# Mechanisms Underlying the Changes in Sporadic E Layers During Sudden Stratospheric Warming
## 3 Results
### Mechanism of Planetary Wave Modulation on Sporadic E Layers  
Analysis of wind fluctuations from the TIMED satellite revealed that the wind field between altitudes of 92.5 and 115 km exhibited a periodicity similar to that of lower atmospheric planetary waves. However, the fluctuations at these altitudes are not directly caused by planetary waves, as vertical propagation to the mesosphere-lower thermosphere (MLT) region presents significant challenges [39; 40]. The authors of reference [40] conducted a comparative analysis of wind field observations from European meteor radars and mid-frequency radars in the USA with adjacent ionosonde foEs observations. Their analysis suggested an indirect modulation mechanism for planetary waves on sporadic E layers, where planetary waves influence tidal winds, which subsequently impact the formation of sporadic E layers. Critical evidence supporting this theory was found in the period analysis of ionosonde foEs observations, which identified harmonic periods indicative of a nonlinear interaction between planetary and tidal waves. Specifically, power spectral peaks were identified at 11 and 13 h, proximal to the 12-h period, and at 21 and 28 h, close to the 24-h period. To further explore the mechanism of planetary wave modulation of sporadic E layers, wavelet decomposition and reconstruction techniques were employed to isolate the diurnal and semidiurnal tidal components from the foEs data. The amplitude changes in the diurnal tidal component (d_amp) and the semidiurnal tidal component (s_amp) during the winter of 2008/2009 are depicted in Figures 9 and 10. In the mesosphere-lower thermosphere (MLT) region, tidal winds exhibit pronounced diurnal and semidiurnal variations, which influence the intensity of changes in sporadic E layers through the wind-shear process [7; 8]. Consequently, the diurnal and semidiurnal  
Figure 9: (**a–c**) represent the amplitude variations in the diurnal tidal component (d_amp) of sporadic E layers at Okinawa, Yamagawa, and Kokubunji during the winter of 2008/2009. The red dashed lines indicate the onset of the warming event, and the blue dashed lines mark the transition from easterly to westerly winds. tidal components of the sporadic E layers mirror the fluctuations in the corresponding tidal wind components. Figures 9 and 10 illustrate that, prior to the onset of SSW, the diurnal and semidiurnal tidal components of daily average sporadic E layers at the three stations displayed minor fluctuations. Similarly to the changes observed in daily average fOEs, these variations remained subtle up until the wind direction reversed. However, following the transition from easterly to westerly winds, the amplitudes of both the diurnal and semidiurnal tidal components in the sporadic E layers at the three stations began to increase significantly. It is also noteworthy that the amplitude changes in the diurnal and semidiurnal tidal components of sporadic E layers showed marked fluctuations. The wavelet spectra of the diurnal and semidiurnal tidal component amplitudes in the sporadic E layers at the three stations are illustrated in Figure 11. Following the transition from easterly to westerly winds, it is apparent that the amplitudes of both diurnal and semidiurnal tidal components in the sporadic E layers fluctuated, closely aligning with the periods of the quasi-six-day and quasi-sixteen-day waves. Thus, the fluctuations observed in the MLT region's wind field were attributed to planetary wave modulation of the amplitudes of diurnal and semidiurnal tides below the MLT, corroborating the findings of the authors of reference [40]. The modulation of these tides by planetary waves, which subsequently impacted the sporadic E layers, arose from the linear interaction between these waveforms. In addition to modulating the amplitude of diurnal tides, planetary waves have been observed to engage in nonlinear interactions with diurnal tides, as reported by several researchers [41; 42].