Focusing of electromagnetic waves by Es-clouds

Journal of Atmospheric and Terrestrial Physics, 1960, Vol. 18, pp. 253 to 258. Pergamon Press Ltd.

Focusing

RESEARCH

NOTES

of electromaguetic

waves

(Received

1 January

Printed in Northern Ireland

by IT,-clouds

1960)

measurements of ionic pulse absorption, it is necessary to determine a calibration constant I,, of the instrument. This is given by the product of the amplitude of the first echo and twice the reflection height. In this formula it is assumed that the reflection coefficient of the ionosphere is unity. IN

Fig. 1. Histogram of I,-values with and without influence of E, for 1.73 MC/S at Lindau, July-September 1957.

In practice I,, can be measured as follows: the amplitude of the first echo from the F-layer is measured every 15 set during the night; mean values for intervals of 15 min are calculated; and these means are plotted in the form of a histogram. It is usually assumed that the large values are due to focusing effects and the small values to defocusing, both caused by varying curvature of the F-layer. If partial reflections from the Es-layer are observed, one would expect values for the P-echo amplitudes to be smaller than those without such partial reflections; and hence, from the formula, smaller lo-values should appear. However, our measurements at Lindau and Tsumeb/SWA occasionally showed larger values under these conditions, provided that the values of the E,-echo lay within 5-20 per cent of the first F-echo amplitude (Figs. l-3). This effect is particularly clear in measurements obtained on 16 September 1958 at Tsumeb (lined fields in Figs. 2 and 3). All attempts to explain this observation by means of focusing effects appear to fail. The objection is that such abnormally high values are never observed in the absence of Es-echoes, although this is much the commoner condition. These observations can, however, be explained by a focusing effect of the E,-clouds. Optical focusing is produced by convex lenses (refractive index n > 1). By analogy,

253

Research notes

Fig. 2. Histogram of I,-values with and without influence of for 2.20 MC/Sat Tsumeb, July-September 1958. wimno

E,

Es-mfl”mcr

Number

total number: 126

Jo

with Es-mflurncr

Fig. 3. Histogram of I,-values with and without influence of E, for 2.75 MC/S at Tsumeb, July-September 1958.

focusing can be expected in the ionosphere (refractive index 12< 1) when there is a concave structure of the level of constant ionization. This analogy is illustrated in Fig. 4. There is another possible explanation. It seems reasonable to believe that the E-layer sometimes consists of clouds with dimensions of the order of kilometres. The cone from the 254

Research

notes

optic3 : n > 1

ionosphere:

Fig. 4. Analogy between an optical lens and the ionosphere.

Fig. 5. Raypaths

of P-layer-echoes with refraction (The horizontal scale is expanded).

by E,-clouds.

transmitter to the first Fresnel zone in the F-layer (Fig. 5) may be free from any E,-clouds for some time, while there are some in the neighbourhood in random positions. The energy within the cone gives normal F-echo amplitudes. But additional energy is reflected from the F-layer within the Fresnel zone and refracted once by the E,-clouds, as shown by ray A Further additions may come from the in Fig. 5. This increases the P-echo amplitude. energy of rays such as B in Fig. 5. In the absence of ES-clouds, all of these energy sources would not contribute to the final amplitude of the F-echo. This theory must be checked by further investigations. It is intended to study the time variation of the effect, and correlate this with the movement of the ES-clouds. Max-Plunck-Inetitut

G. UMLAIJFT

fiir Aeronomie

Institutfiir Ionospharenphysik

Linduu tiber Northeim, Hannover

255