Abstract
Abstract. A techique for filling the gaps of the missing F2-layer critical frequency is proposed and applied for the derivation of the ionospheric weather index, characterizing the degree of disturbance at each particular station. A daily-hourly analysis of ionosonde observations of foF2 for 16 stations at latitude range 37° to 70° N, longitudes of 10° W to 150° E, is performed during the solar minimum, 2006. Missed ionosonde observations are reconstructed by cloning data of another station. The process of gap filling considers hourly values of the F peak density NmF2 (deduced from foF2), normalized to the respective median, and assumes that this ratio remains the same for the parent and cloned data. It is shown that the correlation coefficient between cloned fcF2 and observed foF2 is greater than 0.75 for the positive and negative ionospheric disturbed days during a year at solar minimum, independent of the distance between the stations in high and middle latitudes. The quiet reference is determined as a running daily-hourly median for 27 days, preceding the day of observation calibrated for a seasonal trend with ITU-R foF2 predictions. The hourly deviation DNmF2 is defined as the logarithm of ratio of NmF2/NmF2med. A segmented logarithmic scale of the ionospheric weather index, W, is introduced, so that W=±1 refers to the quiet state, W=±2 to a moderate disturbance, W=±3 to the ionospheric storm, and W=±4 to the extreme or anomalous conditions. The catalog of the ionospheric disturbances for W exceeding ±2 at least during 3 consecutive hours is produced and presented online at the SRC and IZMIRAN web pages. It is found that the moderate disturbance is a prevailing state of the ionospheric weather for all stations. The stormy conditions comprise 1 to 20% of the times which occur more frequently at high latitudes, by night, during equinox and winter.
Highlights
The state of the ionosphere is determined by superposition of the regular changes and stochastic variations of the ionospheric parameters
Using the ratio of NmF2 from the ITU-R map to the observed NmF2med for the preceding month at a specified location and level of solar activity, we extrapolate this ratio with the ITU-R seasonal trend of foF2 for a relative change in the observed reference value to the day of two weeks ahead which provides a calibration coefficient applied to the “past” median
Information technology for quantitative estimates of the ionospheric variability is described. It is based on the ionosonde observations of the F2 layer critical frequency, foF2
Summary
The state of the ionosphere is determined by superposition of the regular changes and stochastic variations of the ionospheric parameters. There is no limit imposed on an increase in foF2 during the positive phase of the ionospheric storm when the critical frequency could exceed the median value by a few hundreds of percent To avoid these disproportions the decimal logarithm of the hourly value of NmF2, normalized by the quiet reference, NmF2med, is taken as a measure of the NmF2 (or foF2) variability (Gulyaeva, 1996, 2002; Field and Rishbeth, 1997; Fuller-Rowell et al, 2000). The results of its application for the derivation of the ionospheric weather index and disturbed periods are discussed below
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