A statistical analysis of occurrence characteristics of Spread-F irregularities over Indian region

• The paper qualitatively interprets the occurrence probability with causative mechanism of Spread-F across Indian latitudes.
• It reconfirms the occurrence probability of Spread-F to be anti correlated to solar activity.
• Geomagnetic storms do affect their occurrence.
• There Spread-F behaviour is complex as it is influenced by TID’s, gravity waves and irregularities generated locally.

We investigate the regularities of a change in Spread-F F probability during day-to-day, under varying solar variability, latitudinal behavior and their response to geomagnetic storm in equatorial and low-mid latitude stations. The occurrence characteristics of Spread-F irregularities, is obtained from daily hourly ionosonde data from a low-mid latitude station, Delhi (28.6°N, 77.2°E), for more than half a solar cycle (2001 to 2007). The latitudinal behavior of Spread-F is studied using ionosonde data from anomaly crest station, Ahmedabad (23.01°N, 72.36°E) and equatorial station, Kodaikanal (10.2°N, 77.5°E) for low, moderate and high solar activity periods. The maximum percentage occurrences of Spread-F were observed during the low solar activity year 2007, we believe, the low plasma and neutral density during 23/24 solar cycle minimum could be an important factor leading to the generation and propagation of TIDs and gravity waves. An anti-solar activity correlation to Spread-F occurrence is reported during all the seasons at different stations which are because of instability generated by the trans-equatorial meridional winds. There is a substantial variation during pre and post midnight hours in F region height from equatorial to low latitudes in response to magnetic disturbances. Concurrence was observed in the occurrence time of Spread-F to different storm events during different storm phases. The established irregularities and their behavior in Indian region are qualitatively interpreted and discussed.