This paper describes some measurements related to the nature of field‐aligned irregularities in the E region. The measurements were performed near the magnetic equator, and the irregularities are attributed to the equatorial electrojet. It is shown that the irregularities are most likely to consist of plane acoustic waves generated by an instability resulting from the flow of current in the electrojet. Reference is made to a recent theory of Farley in which the existence of such waves is predicted. The characteristics of auroral radar echoes are shown to be similar to those of the equatorial echoes. It is inferred that the auroral field‐aligned irregularities are also caused by the flow of electrojets.

Although t h e two-strcam i n s t a b i l i t y t h e o r y [ F a r l e y , 1963a,b] has been q u i t e su-.cessfUl i n accounting for many observed f e a t u r e s of t h e plasma--,;?-:-? 5 y r m~1 7 a r i . t -uies produced i n t h e e q u a t o r i a l e l e c t r o j e t , i n c r e e s e d e m i m~n t z i -* s e n s i t i v i t y and improved ex2erimental techniques of observing ~:~e 11c:er s F c t r a t n of r a d i o echoes from t h e e l e c t r o j e t have r e s u l t e d i n t k x s~l v . t l o n of ve&er i r r e g u l a r i t i e s n o t p r e d i c t e d by t h a t t h e o r y . I n pir~i-ulsr, such h -r e g u l a r i t i e s a r e observed b o t h when a VHF r a d a r l o c a t e d nezr the rnegneiic e q u a t o r i s d i r e c t e d . v e r t i c a l l y , and when t h e r a d a r -is d i r e c t e d o b l l q u e l y i n t h e e q u e t o r i a l plane. I n t h e l a t t e r case, t h e i r r c g u l c t r i t i e s have lower Doppler s h i f t . s t h a n a r e suggested by t h e t h e o r y .E x p e r i n e n t e l s p e c t r a are p r e s e n t e d t h a t demonstrate f e a t u r e s c o n s i s t e n t with :;he f i r s t -o r d e r two-stream t h e o r y for o b s e r v a t i o n s of a well-developed elec'tro;let a t oblique an&s of incidence. However, t h e r e l a t i v e c o n t r i - b u t i o n of t h e weaker i r r e g u l a r i t i e s can be r e s o l v e d i n s p e c t r a f o r i n c ident? zngles approaching t h e z e n i t h or, when t h e e l e c t r o j e t i s comparatively.VCCL, es most tlngles. The s p e c t r a l wldth obtained at v e r t i c a l incidence 5s shorn-t o be narrow for a w e a k e l e c t r o j e t , and t o be broadened as t h e e l e c t r o j e t becomes s t r o n g e r . The weaker i r r e g u l a r i t i e s probably r e s u l t -.-.--irm the n o n -l i n e s r i n t e r a c t i o n among t h e s t r o n g e r i r r e g u l e r i t i e s , as i sJencnszratzd by Dougherty and F a r l e y i19663 i n a companion paper.i v n~% l r 2 s u l t s are p r e s e n t e d t h a t i n d i c a t e and a s y m e t r y n o t p r e d i c t e d by -Lt z r Y~e n r y ; i.-., the upgoin2 i r r e g u l a r i t i e s (observed a t a n g l e s of iiLLic?:nc i t l , > s (L>srr-rvx3 5 : srsles of incidence east o f t h e izni;h). Expert-west of t h e ;enith) are s t r o n g e r tliz 1 t're d~w n g o i n g I r r e g u l a r --

No abstract

Ionospheric propagation via scattering from the F region was sought at 50 Mc/s over a transequatorial path (with midpoint near Huancayo, Peru) employing a 2580‐km transmitter‐receiver separation. Propagation via F scatter was present over this path about 10 per cent of the time, though only at night. A condition closely related to the occurrence of F scatter was the presence of equatorial spread‐F configurations on the Huancayo ionograms. The height of the propagation medium supporting this F scatter was usually identifiable with the lowest height of the associated equatorial spread F on the ionograms. On the basis of an interpretation of experimental results, equatorial spread F is shown to arise from scattering by relatively thin sheets of irregularities in electron density which occur at the bottom of the F layer or as much as 100 km lower. The thickness of these patches is estimated at the order of 50 km. The scattering irregularities comprising these patches are found to be elongated along the earth's magnetic lines of force. Electron irregularities of scale 10 meters or smaller measured in at least one dimension transverse to the magnetic field lines, and 1000 meters or longer measured along the magnetic field lines, are shown to exist as high as 450 km in the nocturnal equatorial F region during equatorial spread‐F conditions. Equatorial spread‐F echoes observed at Huancayo are demonstrated to be arriving at all elevation angles in the magnetic east‐west plane. The geographical extension of a given scattering sheet in the magnetic east‐west direction was at times as great as 1000 km. An observational procedure is suggested for distinguishing two fundamental varieties of spread‐F echoes appearing on the equatorial ionograms. A necessary condition, that the contours of mean electron density be parallel to the magnetic lines of force, is proposed as a controlling factor for the occurrence of spread F in the equatorial ionosphere.

Two features of the daytime sporadic E echoes observed on ionograms obtained near the magnetic equator have been termed ‘equatorial sporadic E’ and ‘equatorial slant sporadic E.’ These are traces that project from the E‐layer trace and have equivalent ranges independent of frequency and proportional to frequency, respectively. Radar studies in Peru indicate that a thin stratum of magnetic‐field‐aligned irregularities embedded in the E layer is responsible for the entire sporadic E configuration. The increased equivalent ranges of the slant trace are attributed to oblique propagation in the east‐west plane orthogonal to the irregularities. By accounting for the refraction and retardation imposed by the E layer, the entire configuration can be simulated in detail. It is established from this analysis that the equatorial electrojet flows within the equatorial E layer, and that its first‐order current variations are coherent over distances of 500 km or more along the magnetic equator.