Ionization Gradients Research Articles

The Poynting Vector in the Ionosphere

The equations of motion of ionospheric electrons in the field of plane electromagnetic waves subject to the frictional force of collision and to the force of the earth's magnetic field are developed in a form permitting graphical calculation of the wave polarization. The complex Poynting vector is then calculated in terms of the polarization, the complex refractive index, and a third function related to the forward tilt of the electric vector. Graphical integration is used to obtain curves representing the direction of energy flow for the ordinary and extraordinary modes, in a parabolic distribution of ionization for fixed values of geomagnetic latitude, collision frequency, and wave frequency for the case of vertical wave propagation. When collision is taken into account, the deflection has a small westward component for both ordinary and extraordinary modes. At zero collision frequency the deflections are in the vertical plane of the earth's field; the ordinary mode bending towards the poles and the extraordinary mode towards the equator. The normal ionization gradient with latitude, together with diurnal expansion and contraction of the ionized region, can explain the diurnal variation in the f <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">z</sup> -f <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0</sup> critical frequency difference as due to the diurnal variation in the total path deflection.

Longitudinal and transverse propagation in Canada

The apparent terrestrial magnetic field in the ionosphere, as calculated from measured critical-frequency differences of the extra-ordinary and ordinary modes, shows large diurnal and seasonal variations. The longitudinal mode, which is often seen at high magnetic latitudes, gives a consistently different field than the transverse mode. These results are explained as due to deflections of the energy path from the vertical in conjunction with variations in horizontal ionization gradients. The direction of the complex Poynting vector is calculated for the different modes. The westward component of the deflection of the longitudinal ordinary mode is shown to be as large as the north-south deflection.

The absorption of long and very-long waves in the ionosphere

Experimental observations on a series of low frequencies have been used to show that models of the ionosphere in which the ionization density increases linearly or parabolically with height are unsuitable for explaining long and very-long-wave reflection phenomena. Agreement with observation can however be obtained if an absorbing or D-region of small ionization density but several kilometers in thickness is assumed to lie below the main reflecting region. A model in which the ionization density increases exponentially with height is found to include such an absorbing region; this model is used to deduce values for the ionization gradient and the electronic collision frequency in the very-long-wave reflecting region.

Lateral Deviation of Radio Waves at Sunrise

THE observation of systematic lateral deviations of radio waves requires that such deviations shall be large enough not to be obscured by the random deviations always occurring. The sunrise period is particularly favourable for such observations, since the relatively rapid changes in ionization at this time produce appreciable horizontal ionization gradients. The direction of these is such as to move the reflexion point of the ray towards the east, and qualitative confirmation of this effect has previously been noted.

Long-distance observations of radio waves of medium frequencies

The frequency-change technique of Appleton and Barnett has been applied to the analysis of the downcoming waves from a distant transmitter. The observations were carried out simultaneously at distances of 25 and 700 km. from the emitter, which operated on a frequency of 1415 kc./sec. It was found that several downcoming waves were present at the more distant receiving station. Each of these waves was identified by using the path-length of the singly reflected wave from the E layer as a reference. In this way it was found that the equivalent heights of both the E and the F layers are relatively stable over the 700-km. transmission path, and do not vary appreciably with the angle of incidence of the wave. The equivalent height of the F layer showed a pronounced minimum at about 3 a.m. each morning. The rate of propagation of the minimum height in the horizontal direction appears to be slower than the rate of sunset propagation in the same direction. The ionization-density in the E layer in the early morning was always greater than 2.4 × 103 electrons per cm3., and during half the period of the observations was less than 8.3 × 103 electrons per cm3. The intermediate layer was observed regularly at sunrise. From the measurements of equivalent heights at different angles of incidence it is concluded that the gradient of ionization at the lower boundary of the E layer is sharp.

The propagation of medium radio waves in the ionosphere

All the available measurements of sky-wave intensities at medium frequencies are collated and expressed as field-strength, distance curves for six typical wavelengths and for distances from 25 to 1000 km. It is shown how this material may be used for the determination of the non-fading radii of broadcasting emitters over country of any effective conductivity. From the observational material an empirical expression for the reflection coefficient of the lower E layer of the ionosphere is derived. It is shown that the observations are incompatible with the existence of a linear or parabolic gradient of ionization in this layer. This incompatibility is not removed by the assumption of an absorbing or D region below the E layer, or by consideration of the variation with height of the collision frequency ν of an electron with the air molecules in the E layer. It is found that the observations can be fully explained if the gradient of ionization is given by the exponential form N = h, where h is the height in km. above the region where ionization first becomes appreciable. This gradient also gives rise to equivalent heights which are in agreement with experience. It is found that ν has a value of 106 collisions per second at a height of 90 km., in close agreement with Chapman's recent estimate. It is shown that the conclusions reached are not affected by use of the ray methods of geometrical optics, or by neglect of the influence of the earth's magnetic field.

Some notes on wireless methods of investigating the electrical structure of the upper atmosphere. I

Various direct wireless methods of measuring the effective height of the atmospheric ionized layer are discussed and compared. For a layer of horizontal stratification, and under conditions for which the influence of the earth's magnetic field may be neglected, the general equivalence of the quantities measured by the various methods is demonstrated. The effective height in such cases is shown to be greater than the maximum height reached by the atmospheric ray. Proposals for using these methods to obtain information concerning the gradient of ionization in the layer are put forward.