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Continuous observations\ud of fluctuations of the geomagnetic field at Sodankylä Geophysical Observatory\ud (L = 5.2) were used for a comprehensive morphological study of the spectral\ud resonance structure (SRS) seen in the background electromagnetic noise in the\ud frequency range of 0.1–4.0 Hz. It is shown that the occurrence rate of SRS is\ud higher in the nighttime than in the daytime. The occurrence rate is higher in\ud winter than in summer. The SRS frequencies and the difference between\ud neighbouring eigenfrequencies (the frequency scale) increase towards nighttime\ud and decrease towards daytime. Both frequency scale and occurrence rate exhibit\ud a clear tendency to decrease from minimum to maximum of the solar activity\ud cycle. It is found that the occurrence rate of SRS decreases when geomagnetic\ud activity increases. The SRS is believed to be a consequence of a resonator for\ud Alfvén waves, which is suggested to exist in the upper ionosphere. According\ud to the theory of the ionospheric Alfvén resonator (IAR), characteristics of\ud SRS crucially depend on electron density in the F-layer maximum, as well as on\ud the altitudinal scale of the density decay above the maximum.We compared the\ud SRS morphological properties with predictions of the IAR theory. The\ud ionospheric parameters needed for calculation were obtained from the ionosphere\ud model (IRI-95), as well as from measurements made with the ionosonde in\ud Sodankylä. We conclude that, indeed, the main morphological properties of SRS\ud are explained on the basis of the IAR theory. The measured parameters of SRS\ud can be used for improving the ionospheric models.  <b>Key words. </b>Ionosphere (auroral\ud ionosphere; wave propagation) – Radio Science (electromagnetic noise and\ud interference

annales geophysicae

Morphology of the spectral resonance structure of the electromagnetic background noise in the range of 0.1–4 Hz at &amp;lt;i&amp;gt;L&amp;lt;/I&amp;gt; = 5.2