Ionospheric irregularities in the low-latitude valley region observed with the Equatorial Atmosphere Radar

Abstract

The geomagnetically low-latitude valley region between the upper E region and the lower F₁ region is studied with the Equatorial Atmosphere Radar (EAR) in Indonesia. Three-meter-scale field-aligned irregularity echoes have been frequently observed in the valley region in association with the equatorial spread F (ESF) in the period from sunset to midnight. The valley region echoes usually appear at above 150 km and propagate downward with time. Rapid beam scanning of the EAR revealed that spatial structure, temporal variation, and drift velocity of the valley region echoes resemble those of ESF, which indicates that the dynamoelectric field in the equatorial F region controls the low-latitude valley region irregularities. Perturbed electric fields associated with ESF map down to the low-latitude valley region and can produce the perturbed plasma density structures as "images" of ESF structures. Image structure is effectively formed at altitudes below 200 km and is a source of the valley region irregularities observed with the EAR. It is suggested that intermediate layers should supply a plasma density gradient for excitation of 3-m-scale irregularities in the valley region through gradient drift instability.