Signatures of the ionospheric Alfvén resonator from AUREOL-3 ULF/ELF fluctuation measurements

From the AUREOL-3 satellite, simultaneous measurements of plasma density and ULF/ELF electric and magnetic fields on March 16, 1982 were obtained. Based on spectra analysis and impedance function calculation of data from 04:48:37 to 04:52:10 UT time (hh:mm:ss), the origin of the field fluctuations below 5.0 Hz is examined. A high coherence exists between the correlated variations of the mutually orthogonal electric and magnetic fields. Below 2.0 Hz an oscillatory phase difference of about ±π implies the excitation of traveling waves. The phase difference of ±π/2 appears at 4.0 Hz, indicative of a standing wave. Taking into account the local magnetic field and plasma density, for the frequencies between 0.3 and 4.0 Hz, the impedance comes very near to the local Alfvén impedance. In this way, the peaks at 0.5 and 4.0 Hz are identified, respectively, as a traveling and a standing shear Alfvén wave, which presumably resulted from the operation of the ionospheric Alfvén resonator (IAR). At the lowest frequency below 0.1 Hz, the approach of the wave impedance to the reciprocal of local height-integrated Pedensen conductivity implies the AUREOL-3 satellite's crossing of spatially structured field-aligned currents closed through the ionosphere, as suggested by Forget et al. (1991. Ionospheric closure of small scale Birkeland currents. Journal of Geophysical Research 96, 1843–1847).