Reconstruction of the ionospheric influence on ground-based observations of a short-duration ULF pulsation event

Abstract

Recent theoretical advances concerning the influence of the ionosphere on ULF pulsations indicate clear differences depending on whether uniform or non-uniform ionospheric conductivity is regarded. To test these new theoretical concepts, simultaneous observations of a Pc5 event on 2 February 1977 made by the Scandinavian Magnetometer Array (SMA) and the STARE radar are analysed. Taking into account a horizontally uniform conductivity, ground magnetic observations below the STARE field of view could not be predicted in a satisfactory manner from the ionospheric electric field measurements. To allow for non-uniform conductivity in our model calculations we analysed the spatial characteristics of the pulsation wave field using both the SMA and STARE data. We thus derived a first-order approximation of the incident wave field from which the electric field of the reflected wave has been computed for various non-uniform conductivity distributions. From the resulting ionospheric and field-aligned currents the ground magnetic field is determined. In a trial-and-error procedure conductivity and partly also the incident electric field have been changed until satisfactory agreement between our model ionospheric electric field and model ground magnetic fields with those observed by the SMA and STARE, respectively, results. From the final model the spatial variation of the ionospheric rotation angle has been derived. It significantly deviates from the earlier predicted 90° value.