Modeling of the seasonal effects of geomagnetic storms in...

2. Analysis of the Experimental Data

[5] The data of the network of ionosondes and digisondes located in the longitudinal sector 90^o-130^o E were used for the analysis (see Table 1). We studied variations in the critical frequencies of the F2 and E_s layers and also in the heights of the F2 -layer maximum during a storm, including the preliminary and recovery phases. The hourly values of f_oF2, h'F, and h_maxF2 averaged over several quiet days of the month were used as the quiet level. The storms observed from May 2003 to march 2004 were considered. During this period, 15 storms were detected and split into seasons. The performed morphological analysis showed that there exist differences in the manifestation of the ionospheric response in different seasons.

Figure 1

2.1. Summer Conditions

[6] Figure 1 shows the variations in f_oF2 during a typical summer storm. The illumination conditions determined the weakly pronounced diurnal behavior at high-latitude Norilsk station with a slight decrease before the midnight. At stations from Zhigansk to Manzhouli, the quiet diurnal behavior of f_oF2 had a maximum around the midnight and a minimum in the morning hours of the local time. With a decrease of latitude, the difference between the minimum and maximum values of f_oF2 increased. At more southern stations, a well pronounced diurnal behavior with a maximum and a minimum in the afternoon and morning hours, respectively, was observed. During the storm at latitudes from Norilsk to Yakutsk, the disturbances were negative with the amplitude of 20-50% of the quiet level. The absence of reflections in Norilsk and Zhigansk is caused by the appearance of screening sporadic layers of the r and a types, whereas the negative values of Df_oF2 in the daytime are related to the G condition when the frequency of the F2 layer is close to the frequency of the F1 layer. In Irkutsk and Manzhouli in the minimum of Dst in the evening hours of the local time, a sharp depletion of f_oF2 and the following negative disturbance continued the entire recovery period. At Beijing, the disturbance was similar to the one observed at Manzhouli. At low-latitude stations, the disturbances were mainly positive. The transition from positive to negative disturbances takes place southward from Beijing (geomagnetic latitude about 28.7^o N).

Figure 2

2.2. Winter Conditions

[7] A winter storm with the initial positive phase in the morning LT hours and minimum values of the index Dst = -140 nT is shown in Figure 2. The value of Kp in this period did not exceed 7. In the initial and main phase in the daytime on 22 January, positive disturbances were observed at all stations from the auroral zone to the equatorial anomaly. At night at Norilsk, Zhigansk, and Yakutsk anomalous reflections both in the F and E regions were observed. At the recovery phase, the disturbances in the daytime hours were negative at high and middle latitudes. The higher the station latitude, the longer lasts the recovery of the undisturbed level of f_oF2 (3 days at Norilsk, 2 days at Zhigansk and Yakutsk, and 1 day at Manzhouli). At Zhigansk, Yakutsk, and Irkutsk after the negative phase, the disturbances again became positive. At night at high latitudes, the disturbances were positive, whereas at middle latitudes they were negative. At low latitudes the disturbances were positive both in the daytime and at night during the entire storm. At Khainan during the main phase of the storm, oscillations of f_oF2 were also observed.

Figure 3

2.3. Equinox Conditions

[8] Figure 3 shows a moderate storm with the minimum value Dst = -96 nT on 15 October at 0000 UT. At the development phase of the storm on 14 October at Norilsk and Yakutsk in the afternoon, a sharp decrease in f_oF2 (so called break in the diurnal behavior) was observed. In the evening and nighttime at Norilsk, screening sporadic layers in the E region and auroral absorption prevailed. During the following two days at the recovery phase of the magnetic storm, the ionization in the F2 layer stayed very low (4 MHz at noon). Oblique reflections from the poleward wall of the trough were seen in the ionograms at Yakutsk. The auroral ionization zone was located northward from Yakutsk during the entire period. At stations Irkutsk and Manzhouli, a depletion of the daytime values of f_oF2 during the main and recovery phases of the storm was the most pronounced effect of the 14-16 October storm. At low latitudes in the recovery phase, positive disturbances were observed. The reversal of the sign occurred in the vicinity of a geomagnetic latitude of 30^o N.