Scale analysis of pre- and post-midnight ESF bubbles at storm time and quiet time

This paper adopts a scale analysis technique to investigate the properties of intermediate-scale plasma structures observed by ROCSAT-1 in the equatorial F-region. A procedure of scale analysis that is developed via the empirical mode decomposition (EMD) method of Hilbert–Huang transform (HHT) technique allows the mutually correlated components in velocity, density and relative density gradient to be identified and extracted. Comparing the three parameters, good match in wave form is found for density and velocity in the scales between kilometers and hundred meters (few kilometers to 300 m). It implies that there are electric fields proportional to density fluctuation −δn/n in the form similar to what is expected for the generalized Rayleigh–Taylor instability. We find that such a one-to-one match holds for various pre- and post-midnight ESF bubbles during quiet and storm times. It, therefore, means that spatial structures of electric field in the intermediate-scale (300 m to few kilometers) correlates to the density structures in a manner of δE ∝  −δn/n that is not necessarily depending on the driving mechanism of ESF bubbles, although it is known that ESF bubbles can be driven by different mechanisms under different space weather conditions. In smaller scales (300–50 m), fluctuation patterns of density and velocity do not correlate to each other any more, the good match is then found in the density gradient ∇_xn/n and velocity. It is known as the manifestation of the Boltzmann relation. We note that the GRT instability related relationship δV_z ∝  −δn/n for irregularities in scale of kilometers holds only for ESF bubbles that occur within ±5 dip latitude, while the Boltzmann relation (δV_z proportional to ∇_xn/n) holds for small-scale irregularities without such a limitation.