Ionospheric Alfvén resonator as seen by Freja satellite

Abstract

Sharp increase of the Alfvén speed above the ionosphere is expected to create a resonant cavity for the Alfvén waves traveling along the geomagnetic field lines (so‐called Ionospheric Alfvén Resonator). The wavelet transform has been used to analyze magnetic and electric fields recorded on Freja satellite. Cross‐phase spectra between field components in the frequency range 0.5–10 Hz reveal a pattern similar to that predicted theoretically for the resonator. The Fourier cross‐phase spectra fail to show any characteristic features. We conclude that this is due to the spatial structure of the resonator which can be examined only with local analysis techniques.