Concerning the onset mechansim for equatorial spread-F

Data obtained by vertical sounding are used to examine the restructuring of the ionosphere just before the onset of the equatorial spread-F. The following phenomena are observed about 30 minutes before the onset of the spread-F: (1) an increase of h(max) F2; (2) an increase of N(max) Fe; (3) a height greater than 250 km at a frequency of 1 MHz; (4) an increase in the vertical gradient of electron density; and (5) an increase in the relative gradient of electron density. The spatial-resonance and gravity-wave mechanisms of spread-F onset are discussed.