Plasma irregularities in the tropical F‐region detected by OI 7774 Å and 6300 Å nightglow measurements

Simultaneous measurements of the permitted OI 7774 Å and the forbidden OI 6300 Å nightglow emissions were carried out at Cachoeira Paulista (geographic 22.7°S, 45.0°W), Brazil, during the period of April 1978 to March 1979. Both the emissions were observed with tilting filter‐type photometers. During spread‐F conditions, on several nights, both the emissions showed simultaneous short‐time large intensity depletions, indicating the presence of large‐scale irregularities in the F‐region. Data for the period September‐October 1978 are presented and discussed. The depletions observed are possibly associated with the passage of F‐region holes or bubbles drifting across the sky.