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 --

The ionospheric modification experiments provide an opportunity to better understand the aeronomy of the natural ionosphere and also afford the control of a naturally occurring plasma, which will make possible further progress in plasma physics. The ionospheric modification by powerful radio waves is analogous to studies of laser and microwave heating of laboratory plasmas (20). " Anomalous" reflectivity effects similar to the observed ionospheric attenuation have already been noted in plasmas modulated by microwaves, and anomalous heating may have been observed in plasmas irradiated by lasers. Contacts have now been established between the workers in these diverse areas, which span a wide range of the electromagnetic spectrum. Perhaps ionospheric modification will also be a valuable technique in radio communications.

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.

Spectral decomposition of the power spectrum of radar echoes at 50 MHz from the equatorial electrojet reveals information as to the phase velocities of several classes of electron density irregularities and permits inferences as to the electron drift velocity and the electron and ion temperatures in the electrojet. In particular, it is shown that the phase velocity of two-stream irregularities is comparable to the ion-acoustic speed and independent of the horizontal phase velocity of coexistent non-two-stream irregularities. The latter can exceed the ion-acoustic speed and is probably a measure of the electron drift velocity. Thus a probable inference is that the electron drift velocity can itself be supersonic, contrary to some existing theoretical opinions. The resttits suggest that there are frequently two or more strata containing non-two-stream irregularities moving at different mean phase velocities. Identification of the phase velocity of the two-stream irregularities with the ion acoustic speed leads to the conclusion that the electron and ion temperatures in the electrojet are comparable, in agreement with incoherent scatter observations of the E region at other latitudes. Thus, these results tend to refute rocket measurements made by Langmuir probes in the electrojet.The equatorial electrojet is a strong current The physical explanation for selection of this embedded in a slightly ionized ionospheric plasma height region relates to the maximization of (the E layer near the magnetic equator). This ionospheric conductivities there [cf. Hanson, current is a flow of electrons across geomagnetic 1965]. The important feature of this height field lines through ions and neutrals that are for region to note for our purposes is that the many purposes considered to be at rest. This electrons are mainly controlled by the geomagflow is characterized as a drift, since an electric netic field, and not by collisions with neutrals, field is required to propel the electrons. A global whereas the reverse is true for ions. Hence in electric field (resulting from tidal forces)known this region the electrons are subject to drift as the dynamo field [Fejer, 1953] is the driving forces but not the ions. Thus the unusual charforce.acteristics of the electrojet result from the Although the dynamo field is present at other combination of the electric field magnification latitudes, its effects are emphasized near the stemming from the special equatorial magnetic magnetic equator, where the east-west dynamo geometry and the existence of an altitude region field sets up a large vertical polarization field. in which the electrons drift through the ions. This vertical electric field, or Hall field, is muchA phenomenon closely related to the electron stronger than the originally imposed east-west drifts is the production of ionospheric irregudynamo field. This electric field E and the larities. Experiments with radar (until recently ambient magnetic field B cause (through E xB mainly at 50 MHz) have distinguished two forces) east-west ...