## 3 Results
The blue lines with stars represent the foF2 parameter. For comparison, the gray lines indicate the 27 day moving average value of foF2. On 16 July, Figure 3a shows the geomagnetic storm development during the period 15-17 July 2003. Figure 3c-e show that the first obvious increase peak of foF2 appeared at 5 UT on 16 July 2003, when the IMF Bz rapidly turned southward. The southward IMF Bz usually produced the eastward PPEF in the equatorial daytime ionosphere. It is likely that the enhanced eastward electric field causes an equatorial fountain effect, transporting plasma from the equatorial region to higher altitudes and latitudes [44]. However, at the  
Figure 2: Geophysical conditions during the period 15–17 July 2003: (**a**) solar wind velocity, (**b**) IMF Bz, (**c**) AE index, (**d**) SYM-H index, and (**e**) Kp index. IMF = interplanetary magnetic field. higher latitude WK545 station, no enhancement of foF2 was observed at the same moment as shown in Figure 3b, suggesting that the range of latitudes affected by this PPEF was limited. After that, Figure 3b-e show that foF2 started to increase at \(\sim\)7-9 UT with a large amplitude compared with that on the reference day, showing a positive storm effect. It seems that the foF2 started to increase first at the lower latitude station, especially in Figure 3c-e. The enhancements of foF2 lasted for more than 10 h from the main phase to the recovery phase of this storm. Then, the decrease in foF2 occurred at \(\sim\)1 UT on 17 July, and this negative storm effect lasted for almost the entire day. Figure 4b-e also present the existence of spread F marked as light purple stars, as recorded by ionograms with 1 h intervals. Here, we did not distinguish the types of spread F. To our surprise, spread F signatures were observed at all four ionosondes every night during the period 15-17 July 2003. After storm onset, Figure 3b-e show that the durations of spread F at WK545, TO536, YG431, and OK426 were 14-21 UT (23-06 JST), 19-23 UT (04-07 JST), 18-24 UT (03-08 JST), and 14-22 UT (23-06 JST) on 16 July, respectively. Spread F at WK545 and OK426 occurred slightly earlier than midnight, while spread F at TO536 and YG431 occurred after midnight. The above spread F tended to occur at 18-24 UT (03-08 JST), which is persisted until the next morning. On 15 and 17 July, spread F tended to mainly occur at 12-18 UT (21-03 JST) and remained continuous, except for the occurrence at YG431 and OK426 where the duration of spread F was not completely continuous on 15 July. Although few spread F lasted into the morning, such as at OK426 on 15 July, they disappeared no later than 06 JST, and slightly earlier than that on 16 July. Figure 3: **(a)** The SYM-H (blue line) and IMF Bz (orange line) indices show geomagnetic storm development during the period 15–17 July 2003. Variations in the F2 layer critical frequency (foF2) at **(b)** WK545, **(c)** TO536, **(d)** YG431, and **(e)** OK426 during the period of 15–17 July 2003, as measured by ionosonde. Gray lines represent the 27-day moving average value as a reference. The light purple stars indicate the spread F obtained from ionograms. Note that JST = UT + 9. To facilitate the study of the evolution and occurrence characteristics of spread F, we further present ionograms with 1 h intervals. Usually, spread F is identified as four types: frequency spread F (FSF), range spread F (RSF), mixed spread F (MSF), and strong range spread F (SSF) [45; 46; 47]. FSF in the ionograms appears as the intensity of scattered echo traces increased with frequency, accompanied by the failure to identify foF2. RSF is scattered echo traces at lower frequency with clear foF2. MSF is a mixture of FSF and RSF. SSF is described as an extended range spread on F region echoes traces that significantly extend beyond the local foF2 value [45]. Combined with the statistical results in Figure 3, Figure 4 first illustrates the evolution of spread F at four ionosondes during the period 17-22 UT (02-07 JST) on 16 July 2003.