4. Discussion

[17]  Lots of interesting morphological features have been revealed in our analysis. We suppose that most of them can be explained in frames of the present-day F2 -layer formation mechanism, that is, via variations of solar EUV radiation, neutral composition, winds, and plasmaspheric fluxes. For instance, daytime long-duration Q disturbances seem can be related with the atomic oxygen abundance variations in the thermosphere [Mikhailov and Schlegel, 2001]. A steady increase of N_mF2 values for some successive nights (Figure 2) may be due to the increasing plasmaspheric flux to the nighttime F2 region as similar effect was observed at Millstone Hill and explained in this way [Mikhailov and Förster, 1999]. The effect of low-occurrence probability for daytime Q disturbances (Figure 4) is very similar to the well-known "forbidden time" effect for F2 -layer negative storm commencements [Mednikova, 1957; Prölss and von Zahn, 1978]. The effect is related to diurnal variations of the meridional thermospheric wind, so its role may turn out to be important in case of Q disturbances as well. Model calculations will be made in future to check possible mechanisms.

[18]  On the other hand, some morphological results may be due to the method of Q disturbances extraction. For instance, it was found that positive Q disturbances were more numerous compared to negative ones. A 27-day running median used in our analysis bears the effects of negative F2 -layer disturbances as their amplitude usually is larger and they are more often compared to positive F2 -layer storms especially at high latitudes. Therefore any normal quiet day is perceived as a positive Q disturbance, while F2 layer should be strongly modified to mark a given day as a negative Q disturbance event. Thus the situation is not symmetric with respect to the two types of disturbances. We have tried to avoid this effect dealing only with large ( d N_mF2 > 40% ) deviations, but such asymmetry, however, takes place in our results, and it can be at least partly attributed to the method used. However, morphological differences between positive and negative Q disturbances really exist. It appears in seasonal variations of their occurrence (Figures 5 and 6). Negative disturbances distinctly cluster around winter months, while the picture is not that clear and depends on conditions for positive disturbances.

[19]  In our previous analysis [Mikhailov and Schlegel, 2001] we found that both types of daytime Q disturbances had a tendency to cluster around equinoxes. The reason for this difference may be due to the method of the Q disturbances extraction. A rough criterion with daily Ap 12 was applied earlier to select Q disturbances. This allowed some negative D disturbances to appear in the list, but these usual negative F2 -layer perturbations exhibit the largest occurrence in the equinoctial periods due to enhanced geomagnetic activity during equinoxes. A more severe criterion to select Q disturbances was used in the present analysis. This does not abolish the conclusion made by Mikhailov and Schlegel [2001] that daytime negative Q disturbances were due to a decrease in atomic oxygen concentration. Usual negative F2 -layer disturbances are also resulted from O/N₂ ratio decrease [e.g., Prölss, 1995]. Changes of the O/N₂ ratio in this case is due to the [O] decrease and [N₂ ] increase (the latter dominates), while in case of Q disturbances we have solely [O] changes. This difference in mechanisms is clearly seen in Figure 7 (left middle) where the percent of time (proportional to the number of disturbances) sharply increases with latitude for usual negative disturbances while it is practically unchanged for Q perturbations. The former are directly related to the auroral activity while the latter are due to other reasons for [O] changes.

[20]  Daytime positive Q disturbances clustering around equinoxes (Figure 6) can be related to the equinoctial transition in atomic oxygen abundance [Mikhailov and Schlegel, 2001; Shepherd et al., 1999]. The most probable reason for such variations is a change in the global circulation pattern accompanied by vertical motions inferred from observations at E region heights [e.g., Ward et al., 1997].

[21]  Daytime negative Q disturbances demonstrate quite different pattern of spatial variations with the amplitude being practically independent on latitude (Figure 9). Again, a well-pronounced longitudinal difference takes place between European and American sectors (Figure 10, bottom), the latter being less disturbed. It should be noted small longitudinal differences between the two sectors for monthly median N_mF2 values according to the empirical IRI 90 model. So, such perturbations (both positive and negative) should be considered as planetary waves disturbing the N_mF2 longitudinal pattern. It would be interesting to check if these waves are propagating or standing. Unfortunately, it is not easy to do as the mechanism of the F2 -layer formation is different in different LT sectors when different processes play the leading role.

[22]  An analysis of the negative Q disturbance case (Figures 9 and 10) shows that the worldwide pattern is characterized by a general N_mF2 decrease on 6 January, although a 30% longitudinal (America/Europe) difference conserves. Such global N_mF2 decrease could be attributed to a decrease in solar EUV ionizing radiation keeping in mind possible day-to-day variations [Hinteregger et al., 1981]. However, in 2 days, N_mF2 restores to median values in the American sector (Figure 10, bottom) but not in the Eurasian one. Therefore such worldwide variations should be attributed to planetary waves in the upper atmosphere accompanied by changes in neutral winds and composition presumably in atomic oxygen abundance [Mikhailov and Schlegel, 2001]. The effect may be also related to quasi-2-day oscillations in the ionosphere [Altadill and Apostolov, 2001; Apostolov et al., 1995; Chen, 1992; Forbes and Zhang, 1997; Forbes et al., 2000; Rishbeth and Mendillo, 2001], which are connected with quasi-2-day oscillation in mesosphere/lower thermosphere winds.

[23]  Analyzing the F2 -layer variability, Rishbeth and Mendillo [2001] ascribe 15% of the variability to meteorological sources. They as well as Forbes et al. [2000] suggest that meteorological sources of the F -layer variability are comparable to the geomagnetic source (each 15-20% of N_mF2 ) being much larger than the solar component. This is close to the estimations by Mendillo and Schatten [1983], who reported a 13-18% variability of daytime TEC values for magnetic QQ (the 5 quietest days of a month) days.

[24]  Obviously, the meteorological component (impact from below) of the F2 -layer day-to-day variability is a very interesting and challenging problem. As the first step, model calculations are required to specify the quantitative contribution of neutral temperature, composition, thermospheric winds, and electric field variations to the observed Q disturbances and to explain the revealed morphology at the agronomic level at least.