PHASE SCINTILLATION AND IRREGULARITIES IN THE NORTHERN POLAR IONOSPHERE

Radio transmissions from Navy Navigation Satellite System (NNSS) satel lites have been used to study phase scintillation and ionospheric irre gularities at northern polar latitudes. The observations were made at Ny Alesund, a station in the vicinity of the daytime cusp and under th e polar cap on the nightside. The analysis concentrates on data obtain ed over an extended period when more than 7000 satellite passes were r ecorded to provide estimates of the sigma phi phase scintillation para meter for some 300,000 data intervals. The results show a marked diffe rence 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 wint er, irregularities are found at all times of day though there is a noo n maximum that extends in a latitudinal belt into the afternoon and ev ening. The premidnight sector shows the highest occurrence levels in t he polar cap. Investigation of signal fading rates indicates that dopp ler shifting resulting from irregularities convecting rapidly towards the station in the premidnight polar cap may contribute to the occurre nce patterns. Under quiet geomagnetic conditions during summer the irr egularities form a belt essentially in the morning sector, following t he auroral oval. There is expansion to lower latitudes with increased geomagnetic activity and increased occurrence in the afternoon. In win ter, irregularities can be seen at all hours even under quiet conditio ns, with the afternoon and evening showing greatest occurrence at time s of geomagnetic disturbance. The pattern of scintillation occurrence with respect to the station indicates that the irregularities may be i n 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 stretch ed in the antisunwards convective flow across the polar cap.