F‐region ionosphere‐investigations at low latitudes

Abstract

AbstractsFurther studies of the F-region, ordinarily composed of the F-layer at night which separates into the F•-and Fz-layers at small zenith-angles of theSun, have been continued at the Huancayo Magnetic Observatory of the Department of Terrestrial Magnetism of the Carnegie !nstituticn of Washington during the past year. The separation of the F•-and Fz-layers appears to occur because of a decrease of virtual height of the F•-layer and a marked rise of virtual height of the Fz-layer. To adequately represent this effect, iso-ionic charts showing the change of virtual height and ionization with time have been constructed. This construction is made possible by a convenient simplification of the virtual height versus frequency graphs obtained from the observed data. the iso-ionic curves of Figure 4 referring to the F•-layer can also be considered approximately as the ion-distribution along the 12th parallel of latitude for this date. To obtain a further idea of the geographical distribution of this layer it is of interest to refer to the evidence obtained at Washington by Kirby, Berkner, and Stuart [4]. The maximum F• critical-frequency of 4500 kc, reported for summer noon at Washington (zenith-angle about !4ø), corresponds very closely to the results given for Huancayo in Figure 7. This provides a powerful argument in view of the flatness of the diurnal characteristic at noon that the north-south distribution of ionization must be similar to the distribution along a parallel for equal zenith-angles. As the zenith-angle of the Sun increases with season to about 60 ø at Washington, the curves and description of the magnitude and character of the F• critical-frequency on the average agree quite well with the iso-ionic chart of Figure 4 for correspondingly greater zenith-angles of the Sun. It seems pr•fi)al)le, therefore, that isoionic curves for the F•-layer drawn along the meridian fr(•m a point directly under the Sun (that is perpendicular to Fig. 4 at 12 h 15 •") would appear approximately similar to Figure 4]J, at least up to about 60 ø (17•). ()n this basis Figure 9 gives the best conception of the geographical distrit)ution of this layer that can 1)e obtained from available data. . /50-IONIC CURVES GIVoeN IN

First Study on the Occurrence Frequency of Equatorial Plasma Bubbles over West Africa Using an All‐Sky Airglow Imager and GNSS Receivers

“…Irregularities in the F region of the equatorial ionosphere have been studied for about a century now, and this makes it one of the oldest researched topics in the field of equatorial aeronomy (Berkner & Wells, ; Chandra & Rastogi, ; Herman, ; Woodman, ). F region irregularities are well known to manifest as equatorial spread F and are generally attributed to the Rayleigh‐Taylor instability (Kelley, ; Kelley et al, ; Mendillo et al, ; Rayleigh, ; Taylor, ; Woodman & La Hoz, ).…”

Simultaneous radar observations of meter‐scale F region irregularities at and off the magnetic equator over India

“…The plasma irregularities in the equatorial F region manifest in the form of diffuse ionograms and are referred to as equatorial spread F (ESF) [ Berkner and Wells , 1934; Booker and Wells , 1938]. ESF is now known to be a manifestation of ionospheric interchange instabilities like the Rayleigh‐Taylor instability and the gradient drift instability responding to the steep density gradient in the nighttime equatorial F region.…”

Electrodynamics of midlatitude spread F: 1. Observations of unstable, gravity wave‐induced ionospheric electric fields at tropical latitudes

“…The phenomenon of spread F has intrigued observers of the midlatitude ionosphere since the earliest days of experimental ionospheric research [Berkner and Wells, 1934;Booker and Wells, 1938]. Until the 1960s, most observers believed that plasma instabilities were responsible for spread F, but since then the focus of research has shifted toward disturbances produced by attoospheric gravity waves (see Bowman [1990] for a recent review).…”