Equatorial Sporadic E and Plasma Instabilities

Abstract

THE radio reflexions from the sporadic E region (E_s) of the ionosphere show unique properties within a narrow range of latitudes over the magnetic equator. Matsushita¹ pointed out that the latitudinal variations of the maximum frequency reflected from the E_s region and the solar daily range of geomagnetic H component each show a sharp maximum within about ± 3° from the magnetic equator, suggesting an association between equatorial E_s and equatorial electrojet currents. Knecht² has described the characteristics of equatorial E_s (E_s−q). First, it regularly occurs during daylight hours in a narrow belt along the magnetic equator; second, it is always transparent to probing by radio waves; third, it usually displays a well defined lower edge lying between 100 and 110 km, giving scattered and diffuse echoes above the principal echo; fourth, in well defined cases the diffuse echoes are contained below a sharp upper boundary that starts at about ƒ₀E and increases in height with increasing frequency. The time of first appearance of the E_s−q in the morning and the time of disappearance in the evening has been shown to be controlled by the age of the Moon, and this effect has been related to the corresponding variations of the equatorial electrojet^2,3. Close correlations have been established between the dips in ƒE_s and the H component of the magnetic field at equatorial latitudes^4,5.