A formula that is both numerically convenient to use and accurate to within 2% for all χ when X > 50 may be used to approximate ch (X, χ). The approximation may be applied to optical depth calculations in nonexponential models of the terrestrial atmosphere, the resulting errors still remaining smaller than 4% for most ionospheric applications.

Measurements of columnar electron contents made at frequent time intervals throughout the night permit the determination of the rate at which the total content decays. It is found that at midlatitudes this rate is correlated with the geomagnetic activity index Kp, while such correlation is negligible at lower latitudes. An annual variation of the decay rate is observed, with a maximum around June. These results are consistent with the theory that the nighttime ionosphere is maintained in part by the downward flux of electrons from the protonosphere.

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A method for the study of planetary ionospheres is presented which is based on the use of radio waves propagated between the earth and a spacecraft on an occulting trajectory beyond the planet. Phase path, group path, or amplitude measurements made during spacecraft immersion into, and emersion from, the occulted zone could be used to deduce vertical electron density profiles at the two limb positions probed by the waves. If two or more harmonically related frequencies are used, the ionospheric measurements are self-calibrating; thus the extreme measurement and computational precision that otherwise would be required are avoided. Furthermore, the use of more than one frequency makes it possible to separate dispersive ionospheric effects from the nondispersive refractive effects of the neutral atmosphere, so that both neutral and ionized regions of the atmosphere could be studied in some detail. It is suggested that this simple technique would be particularly applicable for the initial exploration of planetary atmospheres.

Accurate measurements of the amount of Faraday rotation imposed on VHF telemetry transmissions from a geostationary satellite have been analyzed to study rates of production and loss of electrons in the F region of the ionosphere. The rate of photoionization of atomic oxygen, integrated with respect to height through the ionosphere, was determined at sunrise over Hawaii. The analysis also yielded the magnitude of the linear loss coefficient at an altitude of 300 km. The integrated rate of production of O+ ions at sunrise was used to deduce the total flux of solar EUV radiation in the 165–911 A wavelength range incident on the earth's atmosphere. Atomic oxygen photoionization rates and solar EUV flux values were obtained on a daily basis, and loss coefficients were obtained on a monthly average basis, over the 2‐year period extending from September 1964 through August 1966. The integrated rate of production of O+ was found to average 1.7 × 109 ions/cm² sec at sunrise over Hawaii. The flux of solar EUV radiation incident on the atmosphere was found to average 4.5 × 1010photons/cm² sec. The loss coefficient averaged 1.4 × 10−5 sec−1 at an altitude of 300 km. No seasonal variation was observed in the integrated O+ production rate, but the loss coefficient had a winter‐to‐summer variation of about 2∶1.