R. J. Stening

Modelling the low latitude F region

Abstract The low latitude ionospheric F region is dominated by the Appleton equatorial anomaly. Much of the challenge in modelling the region is associated with correctly incorporating this feature. Its large latitudinal gradients and considerable variability increase the difficulty of the task. This review discusses three types of model: 1. (i) the purely empirical models, 2. (ii) the purely physical or ‘first principles’ models, and 3. (iii) the semi-empirical models which combine elements of the first two. The salient features of the most recent models are examined together with their antecedents. Extensive comparisons with observed data reveal the strengths and weaknesses of the various models. From these, suggestions for future improvements are derived. Particular improvements required include 1. (a) a better empirical model of h m F 2, 2. (b) inclusion of variations due to changes in the equatorial electrojet, and 3. (c) an understanding of variations of various parameters with the solar cycle.

What drives the equatorial electrojet

Abstract There is a controversy over whether the equatorial electrojet is a separate system with its own return currents at low latitudes or it is an integral part of the Sq current system driven by emfs mostly beyond the Sq focus. The arguments of Onwumechili (1989) in support of the first hypothesis are examined, but the second idea is preferred. Ground-based, satellite and rocket data are reviewed together with correlation studies and various computer models. A picture is suggested where there is a relatively constant current system, including both electrojet and Sq, with other superposed current systems mostly driven by semidiurnal tides, which are responsible for both day-to-day and seasonal variations and also for the poor correlation between geomagnetic variations at the electrojet and elsewhere.

Magnetic variations at other latitudes during reverse equatorial electrojet

Abstract Several reverse equatorial electrojet events are examined. Magnetic variations from a sample of worldwide stations on the same days are compared with the quiet daily average for the same month. In most cases departures can be found elsewhere at the same time as the reverse jet. Various wind systems in the dynamo region might produce the observed results but different systems are required on different occasions.

Lunar tide in the equatorial F region vertical ion drift velocity

Vertical ion drift velocity data from Jicamarca have been analyzed for a lunar semidiurnal tide using a least squares fitting method. Amplitudes of up to 6 m s−1 are obtained with phases in agreement with lunar tidal determinations of other associated physical parameters. Variations between season, solar activity, and day to night are also examined. Generally, amplitudes are larger in the southern summer. Much of the phase variation with season is very similar for solar maximum and minimum years. There is a summer to winter phase change that is most distinct at solar maximum nighttime. A day-to-night phase reversal can also been seen in some seasons. Hints of this are also found in the lunar tide in the F region height and in the magnetic variations at Huancayo.

Seasonal changes in the global lunar geomagnetic variation

Abstract The seasonal variation of phase and amplitude of the lunar semidiurnal geomagnetic variation is derived by the method of Winch and Cunningham (1972) for a global distribution of stations during 1964–1965. Large semiannual variations in amplitude are sometimes found and annual maxima are often in February or August. Some sharp phase changes are seen about October–November similar to those reported in solar semidiurnal winds. In several cases the expected phase change in L(H) with latitude is absent, as is the phase change in L(D) between hemispheres. It is suggested that these latter anomalies may be due to the failure of the lunar dynamo in one hemisphere. Several examples of variations with longitude are shown. Most of the anomalies are also revealed in data from a sunspot maximum period analysed by Gupta and Chapman (1968).

Simulating the ionospheric dynamo — II. Equatorial electric fields

Abstract Different wind models are used as input to the equivalent circuit model and their contributions to the ionospheric electric fields in the equatorial region are discussed. The dependencies of the electric field variations on altitude, season and solar activity are presented. The separate contributions of the southward and eastward components of the winds are derived and it is found that the global southward winds are the main contributors to the electric fields and currents in the equatorial ionosphere. It is confirmed that the prereversal enhancement of the eastward electric field is produced by the F region dynamo. The solar cycle variations in the strength of this enhancement seem to be controlled by the F region ion density. The tidal winds of Forbes and Gillette (1982 [A compendium of theoretical atmospheric tidal structure, part I, model description and explicit structure due to realistic thermal and gravitational excitation, AFGL-TR-82-0173(I)]) reproduce most features of the Jicamarca incoherent scatter experiment and of the AE-E and DE-2 satellite observations of the electric fields. The HWM90 empirical wind model failed to produce the observed electric field and it seems the semidiurnal wind is too strong in that model.

Field-aligned currents driven by the ionospheric dynamo

Abstract Field-aligned currents generated by a ‘1,-2’ mode wind system are calculated by the equivalent circuit method. It is found that changes in the current pattern with season are relatively small compared to changes with longitude. Currents at the equinox are of similar magnitude to those at the solstices and there is no reversal in direction of major features between summer and winter. A band of intense field-aligned current density is found at low magnetic latitudes which appears to be responsible for the ΔY variation at equatorial stations.

Analysis of contributions to ionospheric dynamo currents from e.m.f.'s at different latitudes

Abstract The equivalent circuit method (Stening,1968, 1971) is used to calculate contributions to ionospheric currents and fields from e.m.f.s in different latitude bands. Tables are presented showing the different contributions in the case of separate operation of the ‘1,-2’, ‘2.2’ and ‘2.4’ tidal modes. It is suggested that tidal energy may on occasion leak through to the dynamo region in only a restricted region.

Observations of lunar tides in upper atmosphere winds at Poker Flat, Alaska

Abstract The lunar semidiurnal tide is extracted from hourly values of winds in the 75–105 km region measured by the Poker Flat Alaska MST radar used in the meteor mode. Since year-to-year variations are apparent, detailed results for 1983 and 1984 are presented. Inferred vertical wavelengths range from 17 km in March 1983 to 46–55 km in September of 1983 and 1984. The height progression of the phase is frequently too irregular to derive a vertical wavelength. Amplitudes of 3 m s −1 are common and range up to 8 m s −1 . Amplitudes generally are largest at the equinoxes, especially in September, with another maximum in winter sometimes occurring. Reasonable agreement is found with lunar tidal measurements at Saskatoon, and some points of similarity are found with the solar semidiurnal tide at Poker Flat.

Simulating the ionospheric dynamo—I. Simulation model and flux tube integrated conductivities

Abstract An updated version of the equivalent circuit model for simulating the ionospheric dynamo process is described. The contributions of the E and F regions to the flux tube integrated conductivities are compared. The results confirm that the ionospheric electric process is controlled by the E region during daytime but by the F region during nighttime. The F region has a larger effect on the dynamo processes during solar maximum than at solar minimum, and during equinox than in solstice.