High resolution topside in situ data of electron densities and VHF/GHz scintillations in the equatorial region

Abstract

Coordinated in situ measurements of high‐resolution (spatial resolution 35 m) irregularity structures in the topside ionosphere using the retarding potential analyzer (RPA) on the AE‐E satellite and scintillation measurements using the geostationary satellite Marisat transmissions at 257 MHz and 1.54 GHz from Ascension Island were made in December, 1979 during the recent solar maximum period. The in situ irregularity spectra could be classified into two categories: those having or not having spectral breaks. The first category of spectra is characterized by two spectral indices: one above and the other below a scale‐length that lies typically between 500 m and 1 km. The spectral index in the long scale‐length end (10 km to ∼1 km) varies between −1 to −1.5, whereas the index in the short scale‐length end (∼1 km to 70 m) is between −3 to −3.5. In the second category, the power law spectral index of the irregularities ranges between −2 to −2.5 over the entire scale‐length range of 70 m to 10 km. The in situ measurements reveal that in the early evening hours, the E‐W gradient scale‐lengths can be as small as 35 m, the sampling interval of the RPA instrument. In the pre‐midnight and post‐midnight hours, the gradient scale length becomes larger, but scale lengths of several hundred meters are frequently encountered. The temporal variation of 1.54‐GHz scintillation magnitudes are found to track the E‐W bubble structure, scintillations being minimum when the ray traverses the center and large when it crosses the eastern and western walls of the bubble. On the other hand, scintillations at 257 MHz remain saturated during an encounter with a bubble and focussing is often observed, but the autocorrelation interval tracks the bubble structure, being large when the ray path is deep within the bubble where the perturbation is weaker. The power spectra of weak 1.54‐GHz scintillations are found to be consistent with the predictions of weak scatter theory based on irregularities having the observed in situ spectra. Significant differences are noted in the spectra of strongly scattered 257‐MHz scintillations. In this case the decorrelation bandwidth extends beyond 1 Hz, and the spectral slope of the transitional portion of the spectrum becomes as steep as −6. The moderate to strong GHz scintillations sometimes show a dual slope power spectrum with the lower frequencies exhibiting a slope of −1.5 and the higher frequencies showing a slope of −5.5. The observations of steep structures in irregularities with scale‐length of a few hundred meters near midnight, implying relatively slow erosion of sharp gradients are discussed in terms of plasma processes in the topside ionosphere.