Seasonal and longitudinal variations in the occurrence frequency of magnetospheric ionization ducts

Abstract

Seasonal effects in the occurrence frequency of conjugate echoes on top side sounder ionograms recorded at nine telemetry stations, located at various longitudes, are studied by using Alouette 2, Isis 1, and Isis 2 satellite data. The combined data from all telemetry stations (2.8 × 105 ionograms) show minimums in the percentage of ionograms containing conjugate echoes at the equinoxes (2%) and maximums in the percentage of ionograms containing conjugate echoes at the solstices (about 7%). However, data recorded at a single telemetry station may show a maximum at only one of the solstices depending upon the longitude of the station. For example, data recorded at Quito, Ecuador (281°E), show only a December solstice maximum, whereas data recorded at Singapore (104°E) show only a June solstice maximum. A geometric model based on these results and on the observation that conjugate echoes are observed mainly during the nighttime hours is presented. According to this model, the conjugate ducts tend to be created when the angle between the geomagnetic axis and the normal to the plane of the ecliptic is maximum, that is, when the conjugate points of magnetic field lines are most asymmetric with respect to the sun-earth line. Magnetospheric currents flowing from one hemisphere to the other along geomagnetic field lines are suggested as the cause of the field-aligned ionization irregularities which produce the observed conjugate echoes on top side sounder ionograms.