Abstract
Abstract. The coherence scale length, defined as the 50% decorrelation scale length along the magnetic east-west direction, 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, using amplitude scintillation data recorded by two spaced receivers. The average east-west drift of the ground scintillation pattern, during the pre- and post-midnight periods, also calculated from the same observations, shows an almost linear 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 solar flux except for cases with F 10.7-cm solar flux <100. This indicates that, during the initial phase of ESF irregularity development, the irregularity spectrum does not have much variability while further evolution of the spatial structure in ESF irregularities is controlled by factors other than the solar flux.
Highlights
IntroductionA method has been devised for determining the spatial scales present in the ground scintillation pattern from scintillation data recorded by two spaced receivers (Bhattacharyya et al, 2003)
The generation and evolution of equatorial spread F (ESF) irregularities through plasma instabilities (Haerendel, 1973; Ossakow, 1981; Keskinen et al, 1998, 2003; Bhattacharyya, 2004) depends on ambient ionospheric conditions present in the equatorial and low-latitude regions
Use is made of the fact that VC has nonzero values, and variations in V0 are automatically taken into account in the calculation of coherence scale dI, so that these may be directly compared with results obtained in theoretical calculations by Engavale and Bhattacharyya (2005). These authors found that for a given average height of the irregularity layer and weak scintillations, dI increased with increasing value of the irregularity power spectral index
Summary
A method has been devised for determining the spatial scales present in the ground scintillation pattern from scintillation data recorded by two spaced receivers (Bhattacharyya et al, 2003). Estimate the distribution of spatial scale sizes present in the ground diffraction pattern for periods of weak scintillations during a scintillation event. Variation in the functional form of CI (x, 0) determines the distribution of dominant spatial scale size dI in the ground scintillation pattern as the ESF irregularities evolve. The background ionospheric conditions show a great deal of variability from one month to another, which would give rise to different patterns of evolution of the irregularities during these periods This is expected to yield different distributions of spatial scale sizes in the ground scintillation pattern in different seasons. Average values of V0 during pre- and post-midnight periods are plotted as functions of the 10.7-cm solar flux in Figs. 8 and 9, respectively
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