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Ann. Geophys., 21, 779-786, 2003
www.ann-geophys.net/21/779/2003/
© European Geosciences Union 2003
Morphology of the spectral resonance structure of the electromagnetic background noise in the range of 0.1–4 Hz at L = 5.2
A. G. Yahnin1, N. V. Semenova1, A. A. Ostapenko1, J. Kangas2, J. Manninen2, and T. Turunen2
1Polar Geophysical Institute, Apatity, 184200, Russia
2Sodankylä Geophysical Observatory, Sodankylä, Finland
Abstract. Continuous observations
of fluctuations of the geomagnetic field at Sodankylä Geophysical Observatory
(L = 5.2) were used for a comprehensive morphological study of the spectral
resonance structure (SRS) seen in the background electromagnetic noise in the
frequency range of 0.1–4.0 Hz. It is shown that the occurrence rate of SRS is
higher in the nighttime than in the daytime. The occurrence rate is higher in
winter than in summer. The SRS frequencies and the difference between
neighbouring eigenfrequencies (the frequency scale) increase towards nighttime
and decrease towards daytime. Both frequency scale and occurrence rate exhibit
a clear tendency to decrease from minimum to maximum of the solar activity
cycle. It is found that the occurrence rate of SRS decreases when geomagnetic
activity increases. The SRS is believed to be a consequence of a resonator for
Alfvén waves, which is suggested to exist in the upper ionosphere. According
to the theory of the ionospheric Alfvén resonator (IAR), characteristics of
SRS crucially depend on electron density in the F-layer maximum, as well as on
the altitudinal scale of the density decay above the maximum.We compared the
SRS morphological properties with predictions of the IAR theory. The
ionospheric parameters needed for calculation were obtained from the ionosphere
model (IRI-95), as well as from measurements made with the ionosonde in
Sodankylä. We conclude that, indeed, the main morphological properties of SRS
are explained on the basis of the IAR theory. The measured parameters of SRS
can be used for improving the ionospheric models.
Key words. Ionosphere (auroral
ionosphere; wave propagation) – Radio Science (electromagnetic noise and
interference)
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