Typical Daytime Research Articles

Thermal response properties of the Earth's ionospheric plasma

The thermal response of the Earth's ionospheric plasma is calculated for various suddenly applied electron and ion heat sources. The time-dependent coupled electron and ion energy equations are solved by a semi-automatic computational scheme that employs Newton's method for coupled vector systems of non-linear parabolic (second order) partial differential equations in one spatial dimension. First, the electron and composite ion energy equations along a geomagnetic field line are solved with respect to a variety of ionospheric heat sources that include: thermal conduction in the daytime ionosphere; heating by electric fields acting perpendicular to the geomagnetic field line; and heating within a stable auroral red are (SAR-arc). The energy equations are then extended to resolve differential temperature profiles, first for two separate ion species (H +, O +) and then for four separate ion species (H +, He +, N +, O +) in addition to the electron temperature. The electron and individual ion temperatures are calculated for conditions within a night-time SAR-arc excited by heat flowing from the magnetosphere into the ionosphere, and also for typical midlatitude daytime ionospheric conditions. It is shown that in the lower ionosphere all ion species have the same temperature; however, in the topside ionosphere above about 400 km, ion species can display differential temperatures depending upon the balance between thermal conduction, heating by collision with electrons, cooling by collisions with the neutrals, and energy transfer by inter-ion collisions. Both the time evolution and steady-state distribution of such ion temperature differentials are discussed. The results show that below 300km both the electrons and ions respond rapidly (<30s) to variations in direct thermal forcing. Above 600 km the electrons and ions display quite different times to reach steady state, depending on the electron density: when the electron density is low the electrons reach steady state temperatures in 30 s, but typically require 700 s when the density is high; the ions, on the other hand, reach steady state in 700 s when the density is high, and 1500–2500 s when the density is low. Between 300 and 600 km, a variety of thermal structures can exist, depending upon the electron density and the type of thermal forcing; however steady state is generally reached in 200–1000 s.

Theoretical study of cross-valley wind circulation

An analytical model is developed to investigate the interaction between the large scale prevailing air flow and the thermally induced slope wind circulation. The Boussinesq approximation is assumed. The lower order successive approximation solutions are obtained in spectral representation for both typical daytime and night time cases. The results have produced interesting circulation features often observed in mountainous terrain.

Spectral characteristics of HF ground backscatter

A fixed frequency (18.415 MHz) ionospheric backscatter sounder affording coherent detection and an associated data acquisition system were used to collect data in a form suitable for computer analysis. Most of the data collected related to single hop F-layer propagation, with backscatter from both land and sea being observed. Digital techniques were used to derive the Doppler spectra characterizing the backscatter returns, and to display the spectra in formats designed to delineate clearly the relevant spectral characteristics. A statistical analysis of the data revealed that, excluding periods of sunrise and sunset, typical mean daytime ionospheric Doppler shifts fell in the range ± 0.5 Hz, while the associated spectral widths were of the order of 0.3 Hz. The presence of ionospheric disturbances limited the spectral integration periods to values much less than those employed in ground wave propagation studies.

Field Strength Measurements at Ahmedabad of Short Wave Radio Signals from ‘Radio Sri Lanka’, Colombo, on 11·8 MHz

In this paper, results are reported on the studies of the variations in the field intensity of short wave radio signals. An automatic equipment was set up at Ahmedabad (23°01'N, 72°36E; I = 34°N) in July 1966 for recording field strength of short waves transmitted on 11·8 MHz from Colombo (06°54'N, 79°52'E; I = 5°S). The aims of this experiment were to obtain an average diurnal and seasonal variation of the short wave field intensity for a transequatorial propagation over a distance of about 2000 km and to report on some events like SID and SWF. Some typical day-time and night-time records of signal strength are presented, and the results obtained for the two years 1966–68 are discussed.

Radio frequency heating effects on electron density in the lowerEregion

Attention is drawn to the fact that the high-powered radio wave heaters developed for modifying F region ionospheric parameters can have a detectable influence on E region electron densities as well. Anticipated density variations are calculated for typical daytime and nighttime conditions. Furthermore, it is suggested that such variations will perturb the ionospheric dynamo current, thereby leading to a perceptible variation in the associated magnetic field measured on the ground.

Thermal diffusion in the F2-Region of the ionosphere

In order to examine the effects of thermal diffusion on F2-region ion densities we have evaluated ordinary and thermal diffusion coefficients to the second approximation of Chapman and Cowling for a partially ionized atomic oxygen plasma. Our results show that the thermal ambipolar diffusion coefficient, like the ordinary ambipolar diffusion coefficient, is not significantly affected by the electrons. Its value is determined almost entirely by ion-neutral interactions. We have solved the steady state continuity equation of the oxygen ions for typical mid-latitude daytime conditions and find that thermal diffusion acts to reduce the ion density at 600 km by 24 per cent. At altitudes above 300 km diffusion of the minor ions, H +, He + and N + is not influenced by the neutral species, and the ordinary and thermal diffusion coefficients of these ions may be evaluated for a fully ionized plasma. Our solutions of the continuity equations of these ions show that, at 700 km, the effect of thermal diffusion is to reduce the density of H + ions by 46 per cent, to reduce the density of He+ ions by 22 per cent, and to increase the density of N + ions by more than a factor of 2.

Seasonal variations of ionospheric absorption deduced from A3-measurements in the frequency range 100–2000 Kc/s

A3-absorption data in different frequency ranges indicate the existence of real seasonal changes in the plasma structure of the mesosphere region. From enhancements of absorption in winter, dominating on higher frequencies, and in summer, mainly present on lower frequencies, a quasi-semi-annual wave is found on all frequencies. The maxima are thought to be of very different origin. The ‘winter anomaly’ is studied through the pronounced frequency dependence of this phenomenon, which is centered on the months of December and January and is a typical daytime effect situated in the upper part of the D-layer. On the contrary the summer enhancement is probably an effect in the lowest part of the D-region, originating in the pre-sunrise ionisation. If the absorption data are corrected for this (cosmic ray) D-layer attenuation, nearly the same exponent of the zenith angle law is found over the entire frequency range, in spite of the fact that the frequency dependence is markedly different between summer and winter.

The Determination of Ionospheric Electron Distribution

The virtual height versus frequency integral is derived, neglecting absorption and the earth's magnetic field. It is shown how solution of this integral equation can be obtained using the Laplace transformation, and how true height versus frequency can be determined graphically from virtual height versus frequency curves. Application of the method is made to some typical nighttime and daytime ionosphere records.