THE POLARIZATION OF PC5 ULF WAVES AROUND DAWN - A POSSIBLE IONOSPHERIC CONDUCTIVITY GRADIENT EFFECT

In this study over 100 Pc5 events observed on the SAMNET magnetometer array have been catalogued. The ground horizontal polarization of thes e waves has been investigated and it has been shown that there are lar ge differences between the polarization azimuth distributions on eithe r side of noon. In the morning the azimuth distributions are spread ev enly across all possible orientations whereas in the afternoon they ar e predominantly in a North-South orientation. The difference appears t o be the result of a specific polarization pattern that occurred acros s the SAMNET array for 31 of the Pc5 events. These 31 Pc5 events all o ccurred in the morning sector, close to the dawn terminator. It is sug gested that this special polarization pattern is a consequence of iono spheric conductivity gradients that occur at around dawn. This theory is reinforced by the comparison of the local times of these polarizati on variations with the local time of sunrise. By employing ionospheric conductivity data from the EISCAT radar for two of these Pc5 events, this connection has been studied in more detail.