PHASE SCINTILLATION AND IRREGULARITIES IN THE NORTHERN POLAR IONOSPHERE

Radio transmissions from Navy Navigation Satellite System (NNSS) satellites have been used to study phase scintillation and ionospheric irregularities at northern polar latitudes. The observations were made at Ny Alesund, a station in the vicinity of the daytime cusp and under the polar cap on the nightside. The analysis concentrates on data obtained over an extended period when more than 7000 satellite passes were recorded to provide estimates of the sigma phi phase scintillation parameter for some 300,000 data intervals. The results show a marked difference in scintillation and irregularity occurrence with season. During the summer months the main feature is a maximum around magnetic noon, apparently associated with irregularities in the cusp region. In winter, irregularities are found at all times of day though there is a noon maximum that extends in a latitudinal belt into the afternoon and evening. The premidnight sector shows the highest occurrence levels in the polar cap. Investigation of signal fading rates indicates that doppler shifting resulting from irregularities convecting rapidly towards the station in the premidnight polar cap may contribute to the occurrence patterns. Under quiet geomagnetic conditions during summer the irregularities form a belt essentially in the morning sector, following the auroral oval. There is expansion to lower latitudes with increased geomagnetic activity and increased occurrence in the afternoon. In winter, irregularities can be seen at all hours even under quiet conditions, with the afternoon and evening showing greatest occurrence at times of geomagnetic disturbance. The pattern of scintillation occurrence with respect to the station indicates that the irregularities may be in the form of field-aligned sheets of low axial ratio. In contrast to observations of L shell confined sheets at auroral latitudes there are indications from the current work that the irregularities are stretched in the antisunwards convective flow across the polar cap.