First evidence at high latitudes for the ionospheric Alfven resonator

A high-sensitive, two-component induction magnetometer was used to measure natural electromagnetic background noise in the frequency range 0.1-10 Hz. The measurements were carried out in Kilpisjarvi/Finland (geographic latitude 69.02 degrees N, longitude 20.87 degrees E, L similar to 6) in 1993. The average dynamic spectra were derived for four senses of polarization (two linear N-S and E-W and two circular R and L) with a spectral resolution of 0.1 Hz and time resolution of 20 min. During at least 4 out of 12 nights covered by the measurements, clear evidence of spectral resonance structures (SRS) known from midlatitude observations were seen in the magnetic noise spectra. As a rule, 3-4 spectral maxima were observed. The frequency spacing Delta F of SRS between the spectral peaks in the SRS varied between similar to 0.5 and similar to 1.5 Hz. The ratio of amplitudes between spectral maxima and minima constituting the SRS was about 2. The diurnal variation of Delta F did not differ in principle from the one observed at midlatitudes. The sense of polarization of the SRS signal varied between right-handed circular and linear with an arbitrary orientation of the magnetic axis. Electron density measurements in the ionospheric F region support the view that the SRS are due to the effect of the ionospheric Alfven resonator.