## 1. Introduction

[2] Quasiperiodic (QP) diffraction pattern in the scintillation patches have been observed and discussed by a number of researchers since 1965 [*Turnbull and Forsyth*, 1965; *Ireland and Preddey*, 1967; *Kelleher and Martin*, 1975; *Heron*, 1979; *Davies and Whitehead*, 1977; *Hajkowicz et al.*, 1981]. *Franke et al.* [1984] performed a model simulation based on the diffraction theory of *Titheridge* [1971] with a specially structured boundary. They showed a good interpretation of the quasiperiodic diffraction pattern observed in an equatorial VHF scintillation from plasma bubbles. They have shown that the pattern occurs most often at the beginning or the end of a scintillation patch that are associated with the structured walls (edges) of the plasma bubbles. The pattern is consistent with irregularities having east-west scale sizes of a few hundred meters. By adjusting the parameters in their modeling procedure, they have shown that by matching the modeled QP with the observation, it is possible to estimate some irregularity parameters, such as location, the scale size and the drift velocity of the irregularity.

[3] In this paper, we applied a newly developed time-frequency analysis technique, the Hilbert-Huang transform (HHT) to study the QP pattern embedded in the scintillation signal. The new technique allows us to extract relevant information about the irregularities causing the scintillation directly from the data without relying on any modeling efforts. Hilbert-Huang transform (HHT) developed by *Huang et al.* [1998] allows the nonlinear and the nonstationary data to be decomposed into a finite number of “intrinsic mode functions” (IMFs) via empirical mode decomposition (EMD) method. Instantaneous oscillation frequency can be achieved by applying Hilbert transform to each IMF component. Unlike the traditional analysis by fast Fourier transform (FFT) or wavelet transform (WT), HHT does not need a priori frequency bases; it adopts a posterior basis (the IMFs) that comes from the input data itself and are adjusted automatically according to the nonstationary and nonlinear features of the input data. The resulting HHT spectrum has the advantage that it eliminates the need for spurious harmonics to represent nonstationary and nonlinear signals, no additional assumptions on the bases need to be introduced and therefore can represent the spectrum faithfully [*Huang et al.*, 1998].

[4] Scintillation data often show nonstationary characteristics in its fluctuation pattern. Therefore HHT is very suitable for analyzing such data. The data we studied in this paper are the VHF (244 MHz) scintillation data taken at Ascension Island (7°58′S, 14°25′W, 39°S DIP) from a geostationary satellite FLEETSAT (23 W), recorded by two receiving antenna separated 216 m from each other in a east-west baseline. Studying the scintillation data on 25 March 2000, we noticed certain regular oscillation pattern in a period of a few seconds that appeared before midnight. The ion data taken by ROCSAT-1 at 600 km altitude also show a strong bubble occurred above Ascension Island half an hour early. Although the ROCSAT-1 data do not comprise a coincident observation over the Ascension Island, they implied that plasma bubbles could have occurred somewhere around the area over Ascension Island. In the following, we will first use EMD to separate the quasiperiodic diffraction patterns from the scintillation data. Instantaneous frequencies are computed to construct the time-frequency spectrum. Correlation studies are carried out for the IMF components derived from data recorded at the two receivers. Results of these studies are used to estimate several parameters for the irregularities associated with the bubble.