Many days may pass without the observation of a single whistler. On other occasions whistlers may occur at rates exceeding one per second.Synoptic data on the occurrence of whistlers show that whistler activity tends to be greatest at middle latitudes, reaching a maximum in the vicinity of 50 degrees geomagnetic latitude. At the geomagnetic equator whistlers are virtually unknown, and in polar regions their rate is significantly lower than in middle latitudes.Recordings made simultaneously at spaced stations show that a whistler may spread over an area typically about 1000 km in diameter. On occasion process continues, resulting in trains of whistlers at the two e.nds of the path.One can see from the diagram that the ratio between the delays of the members of the echo train will be 1,3, 5, and so forth at the one-hop end of the path, and 2, 4, 6, and so forth at the source end of the path.Although only one path is shown in Fig. 1-2 for purposes of explanation, the ionosphere often contains a number of such paths, which can be excited approximately simultaneously by the signal radiated from a lightning source. Because the lengths of these paths are different, and because the distribution of electron density and magnetic-field strength along them is different, the delays will be different.Often when multiple ducts are present some whistlers appear to result from propagation over a combination of these ducts. These "mixed-path" whistlers are not well understood; it is probable that conditions for coupling between the ends of the ducts are fairly good on certain occasions. A possible explanation is that the ducts terminate well above the F layer, permitting energy to spread out, be reflected from the lower boundary, and enter any other ducts that are present.Nose whistlers. During the preparation period for the International Geophysical Year, observations of whistlers were extended to higher latitudes and to higher frequencies. At high latitudes and at frequencies below 10 kc/s, and at middle latitudes and higher frequencies, a new type of whistler was observed.This whistler had the frequency-time shape sketched in Fig. 1-3, showing a distinct nose at which the delay was a minimum. The frequency of this nose was called the "nose" frequency and these whistlers were called "nose" whistlers. At frequencies above the nose, the frequency of the whistler increased with time.Simultaneously, below the nose, the frequency decreased with time. From an extension of the dispersion theory, it was found that this type of whistler effect, as yet unexplained, holds for both hemispheres.(3) During a magnetic storm the apparent electron density in the magnetosphere, as measured by whistlers, drops greatly. Densities may range all the way from two-thirds to one-tenth of their normal quiet-day values. The explanation of this effect, when it is found, promises to affect theories of the F region and the magnetosphere significantly.Artificially generated whistler-mode signals. The understanding of the phenomenon of whistlers has led natura...
