Post-sunset Rise Research Articles

Evidence of a velocity shear in bulk plasma motion associated with the post‐sunset rise of the equatorial F‐layer

Using the east‐west drift of equatorial spread‐F (ESF) irregularities as tracers for horizontal bulk plasma motion, we show that a velocity shear (with altitude) in east‐west plasma drift exists in the nighttime equatorial F‐layer. When combined with incoherent‐scatter measurements of the vertical motion of the F‐layer, we find evidence that the two‐dimensional plasma flow pattern in the F‐region around E‐region sunset resembles a vortex. The velocity vortex is attributed to (1) a velocity shear that is set up by the F‐region dynamo that begins to function around E‐region sunset, and (2) the postsunset rise of the F‐layer. The existence of a velocity shear verifies the importance of the eastward neutral wind and F‐region dynamo in nighttime equatorial plasma electrodynamics, and allows new interpretation for ESF generation processes.

Late time barium cloud striations and their possible relationship to equatorial spread F

An active experiment has been conducted in space that allows a test for the theory of nonlinear development of striations in large barium clouds. The results are in excellent agreement with a computer simulation of the E × B instability reported by Scannapieco et al. (1976). The power law irregularity spectrum predicted by the theory and verified by the experiment is shown here to be due to wave steepening and not to plasma turbulence. The barium cloud results are remarkably similar to bottomside equatorial spread . A possible role of the E × B instability in bottomside equatorial spread F is discussed as a supplement to the Rayleigh‐Taylor instability during the postsunset rise of the F layer and during anomalous plasma uplifts which occur during geomagnetically active periods.

Range and frequency spreadF at Huancayo

The mean occurrence of frequency type of spreadF at Huancayo has been shown to have practically no solar cycle dependence. The occurrence of range type of spreadF is shown to be inversely related to sunspot number. The range spread occurs mostly around 2100 LT for any of the solar epoch, while frequency spread has maximum occurrence between 2300 and 0000 LT in high sunspot years and between 0000 and 0100 LT in low sunspot years. The seasonal variation in the occurrence of either type of spreadF shows minimum in northern solstices (June months) and maximum occurrence in southern solstices (December months). The post-sunset rise ofF layer is most predominant during high sunspot years. These results point out the inadequacy of the theory of spreadF based entirely on the post-sunset upward rise of theF region after sunset.

Dynamics of the equatorial F-region

This paper reviews the large-scale dynamics of the equatorial F-region. Its main topic is electric fields: Section 1 deals with their origin and effects on the F2-layer, in particular the ‘equatorial fountain’ and the post-sunset rise of the F2-layer. Section 2 deals with neutral air winds and their F2-layer effects, with discussion of what may happen if the horizontal winds are convergent or divergent. Both these sections conclude with a list of current problems. Section 3 deals with aspects of the equatorial F2-layer that are not simply classifiable as ‘electric fields’ or ‘winds’, while Section 4 briefly recapitulates some major unsolved problems.

Equatorial range spreadF and high multiple echoes from theF region

It has been shown that the post-sunset rise of the minimum virtual height (h′F) of theF region of the ionosphere near the magnetic equator during the high sunspot years is an effect of the overall rising of the completeF region. This feature of theF region produces very high order multiple reflections from theF regions. On some occasions strong spreadF is observed simultaneously with the high multiples suggesting theF region to be smooth and devoid of irregularities inside it. The irregularities causing the spreadF during the early stages of its development are suggested to be at heights below theF region.

Some aspects of the behaviour of the night-time ionospheric F-layer at low latitudes

It is shown that at geographic latitudes between 30°N and 30°S substantial ionization remains in the F-layer at midnight, particularly during periods of high sunspot numbers; the midnight values of foF2 are highly correlated with the sunspot numbers. The post-sunset rise in h′ F, characteristic of the equatorial F-layer at sunspot maximum, exhibits a semiannual variation at many stations and is found to depend not only on magnetic latitude but also on geographic latitude.

Postsunset rise of ƒoF2and geomagnetic activity

During the maximum epoch of the present sunspot cycle 19, owing to its unusually high solar activity, some anomalies in the postsunset rise of ƒoF2 at subtropical belts and the postsunset rise of h′F in the equatorial zone have been observed. These phenomena were investigated by researchers mainly on the monthly mean conditions or on magnetically calm days. In this paper, the relations between the postsunset rise of ƒoF2 and h′F and geomagnetic activity have been investigated. A possible explanation of the phenomena is also given.

Evidence on the horizontal diffusion ofF-region lonization along the magnetic lines of force in equatorial latitudes

Wright [1960] has reported that at Bogotá (dip 32°N) the F-region ionization density at night is greater than that in the regions nearer the dip equator. He suggests that horizontal diffusion of equatorial ionization along the earth's magnetic field lines could explain this feature. Recently Rao [1963], examining the ƒ0F2 data for Léopoldville (dip 33°S) and the h′F data for Ibadan (dip 6°S), has found that the period of sunspot maximum a post-sunset increase in ƒ0F2 occurs in the transition zone (30°–40°) around 2100–2200 hours. He has also found that this phenomenon occurs about 2 hours after the postsunset rise in h′F2 occurs at Ibadan, which is in the same longitudinal zone, and he infers that horizontal diffusion is responsible for it.

The postsunset rise offoF2in the transition region and its dependence on the postsunset rise ofh′Fin the equatorial region

In the transition region lying between the equatorial anomaly region and the middle latitudes a characteristic postsunset rise in F/sub 2/-region critical frequencies is observed during years of high sunspot activity. This phenomenon is found to depend on the evening height rise in the equatorial region. The analysis presented substantiates the view that the ionization lifted over the equatorial region is transported along the magnetic lines of force to the transition region. (auth)

Equatorial spreadFin relation to post-sunset height changes and magnetic activity

Equatorial spread F, magnetic activity, and post-sunset rise in h′F are studied in detail as three parameters, and correlations in the variations in each of the three pairs are examined for all the seasons. Partial correlations are sought to examine the independent influence of magnetic activity and h′F changes on spread F. It is observed that the time of maximum spread F on each night is effectively controlled by the magnitude of the h′F change between 1800 and 1900 LT. The results are discussed in the light of the recent theory of spread F outlined by Martyn.

The belt of equatorial spread-F

The morphology of the belt of equatorial spread-F at sunspot maximum is investigated, using I.G.Y. data, for magnetically quiet and magnetically disturbed conditions respectively. The belt is found to extend from about 30°S to 30°N in magnetic latitude and to have a region of very high incidence, exceeding 90 per cent for the early part of the night, about 20° wide in latitude and centred on the dip equator. In Africa and India there is little seasonal variation in quiet-day incidence, but a marked seasonal variation occurs in the American zone where there is a pronounced minimum in local winter. Throughout the whole belt from 30°S to 30°N there is a strong inverse correlation between spread-F incidence and magnetic activity. Moreover the post-sunset rise of the base of the layer, which usually precedes the onset of spread-F shows a marked positive correlation with spread-F incidence within the belt. No existing theory seems adequate to account for all of these facts but it seems likely that hydromagnetic disturbances are involved.