WHISTLER

also called Whistling Atmospheric, gliding high-to-low-frequency sound occasionally emitted by a sensitive audioamplifier. It initially lasts about half a second, may be repeated at equal intervals, and grows progressively longer and fainter with time. The origin of these sounds in the atmosphere is lightning discharges, particularly the associated electromagnetic radiation with frequencies of 300 to 30,000 hertz. These electromagnetic waves cannot be heard directly but are converted into audible sound waves of the same frequency range by the audioamplifier. They are propagated from one hemisphere to another in the ionized portions of the Earth's upper atmosphere (as high as 19,000 to 26,000 km [12,000 to 16,000 miles] at the magnetic equator). Led by the faster, higher-frequency waves, these signals move along the magnetic lines of force until they are reflected at the corresponding geomagnetic latitude in the opposite hemisphere. The arrival at the amplifier of the high-frequency component ahead of the lower-pitched sound accounts for the whistle, and the repeated reflections caused by dispersion and absorption of the waves are responsible for the subsequent fainter and longer whistle falls. Studies of these dispersion effects have been used to determine the electron density at altitudes of 19,000 to 26,000 km, as well as daily, annual, and long-term variations of the electron density in the upper atmosphere.