. Compendium of meteorology. Meteorology. 336 THE UPPER ATMOSPHERE into two or more returned components, and the angle of arrival give information concerning the medium re- sponsible for return of the wave energy. In terms of the penetration frequency /o and the wave frequency /, the index of refraction for elementary purposes is given by .2 = 1 --^ J 2 (2) Clearly the index of refraction is a function of frequency and therefore the ionosphere is a dispersive medium. Usually, also, each successive wave packet emitted is of a radio frequency differing incrementally from that preceding. When thi
. Compendium of meteorology. Meteorology. 336 THE UPPER ATMOSPHERE into two or more returned components, and the angle of arrival give information concerning the medium re- sponsible for return of the wave energy. In terms of the penetration frequency /o and the wave frequency /, the index of refraction for elementary purposes is given by .2 = 1 --^ J 2 (2) Clearly the index of refraction is a function of frequency and therefore the ionosphere is a dispersive medium. Usually, also, each successive wave packet emitted is of a radio frequency differing incrementally from that preceding. When this process is carried out over a large frequency range, the results can be recorded on a graph as in Fig. 2. 5 IE Z liJ Q. tr UJ o a. >- z O 1 o UJ 'H i£' 2 a. CL "^ UJ 1 3 z O o UJ ^^^-. J / cc o I k J J 'i""^'^' u. LiJ q: 1 ~i UJ S J J F^-REGION 2 x â ^â â JJ }E- -REGION EMITTED PULSES !t < en fc < WAVE FREQUENCY ^ TIME REQUIRED TO SWEEP OVER THE FREQUENCY RANGE *â Fig. 2.âTypical data obtained by radio sounding of the ionosphere. This is the usual form taken by data embracing time of flight versus frequency in temperate latitudes during daylight. Splitting of the original wave packet into two or more components is caused by the effect of the earth's magnetic field. Transmission and reception are at verti- cal incidence. This technique is the most widely used and is an elegant development of the original Breit- Tuve experiment. Study of the ionosphere at vertical incidence immensely simplifies the general theory for a special case and, although involving some approxima- tions, has given the bulk of useful information about the ionosphere. This procedure fails at low frequencies and is usually replaced by the Appleton-Barnett phase- interference method. Polarization and angle-of-arrival measurements are quite important but have been little used systematically because of inherent interpretative and instrumental difficulties. More attention should
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