2. Data Analysis

[3]  The main morphological analysis was made over 26 ionosonde stations located in the Eurasian sector using all observations available (Table 1). As the morphology of Q disturbances is expected to depend on latitude, all the stations in accordance with their invariant latitude were conventionally divided in high-latitude (six auroral and subauroral stations), midlatitude (12 stations), and lower-latitude (eight stations) ones. A 27-day f_oF2 running median centered for the day in question rather than usual monthly median was used for the Q disturbance analysis. On one hand, a 27-day running median looks more natural as this period equals to one solar rotation; on the other hand, this saves us from large and unreal disturbance effects in the beginning and in the end of a month as well as at the junction of 2 months especially during the equinoctial periods when changes in the thermosphere and ionosphere are very fast. The advantage of using running f_oF2 median for F2 -layer disturbance analyses was stressed long ago [e.g., Mednikova, 1957]. Q disturbances were referred to hourly ( N_mF2/N_mF2 _med -1 ) deviations more than 40% if all 3-hour Ap indices were 7 for 24 previous hours. This assumption is based on the empirical estimation of the ionosphere reaction to the forcing geomagnetic activity. Some estimates of this time constant for midlatitude F2 region are 0-6 hours for positive disturbances [Zevakina and Kiseleva, 1978b], 12 hours [Wrenn et al., 1987], 15 hours [Wu and Wilkinson, 1995], 6-12 hours [Forbes et al., 2000]; 16-18 hours [Kutiev and Muhtarov, 2001], and 8-20 hours depending on season [Pant and Sridharan, 2001]. Three levels of solar activity were considered using 12-month running mean sunspot number: solar minimum R₁₂ < 50, medium R₁₂ = 50-100, and maximum R₁₂ > 100. The total number of Q disturbances found depends on the latitude of a station and the period of observations available. For instance, at Slough with the longest period of observations the number of negative (257) and positive (1050) Q disturbances. At Arkhangelsk, located in the auroral zone but with a short period of observations, the corresponding numbers are 225 and 667 (see also section 3.1)