High-latitude irregularities in the top side ionosphere based on Isis 1 Thermal Ion Probe data

Thermal ion density irregularities at high latitudes have been investigated by using results from the spherical electrostatic analyzer flown on the polar-orbiting satellite Isis 1. The size of the irregularities is found to be highly variable, dimensions typically ranging from 1 to 140 km. Results from 3500 orbits in the top side ionosphere during the first year of operation have been examined. The daily and seasonal behavior of the equatorial boundary of the high-latitude irregularity zone and its dependence on magnetic activity have been established for the northern and southern hemispheres. The daily variation in the location of the boundary of the irregularity zone is found to be 14°±2° for any level of magnetic activity; the boundary is at the highest latitude near local noon. The irregularities consistently extend across the pole; there is no upper boundary. Systematic differences are found in the mean location of the equatorial boundary in the northern and southern hemispheres. In the south the boundary is on the average 4.8°±0.6° closer to the pole than in the north. Good agreement is found between the direct measurements of the irregularity boundary and scintillations, severe top side spread F and ≤300-eV electron precipitation boundaries, and trough and plasmapause positions on the night side. Significant differences found to exist between the irregularity boundary and plasmapause positions on the day side are considered to be due to their different origins.