3. Morphological Results

Figure 1

[4]  In the beginning, we present some examples of Q disturbances to get an idea of how they look in comparison with usual F2 -layer storm effects. A strong daytime negative disturbance on 23 April 1980 is shown in Figure 1 (top). Note that only daytime period was subjected to the N_mF2 decrease, while N_mF2 values for the whole previous day and nighttime hours of 23/24 April are close to the median. Some residual effect takes place on the next day, 24 April, and again during daytime hours only. Another interesting feature of this type of disturbances is h_mF2 variations calculated using the expression by Bradley and Dudeney [1973]. Unlike usual negative F2 -layer storm effect when h_mF2 always increases, in this case, h_mF2 turns out to be close to the median values.

[5]  A long-duration positive Q disturbance effect is shown in Figure 1 (bottom). A pronounced positive N_mF2 effect lasts for some days both during daytime and nighttime hours. Similar to the previous case, h_mF2 variations are very close to the median values, and this is not observed for a normal F2 -layer positive storm effect.

Figure 2

[6]  Positive (Figure 2, top) and negative (Figure 2, bottom) nighttime Q disturbances are shown in Figure 2. Note that the effect appears only during nighttime hours, while daytime N_mF2 values coincide with the median. An interesting feature of the positive effect (Figure 2, top) is a steady N_mF2 increase for three nights followed by a sharp N_mF2 decrease to the median value on 12 January, although the geomagnetic conditions conserved at a quiet level. Such behavior was revealed for some other cases. Similar tendency with a steady N_mF2 increase for three nights is seen for a negative Q disturbance case (Figure 2, bottom).

[7]  Let us consider morphological results obtained over all stations and periods of observations available.

3.1. Duration and Total Number of Disturbances

Figure 3

[8]  Distributions for the occurrence of positive and negative Q disturbances versus their duration are shown in Figure 3 for high-latitude Lycksele and low-latitude Ashkhabad stations. All levels of solar activity were put together. Short-term ( <3 hours) deviations are seen to be the most numerous, and they may be attributed to short-term ionosphere fluctuations which lie beyond our scope. We are interested in longer disturbances which can be related to background changes in thermospheric parameters. A 3-hour (4 hourly successive f_oF2 values) threshold was accepted for our analysis. The distributions are seen to be broader at high latitudes; that is, the percentage of long (both negative and positive) disturbances increases with latitude. This latitudinal dependence is shown in Table 2. Positive disturbances are seen to be more numerous than negative ones at all latitudes.

[9]  The dependence on solar activity level was analyzed for Slough station by selecting 10 years of solar maximum and 10 years of solar minimum. The total number of cases (solar maximum/solar minimum) is (144/280) for positive disturbances and (38/92) for negative ones. It is seen that (1) positive Q disturbances are more numerous at any level of solar activity and (2) both types of disturbances are more numerous (by 2 times) at solar minimum.

3.2. Occurrence Versus Local Time

Figure 4

[10]  Distributions of the occurrence for negative and positive Q disturbances versus local time (LT) are given in Figure 4. The stations were grouped in accordance with their latitudes as mentioned earlier. Although there is some dependence on latitude, in general, both types of disturbances are the most frequent in the evening and night-early morning LT sectors, and they are rare during daytime. Similar results were obtained earlier for usual negative F2 -layer storms [Mednikova, 1957; Prölss and von Zahn, 1978]. Partly, this is due to the method used as the deviations of > 40% are not frequent in daytime when N_mF2 are large. However, this effect also may have physical explanation as it takes place for usual negative F2 -layer storms [Prölss, 1995]. Therefore three LT time intervals will be considered further in our analysis: daytime (0900-1500 LT), evening (1600-2200 LT), and nighttime-early morning (0100-0400 LT) sectors. A well-pronounced nighttime peak takes place for negative disturbances at high latitudes. The evening peak is forming earlier at lower and later at higher latitudes in case of negative disturbances. Midlatitude stations exhibit the maximal early morning peak for positive disturbances with the decreasing occurrence both to lower and higher latitudes. Contrary to the negative disturbance case, high-latitude stations exhibit the earliest evening peak for positive disturbances. All these morphological features imply different physical mechanisms of their formation.

3.3. Seasonal Dependence

Figure 5

[11]  Seasonal dependence for the occurrence of negative Q disturbances is shown in Figure 5 for daytime (0900-1500 LT) and evening (1600-2200 LT) sectors where the occurrence frequency is the minimal and the maximal, correspondingly. All solar activity levels were combined for daytime hours (Figure 5, left) as the total number of disturbances is small. However, it was possible to consider separately three solar activity levels for the evening hours (Figure 5, right). Negative disturbances are seen to cluster around winter months (November-January) at high and middle latitudes in both LT sectors for all solar activity levels. The pattern is somewhat different for lower-latitude stations. No seasonal dependence takes place for solar medium and minimum in the evening sector, the number of cases being sufficient. There also exists a summer increase of the occurrence in the daytime sector. However, in general, we may conclude that winter season is the most preferable for negative Q disturbances and the revealed stability of the seasonal pattern may help understand physical mechanism of the effect.

