## 1. Introduction

[2] As the radio wave propagates through the ionospheric random media, the irregularities scatter the wave, the phase mixing occurs on the path to the receiver. Thus, scintillation phenomena, temporal phase and amplitude fluctuations in the received wave, are observed in the presence of relative motion of the irregularity and the receiver. It is known that the electron density fluctuation in the ionosphere is a direct quantity of concern in the scintillation phenomenon.

[3] Since the study of ionospheric scintillation belongs to the problem dealing with the wave diffraction and propagation through a random media, the scintillation problem can be solved by finding the description that maps a point in the probability space of fluctuating media onto a point in the probability space of wavefield. Although the description in the analytical method is preferred, no such method is available at this moment. Therefore, by relaxing the requirements in the problem, a more modest answer can be achieved in the mathematical manipulation with some approximations [*Yeh and Liu*, 1982]. *Rino* [1979, 2011] has derived a thin phase screen method for case of weak scintillation caused by the propagation of electromagnetic wave through a turbulent media with some approximations such as the forward propagation approximation, far field approximation and weakly inhomogeneous media. While, to describe the wave propagation through the ionospheric media, *Yeh and Liu* [1982] adopted a parabolic equation approach applicable for various levels of scintillation.

[4] In the past, there are some scintillation studies using the in situ measurements to compare the electron density fluctuations in the ionosphere with the ground scintillation observation. In the study of *Wernik and Liu* [1974], the Rytov solution is carried out using the numerical computation of scintillation with the assumed irregularity spectra. *Basu et al.* [1976]used the OGO-6 ion density measurement and the power law phase screen model with some assumed geophysical parameters to derive the occurrence rate of VHF scintillation in winter. In the report of*Wernik et al.* [2007], a scintillation climatological model for the Northern Hemisphere high-latitude region was obtained using the density observations from DE2 satellite and a closed form expression of the phase screen theory of*Rino* [1979]. The methodology of *Wernik et al.* [2007] was modified by *Liu et al.* [2012]in the study of scintillation morphology in the low-latitude region using the ROCSAT-1 data taken during the years of 2000 to 2003. In the modified method,*Liu et al.* [2012] was able to obtain additional parameter of the outer scale distribution for the irregularity structures. In *Franke and Liu* [1983], the multifrequency scintillations measured at the Ascension Island were used to derive the irregularity model from the numerical simulation under the assumption of weak scintillation. The scintillation spectrum derived from the two component model of the irregularities was shown to be consistent with the spectrum from the scintillation measurement.

[5] Therefore, it is realized that ionospheric scintillation phenomena have been investigated using two approaches in the past. One approach depends on the measured ionospheric electron density distributions. Numerical computations of radio waves propagating through an ionosphere modeled by the measured electron density distributions are used to simulate the scintillation pattern observed on the ground. The second approach uses the scintillation signals recorded on the ground at different locations to derive the statistical properties of the ionospheric irregularities. It is quite obvious that better test of the ionospheric scintillation theory can be achieved if we have coincidental observations of electron density in the ionosphere and scintillation signals on the ground.

[6] Through the efforts of many studies in the past, weak scintillation models, phase screen theory and the Rytov solution, have been verified. As the multiple forward scattering effect dominates the fluctuations of both phase and amplitude, the scintillation becomes strong and the weak scintillation model cannot be applied any longer [*Yeh and Liu*, 1982; *Rino*, 1992]. In this paper, we will study one such rare strong scintillation event with the coincident observation of the ionospheric irregularities to test the strong scintillation model. We will use the ionospheric irregularity density observed by ROCSAT-1 at 600-km altitude and the coincident scintillation observations recorded at the Ascension Island to study the characteristics of interaction between the radio waves and the density irregularities that produces the observed ground scintillation data. Moreover, the relationship of frequency dependence on S4, the normalized variance of the signal intensity, predicted by the weak scintillation theory will be studied in the scintillation measurement. The discussion of multiple scattering effects will be presented with the spectral and time series analyses of the ground measurements. Finally, we will use the parabolic equation method (PEM) to simulate the scintillation on the ground from the wave propagation through the observed irregularity structure in space. The result of the simulation study is then used to discuss the assumptions and limitations of the simulation model by comparing with the ground observation.