NmF2 Values Research Articles

A theoretical study of the effects of quiet‐time electromagnetic drifts on the behavior of thermal plasma at mid‐latitudes

Starting with a depleted protonosphere, the behavior of O+ and H+ at L=.2 is examined theoretically to determine the influence of a quiet time diurnal electromagnetic drift with a 24‐hour period. The conditions are appropriate to sunspot maximum. Large changes in the H+ tube content at L=3.2 occur as the protonosphere is replenished. In the absence of the electromagnetic drift, such changes would imply field‐aligned proton fluxes up to the order of 1010 cm−2 s−1; the actual field‐aligned proton flux is of the order of 108 cm−2 s−1. The combined effect of diurnal drifts and frequency of magnetic storms can produce multiple peaks in the diurnal behavior of the H+ tube content at L=3.2. As the drift lowers the altitude of the O+ layer during daytime, the values of NmF2 decrease owing to the change in the linear loss coefficient. At night, cross‐L gradients in NmF2 are significant for the conditions considered, and the outward drift increases NmF2 values at L=3.2. When the protonosphere is sufficiently replenished, the inward drift causes downward proton fluxes at L=3.2.

Origin of annual and subannual periodicities in F region ionization

The monthly median values of maximum electron density NmF2 in the F region are harmonically analyzed for a number of high‐latitude and auroral zone stations for each hour of the day during 1958 and 1964. In the high‐activity year 1958 the annual maximum appears in December around 70°N (geographic latitude) for a few hours around noon. The December excess diminishes gradually toward the south, though the duration over which it lasts increases toward the dip equator. By 70°S a slight June enhancement is seen around noon. During the remaining hours of the day a December phase is seen around 70°S, having a large amplitude which decreases toward the north, reaching a slight June excess by 70°N. A 6‐monthly component with equinoctial maximums exists at all stations for all hours of the day, the amplitude being somewhat larger in the south than it is in the north. In the low‐sunspot year 1964 the December phase of the annual component is present in the north with low amplitude for a few hours around noon. The same phase prevails in the south. This is followed in local time by a large‐amplitude June phase in the north and a December phase in the south. The 6‐monthly component is predominant during daytime only and has equinoctial maximums. The current theories are shown to be inadequate to explain these results.

Low latitude features of annual and semiannual periodicities in NmF2 in American and Indian zones

The monthly median NmF2 values during the high sunspot period 1958 from a number of low latitude stations in the American zone are subjected to harmonic analysis. The results are presented as a function of local time and are compared with those obtained earlier from the Indian zone. The gross features in the two zones are found to be identical. The semiannual periodicity is shown to have maxima in April and October at all stations for all hours of the day and this is attributed to similar changes in thermospheric atomic oxygen concentration controlling the production of ionization. The annual maximum exhibits systematic changes in the amplitude and phase as a function of latitude and local time. One of the main features is a north-south asymmetry in the amplitude of the annual variation which can also be attributed to larger concentration of atomic oxygen in January than that in July. Further, a geomagnetic control is seen to exist on the annual variation at both the eastern and western longitude zones. This is explainable in terms of plasma exchange between magnetically conjugate latitudes.

Low latitude features of annual and semiannual periodicities in NmF2 in American and Indian zones

The monthly median NmF2 values during the high sunspot period 1958 from a number of low latitude stations in the American zone are subjected to harmonic analysis. The results are presented as a function of local time and are compared with those obtained earlier from the Indian zone. The gross features in the two zones are found to be identical. The semiannual periodicity is shown to have maxima in April and October at all stations for all hours of the day and this is attributed to similar changes in thermospheric atomic oxygen concentration controlling the production of ionization. The annual maximum exhibits systematic changes in the amplitude and phase as a function of latitude and local time. One of the main features is a north-south asymmetry in the amplitude of the annual variation which can also be attributed to larger concentration of atomic oxygen in January than that in July. Further, a geomagnetic control is seen to exist on the annual variation at both the eastern and western longitude zones. This is explainable in terms of plasma exchange between magnetically conjugate latitudes.

Negative ionospheric storms caused by thermospheric winds

The equations of motion for the electrons, ions and neutral particles and the appropriate continuity equations have been solved simultaneously as functions of time, height and latitude in order to investigate theoretically the effects of storm-time thermospheric winds on the ionosphere. It is concluded that the equatorward storm-time winds will result in decreased values of NmF2 at lower latitudes because they interfere with the normal poleward ionization motion associated with the electric field producing the equatorial anomaly.

Seasonal differences in the low-latitude F2-region ionization density caused by [formula omitted] drift and neutral wind

In the low-latitude ionospheric F2-region the electron density distribution is affected by E ̄ × B ̄ drift and neutral wind as well as by production and loss processes and diffusion. At Jicamarca, Peru (dip angle, 2°), the daily variation of the measured vertical drift velocity undergoes changes in amplitude and phase between the December and June solstices. Incorporating models of the observed vertical drift velocity, the electron density as a function of latitude and local time is theoretically calculated by solving the time dependent plasma continuity equation. In this study the effects of east-west drift are specifically ignored. The results are compared with ionosonde observations at Huancayo, Peru (dip angle, 1°), and Tucuman, Argentina (dip angle, −22°), to determine the extent to which seasonal differences in the drift velocity can account for the seasonal differences in the maximum electron density, NmF2, measured at these two stations. Solving the equations with and without a neutral wind, it is found that at Huancayo the seasonal differences in NmF2 are caused primarily by the differences in E ̄ × B ̄ drift and only secondarily by neutral wind and production processes. At Tucuman, both E ̄ × B ̄ drift and neutral wind play important roles in accounting for the seasonal differences. Between the hours of 1200 and 1900 LT for both solstitial periods the meridional neutral wind must be blowing poleward to achieve good agreement between calculated and observed NmF2 values.

On the Construction and Use of a Simple Ionospheric Model

The parameters controlling the development of a simple model of the E and F regions of the ionosphere are discussed. Uses of such models in scientific investigations and engineering applications are given. A computer model is described that will provide estimates of the electron and ion densities in middle latitudes as a function of latitude, longitude, altitude, season, time, and solar activity. Statistical parameters for midlatitude blanketing sporadic E are presented. An analytic expression is used for the CIRA 1965 neutral atmospheric models. The F1 region is treated using a photoequilibrium model with three ionic constituents and 62 ionizing radiation groups. The F2 layer uses theoretical models arranged to fit boundary conditions of NmF2, HmF2 and the electron density at 1000 km. Values of NmF2 are obtained by using the CCIR atlas of ionospheric characteristics. The midlatitude sporadic E layer is treated by presenting three parameters of the layer: the probability of observing sporadic E with a blanketing frequency greater than 0.5 MHz, the average peak electron density when sporadic E is observed, and the standard deviation (assuming log normal distributions for the peak electron density). The restrictions on the model and the availability of data on suitable boundary conditions are discussed. Suggestions for further development of models of the ionosphere are given.