Abstract:
Statistics of spectral resonance structures of the ionospheric Alfven resonances (IAR) in the ULF frequency range 0.1–5.0 Hz is presented on the basis of the analysis of data obtained from July 2000 to December 2002 (2.5 years of observations) at the Karimshino station (Kamchatka, Russia) by the conventional three component search-coil magnetometer. We analyze both the dynamic spectra of three components and polarization spectra in order to distinguish IAR. Average frequency difference ΔF between the adjacent maxima, intensity and occurrence rate of the IAR spectra have been estimated from the averaged spectra.The results are the following: (a) there is an evident seasonal variation in the occurrence rate with maximum at the September–January period and almost complete absence of IAR structures at the spring–early summer time; (b) occurrence maximum in the daily variation is found at 21–23 LT in autumn–winter; almost all the IAR structures are observed at local night-time; (c) averaged ΔF is about 0.2–0.5 Hz in the autumn period but it increases up to 0.5–0.7 Hz in winter; (d) the IAR occurrence rate is in anti-correlation with the Kp index of the global magnetic activity; (e) intensity of D component exceeds essentially that of H component of the IAR structures in a majority of cases. Diurnal variations of resonance frequencies in the H and D components are not always identical.Seasonal variation of IAR occurrence can be explained under the assumption of the existence of the ionospheric source of IAR events. We produce full-wave computations assuming the presence of an ionospheric source with k−b distribution on horizontal wave numbers (b≃5/3) in correspondence with some satellite recordings. The results of the computations are found to coincide with observational IAR spectra rather well.
