Spread F at tropical latitude stations in India

Abstract

<span style="font-size:11.0pt;mso-bidi-font-size: 10.0pt;font-family:" times="" new="" roman","serif";mso-fareast-font-family:"times="" roman";="" mso-ansi-language:en-us;mso-fareast-language:en-us;mso-bidi-language:ar-sa"="" lang="EN-US">Regular radio soundings of the ionosphere 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 in the present paper using the recordings of recently installed Digisonde during June - July 2012. The spread F echoes at Ahmedabad are not due to the <i>in situ</i> produced irregularities as at an equatorial station Thumba. The spread F echoes at Ahmedabad during the June solstices of 2012 are due to reflection (not scattering) from the off-vertical direction that produce multiple traces from off-vertical direction overlapping over the main (vertical) h'-f trace, sometimes giving an appearance of diffuse echoes near the critical frequencies. Presence of medium scale traveling ionospheric disturbances (TIDs) could be a possibility to account for such off-vertical reflections.</span>