Spread F at Tropical Latitude Stations in India Ahmedabad

Regular ionospheric soundings over Ahmedabad were started in 1953 and several studies describe the various features of the ionosphere close to the Equatorial Ionization Anomaly crest. Finer characteristics of the spread F echoes could not be studied earlier due to the very wide pulse of the transmitter. The characteristics of the spread F at Ahmedabad are described using the recordings of recently installed Digisonde. The spread F echoes at Ahmedabad are not due to the in-situ produced irregularities as at an equatorial station Thumba. The spread F echoes at Ahmedabad are due to reflection (not scattering) from the blobs or tongues of ionization transported from equatorial region along the magnetic lines of force. These produce multiple traces from off-vertical direction overlapping over the main vertical p'-f trace, sometimes giving an appearance of diffuse echoes near the critical frequencies.