Association between equatorial and tropical spread-F

Comparati ve study of spread-F is made from the quarter-hourl y ionograms at Huancayo. near the magneti c equator and !3ogota. close to the anomaly crest region in the same longitude sector. The occurrence of equatori al spread-F is seen onl y du ring nights. preceded by very rapid rise of the F-Iayer close to the magneti c equator. Based on quarterhourl y ionospheric data at a closely spaced chain of 8 stations in the A meri can sector th at operated during IGY . the occurrence of spread-F at low lati tudes is shown to be assoc iated w ith the post-sunset intensifi cat ion of the ioni za ti on anomaly. The ratio or/~F2 at Bogota (anomaly crest) to th at at Huuncayo (dip equator) shows sudden intensi fica ti on on spread-F days around 1900 hI'S LT. peak ing betwecn 2000 ~l1ld 2 100 hI'S L T and can be used as a precursor to predict sprend-F near the magneti c equntor. T he onset of spread-F at Bogotn. however. requires add itional conditi on of strong spread-F at Huancayo covering a wide range of altitudes and stro nger uplift o f the F-Iayer. There are indicati ons th at the verti ca l uplift veloc ity of F-Iayer between 1800 and 1900 hI'S L T could be used to predict the onset of equatorial spread-F even at cres t.