The characteristics of ionospheric heating‐produced ELF/VLF waves over 32 hours

In October 1981, ELF/VLF waves were produced in the ionosphere in an alternating sequence of approximately 1, 2, 3, and 5 kHz by modulated HF heating of the ionosphere near Tromsø, Norway, during a 32‐hour period of high geomagnetic disturbance. The apparent source heights, which are derived from the wave phases, show a diurnal variation from about 55 km during the day to about 74 km at night. Other wave parameters such as amplitude, direction, and ellipticity of the ELF polarization ellipse also show a diurnal variation as well as modulation by Pc 5 hydromagnetic waves. We compare the variation of the ELF wave parameters with electric fields measured by the Scandinavian Twin Auroral Radar Experiment (STARE) and with riometer and magnetometer data in an attempt to understand the factors controlling the ELF wave generation process. We are able to successfully model many of the measured wave characteristics. High electron densities at low altitudes were found necessary to explain the daytime measurements.