Association between equatorial and tropical spread-F

Abstract

Comparative study of spread-F is made from the quarter-hourly ionograms at Huancayo, near the magnetic equator and Bogota, close to the anomaly crest region in the same longitude sector. The occurrence of equatorial spread-F is seen only during nights, preceded by very rapid rise of the F-layer close to the magnetic equator. Based on quarter- hourly ionospheric data at a closely spaced chain of 8 stations in the American sector that operated during IGY, the occurrence of spread-F at low latitudes is shown to be associated with the post-sunset intensification of the ionization anomaly. The ratio of f oF2 at Bogota (anomaly crest) to that at Huancayo (dip equator) shows sudden intensi fication on spread-F days around 1900 hrs LT, peaking between 2000 and 2100 hrs LT and can be used as a precursor to predict sprend -F near the magnetic equator. The onset of spread-F at Bogota, however, requires additional condition of strong spread-F at Huancayo covering a wide range of altitudes and stronger uplift of the F-layer. There are indications that the vertical uplift velocity of F-layer between 1800 and 1900 hrs LT could be used to predict the onset of equatorial spread-F even at crest.