A study of the irregularities in the night time equatorial E-region using a Langmuir probe and plasma noise probe

Abstract

A rocket borne Langmuir probe and plasma noise probe experiment conducted at 2300 hr IST on 29 August 1968 from Thumba near the magnetic equator, revealed the existence of electron densities varying between 10³ and 10⁴ per cc in the height range of 95–120 km and a deep valley in the 120–140 km region. The electron density in the valley was of the order of a few hundreds per cc. Large scale structures in the electron density profile in the 95–120 km region were observed with vertical scale sizes of a few kms and a horizontal extent of the order of 50 km. In these structures the electron density was observed to vary by factors lying between 4 and 25. Small scale irregularities with scale sizes lying in the ranges 30–300 m and 1–15 m that were present in the ambient medium were detected during the flight. While these irregularities were found to occur randomly and in the form of bursts most of them were located only in regions where the background electron density exhibits large negative gradients. The positions of occurrence of the 30–300 m irregularities were found to confirm qualitatively that these irregularities are generated by a cross-field instability mechanism which needs a background electron density gradient with a specific sign. While the smaller scale size irregularities (1–15 m) also occur in regions where the cross-field instability mechanism can operate they cannot be directly explained on the basis of the present theories of the cross field instability. Hence either the present theories need modification, or there exists another mechanism which is responsible for these irregularities. However, even if a separate mechanism were to exist, the present data indicates that depending on the relative orientation of the ambient electric field and the electron density gradients, the cross-field instability mechanism can promote or inhibit the production of these irregularities.