregularities is controlled by factors other than the solar flux.