Semidiurnal tide in the E region from incoherent scatter measurements at Arecibo

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Abstract
A five‐pulse technique was implemented for the 430 MHz incoherent scatter radar at Arecibo Observatory (18.3°N), to explore the detailed thermal structure of the E region from 105 to 130 km with an altitude resolution of 3 km. Five days of measurements in Sept–Oct 1970 showed long‐period temperature fluctuations having a downward phase progression. The temperature oscillations are interpreted as manifestations of a semidiurnal tide which is quite stable over a 12‐day period, together with a superimposed spectrum of shorter‐period gravity waves which are randomly phased from day to day. The semidiurnal tide increased to a maximum amplitude of 17% of the mean temperature near 115 km and decreased above this altitude as dissipative effects became important. The vertical wavelength, deduced from the altitude variation of semidiurnal tidal phase, showed a smooth increase from about 20 km at an altitude of 109 km to about 50 km at an altitude of 127 km. No ready interpretation of the observed tidal characteristics was possible in terms of present theories for the semidiurnal tide. Altitude profiles of mean daytime temperature and ion‐neutral collision frequency were also obtained from the measurements. The mean temperature gradient between 115 and 130 km was 15 K/km, which is somewhat larger than that given by current atmospheric models.

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