On the Disappearance of E_s at Kodaikanal and Thumba

Abstract

Disappearance of E_s-q during the daytime hours at the magnetic equatorial stations Kodaikanal and Thumba are found to occur simultaneously whenever there are depressions in the geomagnetic H variation and such events have been found to occur on quiet as well as on disturbed days. The horizontal drift measurements of the ionospheric irregularities at Thumba by closely spaced receiver technique shows eastward drift for the time duration when E_s-q is absent. Statistics of these three events on quiet days show such occurrences mainly in the evening hours (1500–1700 hr local time); the disappearances of E_s-q are more common during the low sunspot years than during the high sunspot years.

The disappearances of E_s around noon hour are rarely seen during E and J months but are seen on a few occasions during D-months. It is concluded that disappearance of E_s-q occurs during periods of counter-electrojet currents.