Thermal plasma density measurements obtained by using the spherical electrostatic analyzer probe carried by the polar orbiting satellite Isis 1 have been spectrally analyzed to provide quantitative results on the amplitude and scale size distribution of the plasma irregularities in the high-latitude top side ionosphere. The range of scale sizes examined, 200 m to over 100 km under normal conditions, is more than an order of magnitude greater than available from ground-based measurements. Data in the altitude range 574--3523 km and from the period February 1969 to April 1972 have been analyzed. Irregularities in the high-latitude region from 40¿ invariant latitude up to the invariant L pole have been studied. In over 90% of the cases examined, the power spectra are found to fit a power law of the form P=P0f-&bgr;, where the index &bgr; varies within the range 1.5x2.5. The power spectral density was integrated over the scale size range 1--10 km in order to analyze quantitatively latitudinal variations of the irregularity amplitude. While the boundaries of the high-latitude irregularity zone on particular orbits examined by using this technique are in agreement with earlier morphological studies, it is also evident that the amplitude of the irregularities within the high-latitude irregularity region itself shows irregular enhancements above a generally raised amplitude level.