Tropical Spread- F

Abstract

It is shown that the equatorial range spread-F is due to scattering of radio waves from a series of levels of large plasma density gradients and that the equatorial frequency spread is the decay process of the range spread following the lifting of the irregularities to higher heights. The spread-F at tropical latitudes is due to the superimposition of additional off-vertical <i>h'-f</i> traces from the patches of irregularities drifting from the equatorial region along the geomagnetic field lines. The range and frequency spread at tropical latitudes are just different manifestations of additional ionogram traces over the normal one. The occurrences of equatorial and low latitude spread are interconnected through a fountain of plasma irregularities similar to the daytime fountain of the plasma causing tropical maxima of F2 critical frequencies.