# Mechanisms Underlying the Changes in Sporadic E Layers During Sudden Stratospheric Warming
## 3 Results  
### Changes in the Sporadic E Layer During the 2008/2009 Winter SSW  
As shown in the Figure 2a,b, during the winter of 2008/2009, the average temperature at 10 hPa and 90\({}^{\circ}\) N exhibited a sharp increase starting on 16 January 2009, reaching a peak on 23 January 2009, and subsequently returning to pre-warming levels over the ensuing 30 days. Concurrently, the average zonal wind at 10 hPa and 60\({}^{\circ}\) N showed a notable decrease. At the peak temperature on 23 January, the predominant eastern winds shifted to westerly, reaching a maximum speed of approximately 30 m/s. The observed changes in average temperature at 10 hPa and 90\({}^{\circ}\) N, along with alterations in the average zonal wind at 10 hPa and 60\({}^{\circ}\) N, characterized this event as a significant SSW event. According to Figure 2c,d, the kp index remained below 4.5, while the F10.7 index fluctuated between 67 and 72 sfu, indicating that the influence of solar and geomagnetic activities on the sporadic E layer during this SSW event can be disregarded. Figure 3 illustrates the daily average foEs variations at Okinawa, Yamagawa, and Kokubunji stations during the winter of 2008/2009. The red dashed line indicates the onset  
Figure 2: (a) Average temperature (T) at 10 hPa and 90\({}^{\circ}\) N during the winter of 2008/2009, (b) average zonal wind speed (Uwind) at 10 hPa and 60\({}^{\circ}\) N, (c) solar activity index (F10.7), (d) geomagnetic activity index (Kp). of the warming, while the blue dashed line marks the transition from easterly to westerly winds. Analysis of Figure 3 reveals that prior to the SSW, the daily average foEs at the three stations remained stable. From the onset of the SSW to the wind reversal, changes in daily average foEs were not significant. However, a noticeable increase in daily average foEs occurred following the wind shift from east to west. Post this transition, the maximum values of daily average foEs at Okinawa, Yamagawa, and Kokubunji reached 4.5 MHz, 4.2 MHz, and 4.3 MHz, respectively, representing increases of 40.47%, 38.98%, and 48.09% relative to the average values prior to the SSW. Notably, following the shift from easterly to westerly winds, the increases in daily average foEs at these stations manifested in two distinct surges, rather than continuously, indicating a periodic pattern. To confirm the presence of this periodicity, a wavelet transform was applied to the daily average foEs data from 9 January to 28 February 2009. The outcomes are illustrated in Figures 4 and 5, where vertical white dashed lines mark the transition from easterly to westerly winds, and horizontal white dashed lines indicate the y-coordinates corresponding to the peak values within the structures depicted in the figures. Analysis of Figures 4 and 5 reveals that subsequent to the wind shift from east to west, the daily average foEs at all three stations exhibited oscillations with approximately 6-day and 16-day periodicities. The oscillations of the 6-day cycle persisted for about 20 days, whereas those of the 16-day cycle commenced following the wind shift and continued through to the end of the winter season. Notably, at the Yamagawa station, the oscillations of the 6-day cycle occurred twice. Figure 3: Daily average foEs variations at Okinawa, Yamagawa, and Kokubunji during the winter of 2008/2009. Red dashed lines indicate the onset of warming, and blue dashed lines mark the transition from easterly to westerly winds. As previously mentioned, tidal winds significantly contribute to the formation of sporadic E (Es) layers, displaying periodic characteristics with cycles of 24, 12, 8, and 6 h. Additionally, variations in the intensity of sporadic E layers, spanning from 2 to 16 days, have been linked to planetary waves [26; 27]. Research conducted by the authors of references [28; 29] identified significant periodic disturbances in the summer coherent backscatter echoes captured by HF and VHF radars, which they attributed to the effects of lower atmospheric planetary waves.