Plasma blobs observed by ground‐based optical and radio techniques in the Brazilian tropical sector

Abstract

Ground‐based optical and radio observations were carried out in the tropical region in Brazil, during the period from October 1998 to September 2000, and on several occasions we detected F‐region plasma blob (localized discrete plasma density enhancement) events. These are the first observations of blobs in the tropical F‐region using combined ground‐based optical and radio techniques. All‐sky images were used to map the spatial extension and temporal location of plasma blobs and ionosonde and photometer measurements were used to measure the plasma densities. Interesting cases of plasma blob events were observed on October 07, 1999 and March 04, 2000 over Cachoeira Paulista (22.7°S, 45.0°W; magnetic latitude 13.25°S, declination 20°W), showing discrete plasma density enhancements near regions of plasma density depletion structures in the OI 630.0 nm emission images. In these two cases, the electron densities were enhanced by a factor of, approximately, 2 above the background level. In this paper we report the first ground‐based observations of the plasma density enhancements, or blobs, and their association with equatorial spread‐F plasma depletions, and suggest a possible mechanism for their generation.