A study of Pc 3 coherence at cusp latitudes

Previous studies have taken data from multi-instrument arrays, both ground-based and in situ, to determine the spatiotemporal characteristics of Pc 3 pulsations (22-100 mHz) at cusp latitudes. Correlations of Pc 3 frequency band magnetometer, photometer, and riometer data have been cited as evidence of a local (ionospheric), rather than a distant (magnetospheric), source for these cusp pulsations. Electron precipitation modulated at Pc 3 frequencies is thought to be the source mechanism for the unstructured portion of the Pc 3 spectrum; such precipitation can modify local ionospheric conductivities, allowing modulated currents to flow at like frequencies. In this paper we undertake a statistical analysis of induction coil magnetometer records taken at Cape Parry and Sachs Harbor, Canada, in May-June 1985. Comparing the distribution functions of interstation X-X and Y-Y polarization and coherence estimates, we arrive at a simple model to estimate an upper bound for the coherence length, O(200 km), of Pc 3 source regions near the cusp. Such a determination is an important constraint in identifying the source of Pc 3 frequency band modulations of cusp latitude electron precipitation.