Figure 6

[12]  Figure 6 gives seasonal dependence for the occurrence of positive Q disturbances for daytime (0900-1500 LT) and nighttime-early morning (0100-0400 LT) sectors. All solar activity levels were put together for daytime hours (Figure 6, left) as the total number of disturbances is small. However, it was possible to consider separately three solar activity levels for the other LT sector (Figure 6, right). Semiannual variations with peaks around equinoxes dominate at high and middle latitudes in the daytime sector, while a well-pronounced summer peak takes place at lower-latitude stations. Seasonal variations are different at different latitudes in the nighttime sector (Figure 6, right). All levels of solar activity demonstrate a pronounced summer peak in the occurrence at high-latitude stations. A very large May peak (106 points of 347) takes place at middle latitudes at high solar activity. On the other hand, no seasonal variations were found at medium and low solar activity at midlatitude stations. No pronounced seasonal variations in the occurrence were revealed at low latitudes. Summarizing one may conclude that the seasonal variation pattern for positive Q disturbance is more complicated and less systematic compared to the negative Q disturbance case. This may tell us that some processes are responsible for the seasonal variation pattern and their contribution varies with geophysical conditions.

3.4. Spatial Variations

Figure 7

[13]  The available set of stations allows us to consider spatial variations of some parameters. Figure 7 gives latitudinal variations of the percent of time occupied by disturbances in three LT sectors. This parameter is related to the number or occurrence frequency of the disturbances. Along with Q disturbances all observed F2 -layer perturbations are considered for a comparison. Only disturbances with d N_mF2 > 40% and duration 3 hours are included. The F2 -layer perturbations marked "All", in fact, present D disturbances related to geomagnetic activity as the share of Q disturbances is small in the total number of perturbations. Polynomial approximation of the variations is made for the sake of obviousness. The variations of D and Q negative disturbances are seen to be quiet different (Figure 7, left). D disturbances demonstrate large and well-pronounced latitudinal variations, but very small (especially during daytime) latitudinal changes take place for Q disturbances. Obviously, this tells about different mechanisms of their formation. The picture is different for positive Q disturbances (Figure 7, right). The character of their latitudinal variation is similar to the D disturbance ones especially in the evening and early morning LT sectors. This tells us that mechanisms of their formation may be similar.

Figure 8

Figure 9

[14]  Figures 8 and 9 give 2-D plots for the amplitude ( N_mF2/N_mF2 _med averaged over 1100-1400 LT time interval) of strong positive (6-9 April 1973) and negative (6-8 January 1970) Q disturbances. All available over the Northern Hemisphere, middle- and high-latitude ionosonde observations were included. Invariant latitudes were used in Figures 8 and 9, but similar results are obtained with geodetic latitudes as well. An obvious difference is seen between the two cases, in particular in the Eurasian longitudinal sector where the number of stations is sufficient. The positive disturbance exhibits mostly latitudinal variations for the amplitude, the latter increasing with latitude. On the contrary, the negative disturbance demonstrates mainly longitudinal variations with the amplitude slightly varying with latitude (compare Figure 7). This difference in the amplitude variations was stressed earlier by Mikhailov and Schlegel [2001] for other cases of positive and negative Q disturbances.

Figure 10

[15]  The 2-D plots were used to analyze longitudinal variations of the amplitude along the F _inv = 60^o latitude. The points were read from the 2-D plots with a step of contour lines and then approximated by a polynomial (Figure 10). In case of the positive disturbance, besides latitudinal variation of the amplitude clearly seen in the Eurasian sector (Figure 8), pronounced longitudinal variations take place especially at high latitudes. The disturbance looks like a wave with the latitudinal increasing amplitude. A relatively stable minimum of the amplitude takes place in the American sector, while the maximum is observed in the Eurasian sector. The peak is seen to move back and forth in its day-to-day variations (Figure 10, top). The front of this wave may be very steep as on 6 April or gently sloping as on 7 April. In the western European sector (where the number of stations is sufficient) the disturbance is seen to be absent on 6 April as it is located a little to the east. In 2 days (8 April) the disturbance covers Europe, and its amplitude reaches the maximum and then starts to decrease on 9 April (Figure 10, top). So this wave demonstrates a complex spatial structure.

[16]  In case of negative disturbance on 6-8 January 1970 (Figure 9), there is practically no latitudinal dependence for the amplitude in any longitudinal sector. Again the disturbance looks like a planetary wave with the minimal deviations located in the American sector and the maximal amplitudes in the European sector. In this case unlike the previous one, the ionosphere seems to shift as a whole simultaneously at all latitudes in a given longitudinal sector, and this is a principle differences between the two types of disturbances. In both cases a pronounced longitudinal difference between the two sectors takes place: the American sector is less disturbed compared to the European one. An additional analysis is needed to check whether this is a propagating wave with a period of one day or a standing one with varying day-to-day positions of its extremes. Although the number of stations available is small in the Western Hemisphere, the difference between the European and American sectors is obvious, and it should be stressed: the disturbance effect is less pronounced in the Western Hemisphere.