Latitude Variation of the Lower Ionosphere

In March and April 1965 the University of Illinois and the GCA Corporation launched five Nike‐Apache rockets from the aircraft carrier U.S.N.S. Croatan as a part of the NASA Mobile Launch Expedition. As the Croatan followed the meridian at 78° west longitude along the west coast of South America, two rockets were launched at 13° south latitude (at the geomagnetic equator), one rocket at 30°S, one at 44°S, and one at 58°S. Launch times were chosen to correspond to a solar zenith angle of 60° at each of the four different latitudes. Each of the payloads carried a radio propagation experiment and a Langmuir probe for themeasurement of electron concentration and collision frequency in the lower ionosphere. Between 70 and 90 km, the electron concentration profiles at 13° and 58°S are nearly coincident, and the profiles at 30° and at 44°S are also nearly coincident. This pattern suggests a geomagnetic anomaly in the D region of the ionosphere comparable with the geomagnetic anomaly in the F region. From 90 to 150 km the profiles show no systematic differences.