Spectral properties of the ionospheric Alfvén resonator observed at a low‐latitude station (<i>L</i> = 1.3)

Abstract

Data of the first half year of operation of a sensitive search coil magnetometer at a remote site in the island of Crete, Greece (35.15°N, 25.20°E), was used to investigate properties of the spectral resonance structure (SRS) of the ionospheric Alfvén resonator (IAR) at L = 1.3. Most of the properties known from earlier reports and a recent paper (A. G. Yahnin et al., Morphology of the spectral resonance structure of the electromagnetic background noise in the range of 0.1–4 Hz at L = 5.2, submitted to Annales Geophysicae , 2002) (hereinafter referred to as paper CP) at mid and high latitude ( L = 2.65 and L = 5.2) could be verified as being valid also at L = 1.3, but several new features were also found. In contrast to mid and high latitudes, SRS signatures were detected every night but not at all during daytime. The average frequency difference Δ f between two adjacent harmonics is very small (0.2 Hz) and does not exhibit a local time dependence from evening to night hours. The seasonal dependence is very weak though distinct. A large variability of Δ f from night to night was found which increases when proceeding from summer to winter. This variability could not be accounted for by standard IAR models employing an International Reference Ionosphere (IRI). Moreover, the modeled Δ f values typically exhibited a systematic offset to higher values as compared to observed ones. It is expected that calibration of the IRI model by local f o F 2 ionosonde measurements will improve the agreement between model and observation, but it cannot explain fully the variability of Δ f and the systematic offset. Most likely the standard IAR model itself requires revision to be fully applicable in the low‐latitude ionosphere.